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Revision 1.18 by root, Thu Oct 26 16:28:33 2006 UTC vs.
Revision 1.39 by root, Wed Aug 5 11:53:16 2009 UTC

…		…
3		3
4	SYNOPSIS	4	SYNOPSIS
5	use IO::AIO;	5	use IO::AIO;
6		6
7	aio_open "/etc/passwd", O_RDONLY, 0, sub {	7	aio_open "/etc/passwd", O_RDONLY, 0, sub {
8	my ($fh) = @_;	8	my $fh = shift
		9	or die "/etc/passwd: $!";
9	...	10	...
10	};	11	};
11		12
12	aio_unlink "/tmp/file", sub { };	13	aio_unlink "/tmp/file", sub { };
13		14
…		…
23	$req->cancel; # cancel request if still in queue	24	$req->cancel; # cancel request if still in queue
24		25
25	my $grp = aio_group sub { print "all stats done\n" };	26	my $grp = aio_group sub { print "all stats done\n" };
26	add $grp aio_stat "..." for ...;	27	add $grp aio_stat "..." for ...;
27		28
		29	# AnyEvent integration (EV, Event, Glib, Tk, POE, urxvt, pureperl...)
		30	use AnyEvent::AIO;
		31
28	# AnyEvent integration	32	# EV integration
29	open my $fh, "<&=" . IO::AIO::poll_fileno or die "$!";	33	my $aio_w = EV::io IO::AIO::poll_fileno, EV::READ, \&IO::AIO::poll_cb;
30	my $w = AnyEvent->io (fh => $fh, poll => 'r', cb => sub { IO::AIO::poll_cb });
31		34
32	# Event integration	35	# Event integration
33	Event->io (fd => IO::AIO::poll_fileno,	36	Event->io (fd => IO::AIO::poll_fileno,
34	poll => 'r',	37	poll => 'r',
35	cb => \&IO::AIO::poll_cb);	38	cb => \&IO::AIO::poll_cb);
…		…
46	Danga::Socket->AddOtherFds (IO::AIO::poll_fileno =>	49	Danga::Socket->AddOtherFds (IO::AIO::poll_fileno =>
47	\&IO::AIO::poll_cb);	50	\&IO::AIO::poll_cb);
48		51
49	DESCRIPTION	52	DESCRIPTION
50	This module implements asynchronous I/O using whatever means your	53	This module implements asynchronous I/O using whatever means your
51	operating system supports.	54	operating system supports. It is implemented as an interface to "libeio"
		55	(<http://software.schmorp.de/pkg/libeio.html>).
		56
		57	Asynchronous means that operations that can normally block your program
		58	(e.g. reading from disk) will be done asynchronously: the operation will
		59	still block, but you can do something else in the meantime. This is
		60	extremely useful for programs that need to stay interactive even when
		61	doing heavy I/O (GUI programs, high performance network servers etc.),
		62	but can also be used to easily do operations in parallel that are
		63	normally done sequentially, e.g. stat'ing many files, which is much
		64	faster on a RAID volume or over NFS when you do a number of stat
		65	operations concurrently.
		66
		67	While most of this works on all types of file descriptors (for example
		68	sockets), using these functions on file descriptors that support
		69	nonblocking operation (again, sockets, pipes etc.) is very inefficient.
		70	Use an event loop for that (such as the EV module): IO::AIO will
		71	naturally fit into such an event loop itself.
52		72
53	In this version, a number of threads are started that execute your	73	In this version, a number of threads are started that execute your
54	requests and signal their completion. You don't need thread support in	74	requests and signal their completion. You don't need thread support in
55	perl, and the threads created by this module will not be visible to	75	perl, and the threads created by this module will not be visible to
56	perl. In the future, this module might make use of the native aio	76	perl. In the future, this module might make use of the native aio
57	functions available on many operating systems. However, they are often	77	functions available on many operating systems. However, they are often
58	not well-supported or restricted (Linux doesn't allow them on normal	78	not well-supported or restricted (GNU/Linux doesn't allow them on normal
59	files currently, for example), and they would only support aio_read and	79	files currently, for example), and they would only support aio_read and
60	aio_write, so the remaining functionality would have to be implemented	80	aio_write, so the remaining functionality would have to be implemented
61	using threads anyway.	81	using threads anyway.
62		82
63	Although the module will work with in the presence of other (Perl-)	83	Although the module will work in the presence of other (Perl-) threads,
64	threads, it is currently not reentrant in any way, so use appropriate	84	it is currently not reentrant in any way, so use appropriate locking
65	locking yourself, always call "poll_cb" from within the same thread, or	85	yourself, always call "poll_cb" from within the same thread, or never
66	never call "poll_cb" (or other "aio_" functions) recursively.	86	call "poll_cb" (or other "aio_" functions) recursively.
		87
		88	EXAMPLE
		89	This is a simple example that uses the EV module and loads /etc/passwd
		90	asynchronously:
		91
		92	use Fcntl;
		93	use EV;
		94	use IO::AIO;
		95
		96	# register the IO::AIO callback with EV
		97	my $aio_w = EV::io IO::AIO::poll_fileno, EV::READ, \&IO::AIO::poll_cb;
		98
		99	# queue the request to open /etc/passwd
		100	aio_open "/etc/passwd", O_RDONLY, 0, sub {
		101	my $fh = shift
		102	or die "error while opening: $!";
		103
		104	# stat'ing filehandles is generally non-blocking
		105	my $size = -s $fh;
		106
		107	# queue a request to read the file
		108	my $contents;
		109	aio_read $fh, 0, $size, $contents, 0, sub {
		110	$_[0] == $size
		111	or die "short read: $!";
		112
		113	close $fh;
		114
		115	# file contents now in $contents
		116	print $contents;
		117
		118	# exit event loop and program
		119	EV::unloop;
		120	};
		121	};
		122
		123	# possibly queue up other requests, or open GUI windows,
		124	# check for sockets etc. etc.
		125
		126	# process events as long as there are some:
		127	EV::loop;
67		128
68	REQUEST ANATOMY AND LIFETIME	129	REQUEST ANATOMY AND LIFETIME
69	Every "aio_*" function creates a request. which is a C data structure	130	Every "aio_*" function creates a request. which is a C data structure
70	not directly visible to Perl.	131	not directly visible to Perl.
71		132
…		…
107	anymore (except possibly for the Perl object, but its connection to	168	anymore (except possibly for the Perl object, but its connection to
108	the actual aio request is severed and calling its methods will	169	the actual aio request is severed and calling its methods will
109	either do nothing or result in a runtime error).	170	either do nothing or result in a runtime error).
110		171
111	FUNCTIONS	172	FUNCTIONS
112	AIO FUNCTIONS	173	AIO REQUEST FUNCTIONS
113	All the "aio_*" calls are more or less thin wrappers around the	174	All the "aio_*" calls are more or less thin wrappers around the syscall
114	syscall with the same name (sans "aio_"). The arguments are similar	175	with the same name (sans "aio_"). The arguments are similar or
115	or identical, and they all accept an additional (and optional)	176	identical, and they all accept an additional (and optional) $callback
116	$callback argument which must be a code reference. This code	177	argument which must be a code reference. This code reference will get
117	reference will get called with the syscall return code (e.g. most	178	called with the syscall return code (e.g. most syscalls return -1 on
118	syscalls return -1 on error, unlike perl, which usually delivers	179	error, unlike perl, which usually delivers "false") as its sole argument
119	"false") as it's sole argument when the given syscall has been	180	after the given syscall has been executed asynchronously.
120	executed asynchronously.
121		181
122	All functions expecting a filehandle keep a copy of the filehandle	182	All functions expecting a filehandle keep a copy of the filehandle
123	internally until the request has finished.	183	internally until the request has finished.
124		184
125	All requests return objects of type IO::AIO::REQ that allow further	185	All functions return request objects of type IO::AIO::REQ that allow
126	manipulation of those requests while they are in-flight.	186	further manipulation of those requests while they are in-flight.
127		187
128	The pathnames you pass to these routines *must* be absolute and	188	The pathnames you pass to these routines *must* be absolute and encoded
129	encoded in byte form. The reason for the former is that at the time	189	as octets. The reason for the former is that at the time the request is
130	the request is being executed, the current working directory could	190	being executed, the current working directory could have changed.
131	have changed. Alternatively, you can make sure that you never change	191	Alternatively, you can make sure that you never change the current
132	the current working directory.	192	working directory anywhere in the program and then use relative paths.
133		193
134	To encode pathnames to byte form, either make sure you either: a)	194	To encode pathnames as octets, either make sure you either: a) always
135	always pass in filenames you got from outside (command line, readdir	195	pass in filenames you got from outside (command line, readdir etc.)
136	etc.), b) are ASCII or ISO 8859-1, c) use the Encode module and	196	without tinkering, b) are ASCII or ISO 8859-1, c) use the Encode module
137	encode your pathnames to the locale (or other) encoding in effect in	197	and encode your pathnames to the locale (or other) encoding in effect in
138	the user environment, d) use Glib::filename_from_unicode on unicode	198	the user environment, d) use Glib::filename_from_unicode on unicode
139	filenames or e) use something else.	199	filenames or e) use something else to ensure your scalar has the correct
		200	contents.
140		201
		202	This works, btw. independent of the internal UTF-8 bit, which IO::AIO
		203	handles correctly whether it is set or not.
		204
141	$prev_pri = aioreq_pri [$pri]	205	$prev_pri = aioreq_pri [$pri]
142	Returns the priority value that would be used for the next	206	Returns the priority value that would be used for the next request
143	request and, if $pri is given, sets the priority for the next	207	and, if $pri is given, sets the priority for the next aio request.
144	aio request.
145		208
146	The default priority is 0, the minimum and maximum priorities	209	The default priority is 0, the minimum and maximum priorities are -4
147	are -4 and 4, respectively. Requests with higher priority will	210	and 4, respectively. Requests with higher priority will be serviced
148	be serviced first.	211	first.
149		212
150	The priority will be reset to 0 after each call to one of the	213	The priority will be reset to 0 after each call to one of the
151	"aio_*" functions.	214	"aio_*" functions.
152		215
153	Example: open a file with low priority, then read something from	216	Example: open a file with low priority, then read something from it
154	it with higher priority so the read request is serviced before	217	with higher priority so the read request is serviced before other
155	other low priority open requests (potentially spamming the	218	low priority open requests (potentially spamming the cache):
156	cache):
157		219
		220	aioreq_pri -3;
		221	aio_open ..., sub {
		222	return unless $_[0];
		223
158	aioreq_pri -3;	224	aioreq_pri -2;
159	aio_open ..., sub {
160	return unless $_[0];
161
162	aioreq_pri -2;
163	aio_read $_[0], ..., sub {	225	aio_read $_[0], ..., sub {
164	...
165	};
166	};
167
168	aioreq_nice $pri_adjust
169	Similar to "aioreq_pri", but subtracts the given value from the
170	current priority, so effects are cumulative.
171
172	aio_open $pathname, $flags, $mode, $callback->($fh)
173	Asynchronously open or create a file and call the callback with
174	a newly created filehandle for the file.
175
176	The pathname passed to "aio_open" must be absolute. See API
177	NOTES, above, for an explanation.
178
179	The $flags argument is a bitmask. See the "Fcntl" module for a
180	list. They are the same as used by "sysopen".
181
182	Likewise, $mode specifies the mode of the newly created file, if
183	it didn't exist and "O_CREAT" has been given, just like perl's
184	"sysopen", except that it is mandatory (i.e. use 0 if you don't
185	create new files, and 0666 or 0777 if you do).
186
187	Example:
188
189	aio_open "/etc/passwd", O_RDONLY, 0, sub {
190	if ($_[0]) {
191	print "open successful, fh is $_[0]\n";
192	...
193	} else {
194	die "open failed: $!\n";
195	}
196	};
197
198	aio_close $fh, $callback->($status)
199	Asynchronously close a file and call the callback with the
200	result code. *WARNING:* although accepted, you should not pass
201	in a perl filehandle here, as perl will likely close the file
202	descriptor another time when the filehandle is destroyed.
203	Normally, you can safely call perls "close" or just let
204	filehandles go out of scope.
205
206	This is supposed to be a bug in the API, so that might change.
207	It's therefore best to avoid this function.
208
209	aio_read $fh,$offset,$length, $data,$dataoffset,
210	$callback->($retval)
211	aio_write $fh,$offset,$length, $data,$dataoffset,
212	$callback->($retval)
213	Reads or writes "length" bytes from the specified "fh" and
214	"offset" into the scalar given by "data" and offset "dataoffset"
215	and calls the callback without the actual number of bytes read
216	(or -1 on error, just like the syscall).
217
218	The $data scalar *MUST NOT* be modified in any way while the
219	request is outstanding. Modifying it can result in segfaults or
220	WW3 (if the necessary/optional hardware is installed).
221
222	Example: Read 15 bytes at offset 7 into scalar $buffer, starting
223	at offset 0 within the scalar:
224
225	aio_read $fh, 7, 15, $buffer, 0, sub {
226	$_[0] > 0 or die "read error: $!";
227	print "read $_[0] bytes: <$buffer>\n";
228	};
229
230	aio_move $srcpath, $dstpath, $callback->($status)
231	Try to move the *file* (directories not supported as either
232	source or destination) from $srcpath to $dstpath and call the
233	callback with the 0 (error) or -1 ok.
234
235	This is a composite request that tries to rename(2) the file
236	first. If rename files with "EXDEV", it creates the destination
237	file with mode 0200 and copies the contents of the source file
238	into it using "aio_sendfile", followed by restoring atime,
239	mtime, access mode and uid/gid, in that order, and unlinking the
240	$srcpath.
241
242	If an error occurs, the partial destination file will be
243	unlinked, if possible, except when setting atime, mtime, access
244	mode and uid/gid, where errors are being ignored.
245
246	aio_sendfile $out_fh, $in_fh, $in_offset, $length,
247	$callback->($retval)
248	Tries to copy $length bytes from $in_fh to $out_fh. It starts
249	reading at byte offset $in_offset, and starts writing at the
250	current file offset of $out_fh. Because of that, it is not safe
251	to issue more than one "aio_sendfile" per $out_fh, as they will
252	interfere with each other.
253
254	This call tries to make use of a native "sendfile" syscall to
255	provide zero-copy operation. For this to work, $out_fh should
256	refer to a socket, and $in_fh should refer to mmap'able file.
257
258	If the native sendfile call fails or is not implemented, it will
259	be emulated, so you can call "aio_sendfile" on any type of
260	filehandle regardless of the limitations of the operating
261	system.
262
263	Please note, however, that "aio_sendfile" can read more bytes
264	from $in_fh than are written, and there is no way to find out
265	how many bytes have been read from "aio_sendfile" alone, as
266	"aio_sendfile" only provides the number of bytes written to
267	$out_fh. Only if the result value equals $length one can assume
268	that $length bytes have been read.
269
270	aio_readahead $fh,$offset,$length, $callback->($retval)
271	"aio_readahead" populates the page cache with data from a file
272	so that subsequent reads from that file will not block on disk
273	I/O. The $offset argument specifies the starting point from
274	which data is to be read and $length specifies the number of
275	bytes to be read. I/O is performed in whole pages, so that
276	offset is effectively rounded down to a page boundary and bytes
277	are read up to the next page boundary greater than or equal to
278	(off-set+length). "aio_readahead" does not read beyond the end
279	of the file. The current file offset of the file is left
280	unchanged.
281
282	If that syscall doesn't exist (likely if your OS isn't Linux) it
283	will be emulated by simply reading the data, which would have a
284	similar effect.
285
286	aio_stat $fh_or_path, $callback->($status)
287	aio_lstat $fh, $callback->($status)
288	Works like perl's "stat" or "lstat" in void context. The
289	callback will be called after the stat and the results will be
290	available using "stat _" or "-s _" etc...
291
292	The pathname passed to "aio_stat" must be absolute. See API
293	NOTES, above, for an explanation.
294
295	Currently, the stats are always 64-bit-stats, i.e. instead of
296	returning an error when stat'ing a large file, the results will
297	be silently truncated unless perl itself is compiled with large
298	file support.
299
300	Example: Print the length of /etc/passwd:
301
302	aio_stat "/etc/passwd", sub {
303	$_[0] and die "stat failed: $!";
304	print "size is ", -s _, "\n";
305	};
306
307	aio_unlink $pathname, $callback->($status)
308	Asynchronously unlink (delete) a file and call the callback with
309	the result code.
310
311	aio_link $srcpath, $dstpath, $callback->($status)
312	Asynchronously create a new link to the existing object at
313	$srcpath at the path $dstpath and call the callback with the
314	result code.
315
316	aio_symlink $srcpath, $dstpath, $callback->($status)
317	Asynchronously create a new symbolic link to the existing object
318	at $srcpath at the path $dstpath and call the callback with the
319	result code.
320
321	aio_rename $srcpath, $dstpath, $callback->($status)
322	Asynchronously rename the object at $srcpath to $dstpath, just
323	as rename(2) and call the callback with the result code.
324
325	aio_rmdir $pathname, $callback->($status)
326	Asynchronously rmdir (delete) a directory and call the callback
327	with the result code.
328
329	aio_readdir $pathname, $callback->($entries)
330	Unlike the POSIX call of the same name, "aio_readdir" reads an
331	entire directory (i.e. opendir + readdir + closedir). The
332	entries will not be sorted, and will NOT include the "." and
333	".." entries.
334
335	The callback a single argument which is either "undef" or an
336	array-ref with the filenames.
337
338	aio_scandir $path, $maxreq, $callback->($dirs, $nondirs)
339	Scans a directory (similar to "aio_readdir") but additionally
340	tries to efficiently separate the entries of directory $path
341	into two sets of names, directories you can recurse into
342	(directories), and ones you cannot recurse into (everything
343	else, including symlinks to directories).
344
345	"aio_scandir" is a composite request that creates of many sub
346	requests_ $maxreq specifies the maximum number of outstanding
347	aio requests that this function generates. If it is "<= 0", then
348	a suitable default will be chosen (currently 6).
349
350	On error, the callback is called without arguments, otherwise it
351	receives two array-refs with path-relative entry names.
352
353	Example:
354
355	aio_scandir $dir, 0, sub {
356	my ($dirs, $nondirs) = @_;
357	print "real directories: @$dirs\n";
358	print "everything else: @$nondirs\n";
359	};
360
361	Implementation notes.
362
363	The "aio_readdir" cannot be avoided, but "stat()"'ing every
364	entry can.
365
366	After reading the directory, the modification time, size etc. of
367	the directory before and after the readdir is checked, and if
368	they match (and isn't the current time), the link count will be
369	used to decide how many entries are directories (if >= 2).
370	Otherwise, no knowledge of the number of subdirectories will be
371	assumed.
372
373	Then entries will be sorted into likely directories (everything
374	without a non-initial dot currently) and likely non-directories
375	(everything else). Then every entry plus an appended "/." will
376	be "stat"'ed, likely directories first. If that succeeds, it
377	assumes that the entry is a directory or a symlink to directory
378	(which will be checked seperately). This is often faster than
379	stat'ing the entry itself because filesystems might detect the
380	type of the entry without reading the inode data (e.g. ext2fs
381	filetype feature).
382
383	If the known number of directories (link count - 2) has been
384	reached, the rest of the entries is assumed to be
385	non-directories.
386
387	This only works with certainty on POSIX (= UNIX) filesystems,
388	which fortunately are the vast majority of filesystems around.
389
390	It will also likely work on non-POSIX filesystems with reduced
391	efficiency as those tend to return 0 or 1 as link counts, which
392	disables the directory counting heuristic.
393
394	aio_fsync $fh, $callback->($status)
395	Asynchronously call fsync on the given filehandle and call the
396	callback with the fsync result code.
397
398	aio_fdatasync $fh, $callback->($status)
399	Asynchronously call fdatasync on the given filehandle and call
400	the callback with the fdatasync result code.
401
402	If this call isn't available because your OS lacks it or it
403	couldn't be detected, it will be emulated by calling "fsync"
404	instead.
405
406	aio_group $callback->(...)
407	This is a very special aio request: Instead of doing something,
408	it is a container for other aio requests, which is useful if you
409	want to bundle many requests into a single, composite, request
410	with a definite callback and the ability to cancel the whole
411	request with its subrequests.
412
413	Returns an object of class IO::AIO::GRP. See its documentation
414	below for more info.
415
416	Example:
417
418	my $grp = aio_group sub {
419	print "all stats done\n";
420	};
421
422	add $grp
423	(aio_stat ...),
424	(aio_stat ...),
425	...;	226	...
426
427	aio_nop $callback->()
428	This is a special request - it does nothing in itself and is
429	only used for side effects, such as when you want to add a dummy
430	request to a group so that finishing the requests in the group
431	depends on executing the given code.
432
433	While this request does nothing, it still goes through the
434	execution phase and still requires a worker thread. Thus, the
435	callback will not be executed immediately but only after other
436	requests in the queue have entered their execution phase. This
437	can be used to measure request latency.
438
439	IO::AIO::aio_busy $fractional_seconds, $callback->() *NOT EXPORTED*
440	Mainly used for debugging and benchmarking, this aio request
441	puts one of the request workers to sleep for the given time.
442
443	While it is theoretically handy to have simple I/O scheduling
444	requests like sleep and file handle readable/writable, the
445	overhead this creates is immense (it blocks a thread for a long
446	time) so do not use this function except to put your application
447	under artificial I/O pressure.
448
449	IO::AIO::REQ CLASS
450	All non-aggregate "aio_*" functions return an object of this class
451	when called in non-void context.
452
453	cancel $req
454	Cancels the request, if possible. Has the effect of skipping
455	execution when entering the execute state and skipping calling
456	the callback when entering the the result state, but will leave
457	the request otherwise untouched. That means that requests that
458	currently execute will not be stopped and resources held by the
459	request will not be freed prematurely.
460
461	cb $req $callback->(...)
462	Replace (or simply set) the callback registered to the request.
463
464	IO::AIO::GRP CLASS
465	This class is a subclass of IO::AIO::REQ, so all its methods apply
466	to objects of this class, too.
467
468	A IO::AIO::GRP object is a special request that can contain multiple
469	other aio requests.
470
471	You create one by calling the "aio_group" constructing function with
472	a callback that will be called when all contained requests have
473	entered the "done" state:
474
475	my $grp = aio_group sub {
476	print "all requests are done\n";
477	};
478
479	You add requests by calling the "add" method with one or more
480	"IO::AIO::REQ" objects:
481
482	$grp->add (aio_unlink "...");
483
484	add $grp aio_stat "...", sub {
485	$_[0] or return $grp->result ("error");
486
487	# add another request dynamically, if first succeeded
488	add $grp aio_open "...", sub {
489	$grp->result ("ok");
490	};	227	};
491	};	228	};
492		229
		230	aioreq_nice $pri_adjust
		231	Similar to "aioreq_pri", but subtracts the given value from the
		232	current priority, so the effect is cumulative.
		233
		234	aio_open $pathname, $flags, $mode, $callback->($fh)
		235	Asynchronously open or create a file and call the callback with a
		236	newly created filehandle for the file.
		237
		238	The pathname passed to "aio_open" must be absolute. See API NOTES,
		239	above, for an explanation.
		240
		241	The $flags argument is a bitmask. See the "Fcntl" module for a list.
		242	They are the same as used by "sysopen".
		243
		244	Likewise, $mode specifies the mode of the newly created file, if it
		245	didn't exist and "O_CREAT" has been given, just like perl's
		246	"sysopen", except that it is mandatory (i.e. use 0 if you don't
		247	create new files, and 0666 or 0777 if you do). Note that the $mode
		248	will be modified by the umask in effect then the request is being
		249	executed, so better never change the umask.
		250
		251	Example:
		252
		253	aio_open "/etc/passwd", O_RDONLY, 0, sub {
		254	if ($_[0]) {
		255	print "open successful, fh is $_[0]\n";
		256	...
		257	} else {
		258	die "open failed: $!\n";
		259	}
		260	};
		261
		262	aio_close $fh, $callback->($status)
		263	Asynchronously close a file and call the callback with the result
		264	code.
		265
		266	Unfortunately, you can't do this to perl. Perl *insists* very
		267	strongly on closing the file descriptor associated with the
		268	filehandle itself.
		269
		270	Therefore, "aio_close" will not close the filehandle - instead it
		271	will use dup2 to overwrite the file descriptor with the write-end of
		272	a pipe (the pipe fd will be created on demand and will be cached).
		273
		274	Or in other words: the file descriptor will be closed, but it will
		275	not be free for reuse until the perl filehandle is closed.
		276
		277	aio_read $fh,$offset,$length, $data,$dataoffset, $callback->($retval)
		278	aio_write $fh,$offset,$length, $data,$dataoffset, $callback->($retval)
		279	Reads or writes $length bytes from or to the specified $fh and
		280	$offset into the scalar given by $data and offset $dataoffset and
		281	calls the callback without the actual number of bytes read (or -1 on
		282	error, just like the syscall).
		283
		284	"aio_read" will, like "sysread", shrink or grow the $data scalar to
		285	offset plus the actual number of bytes read.
		286
		287	If $offset is undefined, then the current file descriptor offset
		288	will be used (and updated), otherwise the file descriptor offset
		289	will not be changed by these calls.
		290
		291	If $length is undefined in "aio_write", use the remaining length of
		292	$data.
		293
		294	If $dataoffset is less than zero, it will be counted from the end of
		295	$data.
		296
		297	The $data scalar *MUST NOT* be modified in any way while the request
		298	is outstanding. Modifying it can result in segfaults or World War
		299	III (if the necessary/optional hardware is installed).
		300
		301	Example: Read 15 bytes at offset 7 into scalar $buffer, starting at
		302	offset 0 within the scalar:
		303
		304	aio_read $fh, 7, 15, $buffer, 0, sub {
		305	$_[0] > 0 or die "read error: $!";
		306	print "read $_[0] bytes: <$buffer>\n";
		307	};
		308
		309	aio_sendfile $out_fh, $in_fh, $in_offset, $length, $callback->($retval)
		310	Tries to copy $length bytes from $in_fh to $out_fh. It starts
		311	reading at byte offset $in_offset, and starts writing at the current
		312	file offset of $out_fh. Because of that, it is not safe to issue
		313	more than one "aio_sendfile" per $out_fh, as they will interfere
		314	with each other.
		315
		316	This call tries to make use of a native "sendfile" syscall to
		317	provide zero-copy operation. For this to work, $out_fh should refer
		318	to a socket, and $in_fh should refer to mmap'able file.
		319
		320	If the native sendfile call fails or is not implemented, it will be
		321	emulated, so you can call "aio_sendfile" on any type of filehandle
		322	regardless of the limitations of the operating system.
		323
		324	Please note, however, that "aio_sendfile" can read more bytes from
		325	$in_fh than are written, and there is no way to find out how many
		326	bytes have been read from "aio_sendfile" alone, as "aio_sendfile"
		327	only provides the number of bytes written to $out_fh. Only if the
		328	result value equals $length one can assume that $length bytes have
		329	been read.
		330
		331	aio_readahead $fh,$offset,$length, $callback->($retval)
		332	"aio_readahead" populates the page cache with data from a file so
		333	that subsequent reads from that file will not block on disk I/O. The
		334	$offset argument specifies the starting point from which data is to
		335	be read and $length specifies the number of bytes to be read. I/O is
		336	performed in whole pages, so that offset is effectively rounded down
		337	to a page boundary and bytes are read up to the next page boundary
		338	greater than or equal to (off-set+length). "aio_readahead" does not
		339	read beyond the end of the file. The current file offset of the file
		340	is left unchanged.
		341
		342	If that syscall doesn't exist (likely if your OS isn't Linux) it
		343	will be emulated by simply reading the data, which would have a
		344	similar effect.
		345
		346	aio_stat $fh_or_path, $callback->($status)
		347	aio_lstat $fh, $callback->($status)
		348	Works like perl's "stat" or "lstat" in void context. The callback
		349	will be called after the stat and the results will be available
		350	using "stat _" or "-s _" etc...
		351
		352	The pathname passed to "aio_stat" must be absolute. See API NOTES,
		353	above, for an explanation.
		354
		355	Currently, the stats are always 64-bit-stats, i.e. instead of
		356	returning an error when stat'ing a large file, the results will be
		357	silently truncated unless perl itself is compiled with large file
		358	support.
		359
		360	Example: Print the length of /etc/passwd:
		361
		362	aio_stat "/etc/passwd", sub {
		363	$_[0] and die "stat failed: $!";
		364	print "size is ", -s _, "\n";
		365	};
		366
		367	aio_utime $fh_or_path, $atime, $mtime, $callback->($status)
		368	Works like perl's "utime" function (including the special case of
		369	$atime and $mtime being undef). Fractional times are supported if
		370	the underlying syscalls support them.
		371
		372	When called with a pathname, uses utimes(2) if available, otherwise
		373	utime(2). If called on a file descriptor, uses futimes(2) if
		374	available, otherwise returns ENOSYS, so this is not portable.
		375
		376	Examples:
		377
		378	# set atime and mtime to current time (basically touch(1)):
		379	aio_utime "path", undef, undef;
		380	# set atime to current time and mtime to beginning of the epoch:
		381	aio_utime "path", time, undef; # undef==0
		382
		383	aio_chown $fh_or_path, $uid, $gid, $callback->($status)
		384	Works like perl's "chown" function, except that "undef" for either
		385	$uid or $gid is being interpreted as "do not change" (but -1 can
		386	also be used).
		387
		388	Examples:
		389
		390	# same as "chown root path" in the shell:
		391	aio_chown "path", 0, -1;
		392	# same as above:
		393	aio_chown "path", 0, undef;
		394
		395	aio_truncate $fh_or_path, $offset, $callback->($status)
		396	Works like truncate(2) or ftruncate(2).
		397
		398	aio_chmod $fh_or_path, $mode, $callback->($status)
		399	Works like perl's "chmod" function.
		400
		401	aio_unlink $pathname, $callback->($status)
		402	Asynchronously unlink (delete) a file and call the callback with the
		403	result code.
		404
		405	aio_mknod $path, $mode, $dev, $callback->($status)
		406	[EXPERIMENTAL]
		407
		408	Asynchronously create a device node (or fifo). See mknod(2).
		409
		410	The only (POSIX-) portable way of calling this function is:
		411
		412	aio_mknod $path, IO::AIO::S_IFIFO | $mode, 0, sub { ...
		413
		414	aio_link $srcpath, $dstpath, $callback->($status)
		415	Asynchronously create a new link to the existing object at $srcpath
		416	at the path $dstpath and call the callback with the result code.
		417
		418	aio_symlink $srcpath, $dstpath, $callback->($status)
		419	Asynchronously create a new symbolic link to the existing object at
		420	$srcpath at the path $dstpath and call the callback with the result
		421	code.
		422
		423	aio_readlink $path, $callback->($link)
		424	Asynchronously read the symlink specified by $path and pass it to
		425	the callback. If an error occurs, nothing or undef gets passed to
		426	the callback.
		427
		428	aio_rename $srcpath, $dstpath, $callback->($status)
		429	Asynchronously rename the object at $srcpath to $dstpath, just as
		430	rename(2) and call the callback with the result code.
		431
		432	aio_mkdir $pathname, $mode, $callback->($status)
		433	Asynchronously mkdir (create) a directory and call the callback with
		434	the result code. $mode will be modified by the umask at the time the
		435	request is executed, so do not change your umask.
		436
		437	aio_rmdir $pathname, $callback->($status)
		438	Asynchronously rmdir (delete) a directory and call the callback with
		439	the result code.
		440
		441	aio_readdir $pathname, $callback->($entries)
		442	Unlike the POSIX call of the same name, "aio_readdir" reads an
		443	entire directory (i.e. opendir + readdir + closedir). The entries
		444	will not be sorted, and will NOT include the "." and ".." entries.
		445
		446	The callback is passed a single argument which is either "undef" or
		447	an array-ref with the filenames.
		448
		449	aio_readdirx $pathname, $flags, $callback->($entries, $flags)
		450	Quite similar to "aio_readdir", but the $flags argument allows to
		451	tune behaviour and output format. In case of an error, $entries will
		452	be "undef".
		453
		454	The flags are a combination of the following constants, ORed
		455	together (the flags will also be passed to the callback, possibly
		456	modified):
		457
		458	IO::AIO::READDIR_DENTS
		459	When this flag is off, then the callback gets an arrayref with
		460	of names only (as with "aio_readdir"), otherwise it gets an
		461	arrayref with "[$name, $type, $inode]" arrayrefs, each
		462	describing a single directory entry in more detail.
		463
		464	$name is the name of the entry.
		465
		466	$type is one of the "IO::AIO::DT_xxx" constants:
		467
		468	"IO::AIO::DT_UNKNOWN", "IO::AIO::DT_FIFO", "IO::AIO::DT_CHR",
		469	"IO::AIO::DT_DIR", "IO::AIO::DT_BLK", "IO::AIO::DT_REG",
		470	"IO::AIO::DT_LNK", "IO::AIO::DT_SOCK", "IO::AIO::DT_WHT".
		471
		472	"IO::AIO::DT_UNKNOWN" means just that: readdir does not know. If
		473	you need to know, you have to run stat yourself. Also, for speed
		474	reasons, the $type scalars are read-only: you can not modify
		475	them.
		476
		477	$inode is the inode number (which might not be exact on systems
		478	with 64 bit inode numbers and 32 bit perls). This field has
		479	unspecified content on systems that do not deliver the inode
		480	information.
		481
		482	IO::AIO::READDIR_DIRS_FIRST
		483	When this flag is set, then the names will be returned in an
		484	order where likely directories come first. This is useful when
		485	you need to quickly find directories, or you want to find all
		486	directories while avoiding to stat() each entry.
		487
		488	If the system returns type information in readdir, then this is
		489	used to find directories directly. Otherwise, likely directories
		490	are files beginning with ".", or otherwise files with no dots,
		491	of which files with short names are tried first.
		492
		493	IO::AIO::READDIR_STAT_ORDER
		494	When this flag is set, then the names will be returned in an
		495	order suitable for stat()'ing each one. That is, when you plan
		496	to stat() all files in the given directory, then the returned
		497	order will likely be fastest.
		498
		499	If both this flag and "IO::AIO::READDIR_DIRS_FIRST" are
		500	specified, then the likely dirs come first, resulting in a less
		501	optimal stat order.
		502
		503	IO::AIO::READDIR_FOUND_UNKNOWN
		504	This flag should not be set when calling "aio_readdirx".
		505	Instead, it is being set by "aio_readdirx", when any of the
		506	$type's found were "IO::AIO::DT_UNKNOWN". The absense of this
		507	flag therefore indicates that all $type's are known, which can
		508	be used to speed up some algorithms.
		509
		510	aio_load $path, $data, $callback->($status)
		511	This is a composite request that tries to fully load the given file
		512	into memory. Status is the same as with aio_read.
		513
		514	aio_copy $srcpath, $dstpath, $callback->($status)
		515	Try to copy the *file* (directories not supported as either source
		516	or destination) from $srcpath to $dstpath and call the callback with
		517	the 0 (error) or -1 ok.
		518
		519	This is a composite request that creates the destination file with
		520	mode 0200 and copies the contents of the source file into it using
		521	"aio_sendfile", followed by restoring atime, mtime, access mode and
		522	uid/gid, in that order.
		523
		524	If an error occurs, the partial destination file will be unlinked,
		525	if possible, except when setting atime, mtime, access mode and
		526	uid/gid, where errors are being ignored.
		527
		528	aio_move $srcpath, $dstpath, $callback->($status)
		529	Try to move the *file* (directories not supported as either source
		530	or destination) from $srcpath to $dstpath and call the callback with
		531	the 0 (error) or -1 ok.
		532
		533	This is a composite request that tries to rename(2) the file first;
		534	if rename fails with "EXDEV", it copies the file with "aio_copy"
		535	and, if that is successful, unlinks the $srcpath.
		536
		537	aio_scandir $path, $maxreq, $callback->($dirs, $nondirs)
		538	Scans a directory (similar to "aio_readdir") but additionally tries
		539	to efficiently separate the entries of directory $path into two sets
		540	of names, directories you can recurse into (directories), and ones
		541	you cannot recurse into (everything else, including symlinks to
		542	directories).
		543
		544	"aio_scandir" is a composite request that creates of many sub
		545	requests_ $maxreq specifies the maximum number of outstanding aio
		546	requests that this function generates. If it is "<= 0", then a
		547	suitable default will be chosen (currently 4).
		548
		549	On error, the callback is called without arguments, otherwise it
		550	receives two array-refs with path-relative entry names.
		551
		552	Example:
		553
		554	aio_scandir $dir, 0, sub {
		555	my ($dirs, $nondirs) = @_;
		556	print "real directories: @$dirs\n";
		557	print "everything else: @$nondirs\n";
		558	};
		559
		560	Implementation notes.
		561
		562	The "aio_readdir" cannot be avoided, but "stat()"'ing every entry
		563	can.
		564
		565	If readdir returns file type information, then this is used directly
		566	to find directories.
		567
		568	Otherwise, after reading the directory, the modification time, size
		569	etc. of the directory before and after the readdir is checked, and
		570	if they match (and isn't the current time), the link count will be
		571	used to decide how many entries are directories (if >= 2).
		572	Otherwise, no knowledge of the number of subdirectories will be
		573	assumed.
		574
		575	Then entries will be sorted into likely directories a non-initial
		576	dot currently) and likely non-directories (see "aio_readdirx"). Then
		577	every entry plus an appended "/." will be "stat"'ed, likely
		578	directories first, in order of their inode numbers. If that
		579	succeeds, it assumes that the entry is a directory or a symlink to
		580	directory (which will be checked seperately). This is often faster
		581	than stat'ing the entry itself because filesystems might detect the
		582	type of the entry without reading the inode data (e.g. ext2fs
		583	filetype feature), even on systems that cannot return the filetype
		584	information on readdir.
		585
		586	If the known number of directories (link count - 2) has been
		587	reached, the rest of the entries is assumed to be non-directories.
		588
		589	This only works with certainty on POSIX (= UNIX) filesystems, which
		590	fortunately are the vast majority of filesystems around.
		591
		592	It will also likely work on non-POSIX filesystems with reduced
		593	efficiency as those tend to return 0 or 1 as link counts, which
		594	disables the directory counting heuristic.
		595
		596	aio_rmtree $path, $callback->($status)
		597	Delete a directory tree starting (and including) $path, return the
		598	status of the final "rmdir" only. This is a composite request that
		599	uses "aio_scandir" to recurse into and rmdir directories, and unlink
		600	everything else.
		601
		602	aio_sync $callback->($status)
		603	Asynchronously call sync and call the callback when finished.
		604
		605	aio_fsync $fh, $callback->($status)
		606	Asynchronously call fsync on the given filehandle and call the
		607	callback with the fsync result code.
		608
		609	aio_fdatasync $fh, $callback->($status)
		610	Asynchronously call fdatasync on the given filehandle and call the
		611	callback with the fdatasync result code.
		612
		613	If this call isn't available because your OS lacks it or it couldn't
		614	be detected, it will be emulated by calling "fsync" instead.
		615
		616	aio_sync_file_range $fh, $offset, $nbytes, $flags, $callback->($status)
		617	Sync the data portion of the file specified by $offset and $length
		618	to disk (but NOT the metadata), by calling the Linux-specific
		619	sync_file_range call. If sync_file_range is not available or it
		620	returns ENOSYS, then fdatasync or fsync is being substituted.
		621
		622	$flags can be a combination of
		623	"IO::AIO::SYNC_FILE_RANGE_WAIT_BEFORE",
		624	"IO::AIO::SYNC_FILE_RANGE_WRITE" and
		625	"IO::AIO::SYNC_FILE_RANGE_WAIT_AFTER": refer to the sync_file_range
		626	manpage for details.
		627
		628	aio_pathsync $path, $callback->($status)
		629	This request tries to open, fsync and close the given path. This is
		630	a composite request intended to sync directories after directory
		631	operations (E.g. rename). This might not work on all operating
		632	systems or have any specific effect, but usually it makes sure that
		633	directory changes get written to disc. It works for anything that
		634	can be opened for read-only, not just directories.
		635
		636	Future versions of this function might fall back to other methods
		637	when "fsync" on the directory fails (such as calling "sync").
		638
		639	Passes 0 when everything went ok, and -1 on error.
		640
		641	aio_group $callback->(...)
		642	This is a very special aio request: Instead of doing something, it
		643	is a container for other aio requests, which is useful if you want
		644	to bundle many requests into a single, composite, request with a
		645	definite callback and the ability to cancel the whole request with
		646	its subrequests.
		647
		648	Returns an object of class IO::AIO::GRP. See its documentation below
		649	for more info.
		650
		651	Example:
		652
		653	my $grp = aio_group sub {
		654	print "all stats done\n";
		655	};
		656
		657	add $grp
		658	(aio_stat ...),
		659	(aio_stat ...),
		660	...;
		661
		662	aio_nop $callback->()
		663	This is a special request - it does nothing in itself and is only
		664	used for side effects, such as when you want to add a dummy request
		665	to a group so that finishing the requests in the group depends on
		666	executing the given code.
		667
		668	While this request does nothing, it still goes through the execution
		669	phase and still requires a worker thread. Thus, the callback will
		670	not be executed immediately but only after other requests in the
		671	queue have entered their execution phase. This can be used to
		672	measure request latency.
		673
		674	IO::AIO::aio_busy $fractional_seconds, $callback->() *NOT EXPORTED*
		675	Mainly used for debugging and benchmarking, this aio request puts
		676	one of the request workers to sleep for the given time.
		677
		678	While it is theoretically handy to have simple I/O scheduling
		679	requests like sleep and file handle readable/writable, the overhead
		680	this creates is immense (it blocks a thread for a long time) so do
		681	not use this function except to put your application under
		682	artificial I/O pressure.
		683
		684	IO::AIO::REQ CLASS
		685	All non-aggregate "aio_*" functions return an object of this class when
		686	called in non-void context.
		687
		688	cancel $req
		689	Cancels the request, if possible. Has the effect of skipping
		690	execution when entering the execute state and skipping calling the
		691	callback when entering the the result state, but will leave the
		692	request otherwise untouched (with the exception of readdir). That
		693	means that requests that currently execute will not be stopped and
		694	resources held by the request will not be freed prematurely.
		695
		696	cb $req $callback->(...)
		697	Replace (or simply set) the callback registered to the request.
		698
		699	IO::AIO::GRP CLASS
		700	This class is a subclass of IO::AIO::REQ, so all its methods apply to
		701	objects of this class, too.
		702
		703	A IO::AIO::GRP object is a special request that can contain multiple
		704	other aio requests.
		705
		706	You create one by calling the "aio_group" constructing function with a
		707	callback that will be called when all contained requests have entered
		708	the "done" state:
		709
		710	my $grp = aio_group sub {
		711	print "all requests are done\n";
		712	};
		713
		714	You add requests by calling the "add" method with one or more
		715	"IO::AIO::REQ" objects:
		716
		717	$grp->add (aio_unlink "...");
		718
		719	add $grp aio_stat "...", sub {
		720	$_[0] or return $grp->result ("error");
		721
		722	# add another request dynamically, if first succeeded
		723	add $grp aio_open "...", sub {
		724	$grp->result ("ok");
		725	};
		726	};
		727
493	This makes it very easy to create composite requests (see the source	728	This makes it very easy to create composite requests (see the source of
494	of "aio_move" for an application) that work and feel like simple	729	"aio_move" for an application) that work and feel like simple requests.
		730
		731	* The IO::AIO::GRP objects will be cleaned up during calls to
		732	"IO::AIO::poll_cb", just like any other request.
		733
		734	* They can be canceled like any other request. Canceling will cancel
		735	not only the request itself, but also all requests it contains.
		736
		737	* They can also can also be added to other IO::AIO::GRP objects.
		738
		739	* You must not add requests to a group from within the group callback
		740	(or any later time).
		741
		742	Their lifetime, simplified, looks like this: when they are empty, they
		743	will finish very quickly. If they contain only requests that are in the
		744	"done" state, they will also finish. Otherwise they will continue to
		745	exist.
		746
		747	That means after creating a group you have some time to add requests
		748	(precisely before the callback has been invoked, which is only done
		749	within the "poll_cb"). And in the callbacks of those requests, you can
		750	add further requests to the group. And only when all those requests have
		751	finished will the the group itself finish.
		752
		753	add $grp ...
		754	$grp->add (...)
		755	Add one or more requests to the group. Any type of IO::AIO::REQ can
		756	be added, including other groups, as long as you do not create
		757	circular dependencies.
		758
		759	Returns all its arguments.
		760
		761	$grp->cancel_subs
		762	Cancel all subrequests and clears any feeder, but not the group
		763	request itself. Useful when you queued a lot of events but got a
		764	result early.
		765
		766	$grp->result (...)
		767	Set the result value(s) that will be passed to the group callback
		768	when all subrequests have finished and set the groups errno to the
		769	current value of errno (just like calling "errno" without an error
		770	number). By default, no argument will be passed and errno is zero.
		771
		772	$grp->errno ([$errno])
		773	Sets the group errno value to $errno, or the current value of errno
		774	when the argument is missing.
		775
		776	Every aio request has an associated errno value that is restored
		777	when the callback is invoked. This method lets you change this value
		778	from its default (0).
		779
		780	Calling "result" will also set errno, so make sure you either set $!
		781	before the call to "result", or call c<errno> after it.
		782
		783	feed $grp $callback->($grp)
		784	Sets a feeder/generator on this group: every group can have an
		785	attached generator that generates requests if idle. The idea behind
		786	this is that, although you could just queue as many requests as you
		787	want in a group, this might starve other requests for a potentially
		788	long time. For example, "aio_scandir" might generate hundreds of
		789	thousands "aio_stat" requests, delaying any later requests for a
		790	long time.
		791
		792	To avoid this, and allow incremental generation of requests, you can
		793	instead a group and set a feeder on it that generates those
		794	requests. The feed callback will be called whenever there are few
		795	enough (see "limit", below) requests active in the group itself and
		796	is expected to queue more requests.
		797
		798	The feed callback can queue as many requests as it likes (i.e. "add"
		799	does not impose any limits).
		800
		801	If the feed does not queue more requests when called, it will be
		802	automatically removed from the group.
		803
		804	If the feed limit is 0 when this method is called, it will be set to
		805	2 automatically.
		806
		807	Example:
		808
		809	# stat all files in @files, but only ever use four aio requests concurrently:
		810
		811	my $grp = aio_group sub { print "finished\n" };
		812	limit $grp 4;
		813	feed $grp sub {
		814	my $file = pop @files
		815	or return;
		816
		817	add $grp aio_stat $file, sub { ... };
		818	};
		819
		820	limit $grp $num
		821	Sets the feeder limit for the group: The feeder will be called
		822	whenever the group contains less than this many requests.
		823
		824	Setting the limit to 0 will pause the feeding process.
		825
		826	The default value for the limit is 0, but note that setting a feeder
		827	automatically bumps it up to 2.
		828
		829	SUPPORT FUNCTIONS
		830	EVENT PROCESSING AND EVENT LOOP INTEGRATION
		831	$fileno = IO::AIO::poll_fileno
		832	Return the *request result pipe file descriptor*. This filehandle
		833	must be polled for reading by some mechanism outside this module
		834	(e.g. EV, Glib, select and so on, see below or the SYNOPSIS). If the
		835	pipe becomes readable you have to call "poll_cb" to check the
		836	results.
		837
		838	See "poll_cb" for an example.
		839
		840	IO::AIO::poll_cb
		841	Process some outstanding events on the result pipe. You have to call
		842	this regularly. Returns 0 if all events could be processed, or -1 if
		843	it returned earlier for whatever reason. Returns immediately when no
		844	events are outstanding. The amount of events processed depends on
		845	the settings of "IO::AIO::max_poll_req" and
		846	"IO::AIO::max_poll_time".
		847
		848	If not all requests were processed for whatever reason, the
		849	filehandle will still be ready when "poll_cb" returns, so normally
		850	you don't have to do anything special to have it called later.
		851
		852	Example: Install an Event watcher that automatically calls
		853	IO::AIO::poll_cb with high priority (more examples can be found in
		854	the SYNOPSIS section, at the top of this document):
		855
		856	Event->io (fd => IO::AIO::poll_fileno,
		857	poll => 'r', async => 1,
		858	cb => \&IO::AIO::poll_cb);
		859
		860	IO::AIO::max_poll_reqs $nreqs
		861	IO::AIO::max_poll_time $seconds
		862	These set the maximum number of requests (default 0, meaning
		863	infinity) that are being processed by "IO::AIO::poll_cb" in one
		864	call, respectively the maximum amount of time (default 0, meaning
		865	infinity) spent in "IO::AIO::poll_cb" to process requests (more
		866	correctly the mininum amount of time "poll_cb" is allowed to use).
		867
		868	Setting "max_poll_time" to a non-zero value creates an overhead of
		869	one syscall per request processed, which is not normally a problem
		870	unless your callbacks are really really fast or your OS is really
		871	really slow (I am not mentioning Solaris here). Using
		872	"max_poll_reqs" incurs no overhead.
		873
		874	Setting these is useful if you want to ensure some level of
		875	interactiveness when perl is not fast enough to process all requests
		876	in time.
		877
		878	For interactive programs, values such as 0.01 to 0.1 should be fine.
		879
		880	Example: Install an Event watcher that automatically calls
		881	IO::AIO::poll_cb with low priority, to ensure that other parts of
		882	the program get the CPU sometimes even under high AIO load.
		883
		884	# try not to spend much more than 0.1s in poll_cb
		885	IO::AIO::max_poll_time 0.1;
		886
		887	# use a low priority so other tasks have priority
		888	Event->io (fd => IO::AIO::poll_fileno,
		889	poll => 'r', nice => 1,
		890	cb => &IO::AIO::poll_cb);
		891
		892	IO::AIO::poll_wait
		893	If there are any outstanding requests and none of them in the result
		894	phase, wait till the result filehandle becomes ready for reading
		895	(simply does a "select" on the filehandle. This is useful if you
		896	want to synchronously wait for some requests to finish).
		897
		898	See "nreqs" for an example.
		899
		900	IO::AIO::poll
		901	Waits until some requests have been handled.
		902
		903	Returns the number of requests processed, but is otherwise strictly
		904	equivalent to:
		905
		906	IO::AIO::poll_wait, IO::AIO::poll_cb
		907
		908	IO::AIO::flush
		909	Wait till all outstanding AIO requests have been handled.
		910
		911	Strictly equivalent to:
		912
		913	IO::AIO::poll_wait, IO::AIO::poll_cb
		914	while IO::AIO::nreqs;
		915
		916	CONTROLLING THE NUMBER OF THREADS
		917	IO::AIO::min_parallel $nthreads
		918	Set the minimum number of AIO threads to $nthreads. The current
		919	default is 8, which means eight asynchronous operations can execute
		920	concurrently at any one time (the number of outstanding requests,
		921	however, is unlimited).
		922
		923	IO::AIO starts threads only on demand, when an AIO request is queued
		924	and no free thread exists. Please note that queueing up a hundred
		925	requests can create demand for a hundred threads, even if it turns
		926	out that everything is in the cache and could have been processed
		927	faster by a single thread.
		928
		929	It is recommended to keep the number of threads relatively low, as
		930	some Linux kernel versions will scale negatively with the number of
		931	threads (higher parallelity => MUCH higher latency). With current
		932	Linux 2.6 versions, 4-32 threads should be fine.
		933
		934	Under most circumstances you don't need to call this function, as
		935	the module selects a default that is suitable for low to moderate
		936	load.
		937
		938	IO::AIO::max_parallel $nthreads
		939	Sets the maximum number of AIO threads to $nthreads. If more than
		940	the specified number of threads are currently running, this function
		941	kills them. This function blocks until the limit is reached.
		942
		943	While $nthreads are zero, aio requests get queued but not executed
		944	until the number of threads has been increased again.
		945
		946	This module automatically runs "max_parallel 0" at program end, to
		947	ensure that all threads are killed and that there are no outstanding
495	requests.	948	requests.
496		949
497	* The IO::AIO::GRP objects will be cleaned up during calls to
498	"IO::AIO::poll_cb", just like any other request.
499	* They can be canceled like any other request. Canceling will cancel
500	not only the request itself, but also all requests it contains.
501	* They can also can also be added to other IO::AIO::GRP objects.
502	* You must not add requests to a group from within the group
503	callback (or any later time).
504
505	Their lifetime, simplified, looks like this: when they are empty,
506	they will finish very quickly. If they contain only requests that
507	are in the "done" state, they will also finish. Otherwise they will
508	continue to exist.
509
510	That means after creating a group you have some time to add
511	requests. And in the callbacks of those requests, you can add
512	further requests to the group. And only when all those requests have
513	finished will the the group itself finish.
514
515	add $grp ...
516	$grp->add (...)
517	Add one or more requests to the group. Any type of IO::AIO::REQ
518	can be added, including other groups, as long as you do not
519	create circular dependencies.
520
521	Returns all its arguments.
522
523	$grp->cancel_subs
524	Cancel all subrequests and clears any feeder, but not the group
525	request itself. Useful when you queued a lot of events but got a
526	result early.
527
528	$grp->result (...)
529	Set the result value(s) that will be passed to the group
530	callback when all subrequests have finished and set thre groups
531	errno to the current value of errno (just like calling "errno"
532	without an error number). By default, no argument will be passed
533	and errno is zero.
534
535	$grp->errno ([$errno])
536	Sets the group errno value to $errno, or the current value of
537	errno when the argument is missing.
538
539	Every aio request has an associated errno value that is restored
540	when the callback is invoked. This method lets you change this
541	value from its default (0).
542
543	Calling "result" will also set errno, so make sure you either
544	set $! before the call to "result", or call c<errno> after it.
545
546	feed $grp $callback->($grp)
547	Sets a feeder/generator on this group: every group can have an
548	attached generator that generates requests if idle. The idea
549	behind this is that, although you could just queue as many
550	requests as you want in a group, this might starve other
551	requests for a potentially long time. For example, "aio_scandir"
552	might generate hundreds of thousands "aio_stat" requests,
553	delaying any later requests for a long time.
554
555	To avoid this, and allow incremental generation of requests, you
556	can instead a group and set a feeder on it that generates those
557	requests. The feed callback will be called whenever there are
558	few enough (see "limit", below) requests active in the group
559	itself and is expected to queue more requests.
560
561	The feed callback can queue as many requests as it likes (i.e.
562	"add" does not impose any limits).
563
564	If the feed does not queue more requests when called, it will be
565	automatically removed from the group.
566
567	If the feed limit is 0, it will be set to 2 automatically.
568
569	Example:
570
571	# stat all files in @files, but only ever use four aio requests concurrently:
572
573	my $grp = aio_group sub { print "finished\n" };
574	limit $grp 4;
575	feed $grp sub {
576	my $file = pop @files
577	or return;
578
579	add $grp aio_stat $file, sub { ... };
580	};
581
582	limit $grp $num
583	Sets the feeder limit for the group: The feeder will be called
584	whenever the group contains less than this many requests.
585
586	Setting the limit to 0 will pause the feeding process.
587
588	SUPPORT FUNCTIONS
589	$fileno = IO::AIO::poll_fileno
590	Return the *request result pipe file descriptor*. This
591	filehandle must be polled for reading by some mechanism outside
592	this module (e.g. Event or select, see below or the SYNOPSIS).
593	If the pipe becomes readable you have to call "poll_cb" to check
594	the results.
595
596	See "poll_cb" for an example.
597
598	IO::AIO::poll_cb
599	Process all outstanding events on the result pipe. You have to
600	call this regularly. Returns the number of events processed.
601	Returns immediately when no events are outstanding.
602
603	If not all requests were processed for whatever reason, the
604	filehandle will still be ready when "poll_cb" returns.
605
606	Example: Install an Event watcher that automatically calls
607	IO::AIO::poll_cb with high priority:
608
609	Event->io (fd => IO::AIO::poll_fileno,
610	poll => 'r', async => 1,
611	cb => \&IO::AIO::poll_cb);
612
613	IO::AIO::poll_some $max_requests
614	Similar to "poll_cb", but only processes up to $max_requests
615	requests at a time.
616
617	Useful if you want to ensure some level of interactiveness when
618	perl is not fast enough to process all requests in time.
619
620	Example: Install an Event watcher that automatically calls
621	IO::AIO::poll_some with low priority, to ensure that other parts
622	of the program get the CPU sometimes even under high AIO load.
623
624	Event->io (fd => IO::AIO::poll_fileno,
625	poll => 'r', nice => 1,
626	cb => sub { IO::AIO::poll_some 256 });
627
628	IO::AIO::poll_wait
629	Wait till the result filehandle becomes ready for reading
630	(simply does a "select" on the filehandle. This is useful if you
631	want to synchronously wait for some requests to finish).
632
633	See "nreqs" for an example.
634
635	IO::AIO::nreqs
636	Returns the number of requests currently in the ready, execute
637	or pending states (i.e. for which their callback has not been
638	invoked yet).
639
640	Example: wait till there are no outstanding requests anymore:
641
642	IO::AIO::poll_wait, IO::AIO::poll_cb
643	while IO::AIO::nreqs;
644
645	IO::AIO::nready
646	Returns the number of requests currently in the ready state (not
647	yet executed).
648
649	IO::AIO::npending
650	Returns the number of requests currently in the pending state
651	(executed, but not yet processed by poll_cb).
652
653	IO::AIO::flush
654	Wait till all outstanding AIO requests have been handled.
655
656	Strictly equivalent to:
657
658	IO::AIO::poll_wait, IO::AIO::poll_cb
659	while IO::AIO::nreqs;
660
661	IO::AIO::poll
662	Waits until some requests have been handled.
663
664	Strictly equivalent to:
665
666	IO::AIO::poll_wait, IO::AIO::poll_cb
667	if IO::AIO::nreqs;
668
669	IO::AIO::min_parallel $nthreads
670	Set the minimum number of AIO threads to $nthreads. The current
671	default is 8, which means eight asynchronous operations can
672	execute concurrently at any one time (the number of outstanding
673	requests, however, is unlimited).
674
675	IO::AIO starts threads only on demand, when an AIO request is
676	queued and no free thread exists.
677
678	It is recommended to keep the number of threads relatively low,
679	as some Linux kernel versions will scale negatively with the
680	number of threads (higher parallelity => MUCH higher latency).
681	With current Linux 2.6 versions, 4-32 threads should be fine.
682
683	Under most circumstances you don't need to call this function,
684	as the module selects a default that is suitable for low to
685	moderate load.
686
687	IO::AIO::max_parallel $nthreads
688	Sets the maximum number of AIO threads to $nthreads. If more
689	than the specified number of threads are currently running, this
690	function kills them. This function blocks until the limit is
691	reached.
692
693	While $nthreads are zero, aio requests get queued but not
694	executed until the number of threads has been increased again.
695
696	This module automatically runs "max_parallel 0" at program end,
697	to ensure that all threads are killed and that there are no
698	outstanding requests.
699
700	Under normal circumstances you don't need to call this function.	950	Under normal circumstances you don't need to call this function.
701		951
		952	IO::AIO::max_idle $nthreads
		953	Limit the number of threads (default: 4) that are allowed to idle
		954	(i.e., threads that did not get a request to process within 10
		955	seconds). That means if a thread becomes idle while $nthreads other
		956	threads are also idle, it will free its resources and exit.
		957
		958	This is useful when you allow a large number of threads (e.g. 100 or
		959	1000) to allow for extremely high load situations, but want to free
		960	resources under normal circumstances (1000 threads can easily
		961	consume 30MB of RAM).
		962
		963	The default is probably ok in most situations, especially if thread
		964	creation is fast. If thread creation is very slow on your system you
		965	might want to use larger values.
		966
702	$oldmaxreqs = IO::AIO::max_outstanding $maxreqs	967	IO::AIO::max_outstanding $maxreqs
703	This is a very bad function to use in interactive programs	968	This is a very bad function to use in interactive programs because
704	because it blocks, and a bad way to reduce concurrency because	969	it blocks, and a bad way to reduce concurrency because it is
705	it is inexact: Better use an "aio_group" together with a feed	970	inexact: Better use an "aio_group" together with a feed callback.
706	callback.
707		971
708	Sets the maximum number of outstanding requests to $nreqs. If	972	Sets the maximum number of outstanding requests to $nreqs. If you do
709	you to queue up more than this number of requests, the next call	973	queue up more than this number of requests, the next call to the
710	to the "poll_cb" (and "poll_some" and other functions calling	974	"poll_cb" (and "poll_some" and other functions calling "poll_cb")
711	"poll_cb") function will block until the limit is no longer	975	function will block until the limit is no longer exceeded.
712	exceeded.
713		976
714	The default value is very large, so there is no practical limit	977	The default value is very large, so there is no practical limit on
715	on the number of outstanding requests.	978	the number of outstanding requests.
716		979
717	You can still queue as many requests as you want. Therefore,	980	You can still queue as many requests as you want. Therefore,
718	"max_oustsanding" is mainly useful in simple scripts (with low	981	"max_outstanding" is mainly useful in simple scripts (with low
719	values) or as a stop gap to shield against fatal memory overflow	982	values) or as a stop gap to shield against fatal memory overflow
720	(with large values).	983	(with large values).
		984
		985	STATISTICAL INFORMATION
		986	IO::AIO::nreqs
		987	Returns the number of requests currently in the ready, execute or
		988	pending states (i.e. for which their callback has not been invoked
		989	yet).
		990
		991	Example: wait till there are no outstanding requests anymore:
		992
		993	IO::AIO::poll_wait, IO::AIO::poll_cb
		994	while IO::AIO::nreqs;
		995
		996	IO::AIO::nready
		997	Returns the number of requests currently in the ready state (not yet
		998	executed).
		999
		1000	IO::AIO::npending
		1001	Returns the number of requests currently in the pending state
		1002	(executed, but not yet processed by poll_cb).
		1003
		1004	MISCELLANEOUS FUNCTIONS
		1005	IO::AIO implements some functions that might be useful, but are not
		1006	asynchronous.
		1007
		1008	IO::AIO::sendfile $ofh, $ifh, $offset, $count
		1009	Calls the "eio_sendfile_sync" function, which is like
		1010	"aio_sendfile", but is blocking (this makes most sense if you know
		1011	the input data is likely cached already and the output filehandle is
		1012	set to non-blocking operations).
		1013
		1014	Returns the number of bytes copied, or -1 on error.
		1015
		1016	IO::AIO::fadvise $fh, $offset, $len, $advice
		1017	Simply calls the "posix_fadvise" function (see it's manpage for
		1018	details). The following advice constants are avaiable:
		1019	"IO::AIO::FADV_NORMAL", "IO::AIO::FADV_SEQUENTIAL",
		1020	"IO::AIO::FADV_RANDOM", "IO::AIO::FADV_NOREUSE",
		1021	"IO::AIO::FADV_WILLNEED", "IO::AIO::FADV_DONTNEED".
		1022
		1023	On systems that do not implement "posix_fadvise", this function
		1024	returns ENOSYS, otherwise the return value of "posix_fadvise".
721		1025
722	FORK BEHAVIOUR	1026	FORK BEHAVIOUR
723	This module should do "the right thing" when the process using it	1027	This module should do "the right thing" when the process using it forks:
724	forks:
725		1028
726	Before the fork, IO::AIO enters a quiescent state where no requests	1029	Before the fork, IO::AIO enters a quiescent state where no requests can
727	can be added in other threads and no results will be processed.	1030	be added in other threads and no results will be processed. After the
728	After the fork the parent simply leaves the quiescent state and	1031	fork the parent simply leaves the quiescent state and continues
729	continues request/result processing, while the child frees the	1032	request/result processing, while the child frees the request/result
730	request/result queue (so that the requests started before the fork	1033	queue (so that the requests started before the fork will only be handled
731	will only be handled in the parent). Threads will be started on	1034	in the parent). Threads will be started on demand until the limit set in
732	demand until the limit set in the parent process has been reached	1035	the parent process has been reached again.
733	again.
734		1036
735	In short: the parent will, after a short pause, continue as if fork	1037	In short: the parent will, after a short pause, continue as if fork had
736	had not been called, while the child will act as if IO::AIO has not	1038	not been called, while the child will act as if IO::AIO has not been
737	been used yet.	1039	used yet.
738		1040
739	MEMORY USAGE	1041	MEMORY USAGE
740	Per-request usage:	1042	Per-request usage:
741		1043
742	Each aio request uses - depending on your architecture - around	1044	Each aio request uses - depending on your architecture - around 100-200
743	100-200 bytes of memory. In addition, stat requests need a stat	1045	bytes of memory. In addition, stat requests need a stat buffer (possibly
744	buffer (possibly a few hundred bytes), readdir requires a result	1046	a few hundred bytes), readdir requires a result buffer and so on. Perl
745	buffer and so on. Perl scalars and other data passed into aio	1047	scalars and other data passed into aio requests will also be locked and
746	requests will also be locked and will consume memory till the	1048	will consume memory till the request has entered the done state.
747	request has entered the done state.
748		1049
749	This is now awfully much, so queuing lots of requests is not usually	1050	This is not awfully much, so queuing lots of requests is not usually a
750	a problem.	1051	problem.
751		1052
752	Per-thread usage:	1053	Per-thread usage:
753		1054
754	In the execution phase, some aio requests require more memory for	1055	In the execution phase, some aio requests require more memory for
755	temporary buffers, and each thread requires a stack and other data	1056	temporary buffers, and each thread requires a stack and other data
756	structures (usually around 16k-128k, depending on the OS).	1057	structures (usually around 16k-128k, depending on the OS).
757		1058
758	KNOWN BUGS	1059	KNOWN BUGS
759	Known bugs will be fixed in the next release.	1060	Known bugs will be fixed in the next release.
760		1061
761	SEE ALSO	1062	SEE ALSO
762	Coro::AIO.	1063	AnyEvent::AIO for easy integration into event loops, Coro::AIO for a
		1064	more natural syntax.
763		1065
764	AUTHOR	1066	AUTHOR
765	Marc Lehmann <schmorp@schmorp.de>	1067	Marc Lehmann <schmorp@schmorp.de>
766	http://home.schmorp.de/	1068	http://home.schmorp.de/
767		1069

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