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Revision 1.18 by root, Thu Oct 26 16:28:33 2006 UTC vs.
Revision 1.34 by root, Thu Nov 20 09:01:40 2008 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 $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);
…		…
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.
		55
		56	Asynchronous means that operations that can normally block your program
		57	(e.g. reading from disk) will be done asynchronously: the operation will
		58	still block, but you can do something else in the meantime. This is
		59	extremely useful for programs that need to stay interactive even when
		60	doing heavy I/O (GUI programs, high performance network servers etc.),
		61	but can also be used to easily do operations in parallel that are
		62	normally done sequentially, e.g. stat'ing many files, which is much
		63	faster on a RAID volume or over NFS when you do a number of stat
		64	operations concurrently.
		65
		66	While most of this works on all types of file descriptors (for example
		67	sockets), using these functions on file descriptors that support
		68	nonblocking operation (again, sockets, pipes etc.) is very inefficient.
		69	Use an event loop for that (such as the Event module): IO::AIO will
		70	naturally fit into such an event loop itself.
52		71
53	In this version, a number of threads are started that execute your	72	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	73	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	74	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	75	perl. In the future, this module might make use of the native aio
57	functions available on many operating systems. However, they are often	76	functions available on many operating systems. However, they are often
58	not well-supported or restricted (Linux doesn't allow them on normal	77	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	78	files currently, for example), and they would only support aio_read and
60	aio_write, so the remaining functionality would have to be implemented	79	aio_write, so the remaining functionality would have to be implemented
61	using threads anyway.	80	using threads anyway.
62		81
63	Although the module will work with in the presence of other (Perl-)	82	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	83	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	84	yourself, always call "poll_cb" from within the same thread, or never
66	never call "poll_cb" (or other "aio_" functions) recursively.	85	call "poll_cb" (or other "aio_" functions) recursively.
		86
		87	EXAMPLE
		88	This is a simple example that uses the Event module and loads
		89	/etc/passwd asynchronously:
		90
		91	use Fcntl;
		92	use Event;
		93	use IO::AIO;
		94
		95	# register the IO::AIO callback with Event
		96	Event->io (fd => IO::AIO::poll_fileno,
		97	poll => 'r',
		98	cb => \&IO::AIO::poll_cb);
		99
		100	# queue the request to open /etc/passwd
		101	aio_open "/etc/passwd", O_RDONLY, 0, sub {
		102	my $fh = shift
		103	or die "error while opening: $!";
		104
		105	# stat'ing filehandles is generally non-blocking
		106	my $size = -s $fh;
		107
		108	# queue a request to read the file
		109	my $contents;
		110	aio_read $fh, 0, $size, $contents, 0, sub {
		111	$_[0] == $size
		112	or die "short read: $!";
		113
		114	close $fh;
		115
		116	# file contents now in $contents
		117	print $contents;
		118
		119	# exit event loop and program
		120	Event::unloop;
		121	};
		122	};
		123
		124	# possibly queue up other requests, or open GUI windows,
		125	# check for sockets etc. etc.
		126
		127	# process events as long as there are some:
		128	Event::loop;
67		129
68	REQUEST ANATOMY AND LIFETIME	130	REQUEST ANATOMY AND LIFETIME
69	Every "aio_*" function creates a request. which is a C data structure	131	Every "aio_*" function creates a request. which is a C data structure
70	not directly visible to Perl.	132	not directly visible to Perl.
71		133
…		…
107	anymore (except possibly for the Perl object, but its connection to	169	anymore (except possibly for the Perl object, but its connection to
108	the actual aio request is severed and calling its methods will	170	the actual aio request is severed and calling its methods will
109	either do nothing or result in a runtime error).	171	either do nothing or result in a runtime error).
110		172
111	FUNCTIONS	173	FUNCTIONS
112	AIO FUNCTIONS	174	AIO REQUEST FUNCTIONS
113	All the "aio_*" calls are more or less thin wrappers around the	175	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	176	with the same name (sans "aio_"). The arguments are similar or
115	or identical, and they all accept an additional (and optional)	177	identical, and they all accept an additional (and optional) $callback
116	$callback argument which must be a code reference. This code	178	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	179	called with the syscall return code (e.g. most syscalls return -1 on
118	syscalls return -1 on error, unlike perl, which usually delivers	180	error, unlike perl, which usually delivers "false") as its sole argument
119	"false") as it's sole argument when the given syscall has been	181	after the given syscall has been executed asynchronously.
120	executed asynchronously.
121		182
122	All functions expecting a filehandle keep a copy of the filehandle	183	All functions expecting a filehandle keep a copy of the filehandle
123	internally until the request has finished.	184	internally until the request has finished.
124		185
125	All requests return objects of type IO::AIO::REQ that allow further	186	All functions return request objects of type IO::AIO::REQ that allow
126	manipulation of those requests while they are in-flight.	187	further manipulation of those requests while they are in-flight.
127		188
128	The pathnames you pass to these routines *must* be absolute and	189	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	190	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	191	being executed, the current working directory could have changed.
131	have changed. Alternatively, you can make sure that you never change	192	Alternatively, you can make sure that you never change the current
132	the current working directory.	193	working directory anywhere in the program and then use relative paths.
133		194
134	To encode pathnames to byte form, either make sure you either: a)	195	To encode pathnames as octets, either make sure you either: a) always
135	always pass in filenames you got from outside (command line, readdir	196	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	197	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	198	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	199	the user environment, d) use Glib::filename_from_unicode on unicode
139	filenames or e) use something else.	200	filenames or e) use something else to ensure your scalar has the correct
		201	contents.
140		202
		203	This works, btw. independent of the internal UTF-8 bit, which IO::AIO
		204	handles correctly whether it is set or not.
		205
141	$prev_pri = aioreq_pri [$pri]	206	$prev_pri = aioreq_pri [$pri]
142	Returns the priority value that would be used for the next	207	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	208	and, if $pri is given, sets the priority for the next aio request.
144	aio request.
145		209
146	The default priority is 0, the minimum and maximum priorities	210	The default priority is 0, the minimum and maximum priorities are -4
147	are -4 and 4, respectively. Requests with higher priority will	211	and 4, respectively. Requests with higher priority will be serviced
148	be serviced first.	212	first.
149		213
150	The priority will be reset to 0 after each call to one of the	214	The priority will be reset to 0 after each call to one of the
151	"aio_*" functions.	215	"aio_*" functions.
152		216
153	Example: open a file with low priority, then read something from	217	Example: open a file with low priority, then read something from it
154	it with higher priority so the read request is serviced before	218	with higher priority so the read request is serviced before other
155	other low priority open requests (potentially spamming the	219	low priority open requests (potentially spamming the cache):
156	cache):
157		220
		221	aioreq_pri -3;
		222	aio_open ..., sub {
		223	return unless $_[0];
		224
158	aioreq_pri -3;	225	aioreq_pri -2;
159	aio_open ..., sub {
160	return unless $_[0];
161
162	aioreq_pri -2;
163	aio_read $_[0], ..., sub {	226	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	...;	227	...
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	};	228	};
491	};	229	};
492		230
		231	aioreq_nice $pri_adjust
		232	Similar to "aioreq_pri", but subtracts the given value from the
		233	current priority, so the effect is cumulative.
		234
		235	aio_open $pathname, $flags, $mode, $callback->($fh)
		236	Asynchronously open or create a file and call the callback with a
		237	newly created filehandle for the file.
		238
		239	The pathname passed to "aio_open" must be absolute. See API NOTES,
		240	above, for an explanation.
		241
		242	The $flags argument is a bitmask. See the "Fcntl" module for a list.
		243	They are the same as used by "sysopen".
		244
		245	Likewise, $mode specifies the mode of the newly created file, if it
		246	didn't exist and "O_CREAT" has been given, just like perl's
		247	"sysopen", except that it is mandatory (i.e. use 0 if you don't
		248	create new files, and 0666 or 0777 if you do). Note that the $mode
		249	will be modified by the umask in effect then the request is being
		250	executed, so better never change the umask.
		251
		252	Example:
		253
		254	aio_open "/etc/passwd", O_RDONLY, 0, sub {
		255	if ($_[0]) {
		256	print "open successful, fh is $_[0]\n";
		257	...
		258	} else {
		259	die "open failed: $!\n";
		260	}
		261	};
		262
		263	aio_close $fh, $callback->($status)
		264	Asynchronously close a file and call the callback with the result
		265	code.
		266
		267	Unfortunately, you can't do this to perl. Perl *insists* very
		268	strongly on closing the file descriptor associated with the
		269	filehandle itself.
		270
		271	Therefore, "aio_close" will not close the filehandle - instead it
		272	will use dup2 to overwrite the file descriptor with the write-end of
		273	a pipe (the pipe fd will be created on demand and will be cached).
		274
		275	Or in other words: the file descriptor will be closed, but it will
		276	not be free for reuse until the perl filehandle is closed.
		277
		278	aio_read $fh,$offset,$length, $data,$dataoffset, $callback->($retval)
		279	aio_write $fh,$offset,$length, $data,$dataoffset, $callback->($retval)
		280	Reads or writes $length bytes from the specified $fh and $offset
		281	into the scalar given by $data and offset $dataoffset and calls the
		282	callback without the actual number of bytes read (or -1 on error,
		283	just like the syscall).
		284
		285	If $offset is undefined, then the current file descriptor offset
		286	will be used (and updated), otherwise the file descriptor offset
		287	will not be changed by these calls.
		288
		289	If $length is undefined in "aio_write", use the remaining length of
		290	$data.
		291
		292	If $dataoffset is less than zero, it will be counted from the end of
		293	$data.
		294
		295	The $data scalar *MUST NOT* be modified in any way while the request
		296	is outstanding. Modifying it can result in segfaults or World War
		297	III (if the necessary/optional hardware is installed).
		298
		299	Example: Read 15 bytes at offset 7 into scalar $buffer, starting at
		300	offset 0 within the scalar:
		301
		302	aio_read $fh, 7, 15, $buffer, 0, sub {
		303	$_[0] > 0 or die "read error: $!";
		304	print "read $_[0] bytes: <$buffer>\n";
		305	};
		306
		307	aio_sendfile $out_fh, $in_fh, $in_offset, $length, $callback->($retval)
		308	Tries to copy $length bytes from $in_fh to $out_fh. It starts
		309	reading at byte offset $in_offset, and starts writing at the current
		310	file offset of $out_fh. Because of that, it is not safe to issue
		311	more than one "aio_sendfile" per $out_fh, as they will interfere
		312	with each other.
		313
		314	This call tries to make use of a native "sendfile" syscall to
		315	provide zero-copy operation. For this to work, $out_fh should refer
		316	to a socket, and $in_fh should refer to mmap'able file.
		317
		318	If the native sendfile call fails or is not implemented, it will be
		319	emulated, so you can call "aio_sendfile" on any type of filehandle
		320	regardless of the limitations of the operating system.
		321
		322	Please note, however, that "aio_sendfile" can read more bytes from
		323	$in_fh than are written, and there is no way to find out how many
		324	bytes have been read from "aio_sendfile" alone, as "aio_sendfile"
		325	only provides the number of bytes written to $out_fh. Only if the
		326	result value equals $length one can assume that $length bytes have
		327	been read.
		328
		329	aio_readahead $fh,$offset,$length, $callback->($retval)
		330	"aio_readahead" populates the page cache with data from a file so
		331	that subsequent reads from that file will not block on disk I/O. The
		332	$offset argument specifies the starting point from which data is to
		333	be read and $length specifies the number of bytes to be read. I/O is
		334	performed in whole pages, so that offset is effectively rounded down
		335	to a page boundary and bytes are read up to the next page boundary
		336	greater than or equal to (off-set+length). "aio_readahead" does not
		337	read beyond the end of the file. The current file offset of the file
		338	is left unchanged.
		339
		340	If that syscall doesn't exist (likely if your OS isn't Linux) it
		341	will be emulated by simply reading the data, which would have a
		342	similar effect.
		343
		344	aio_stat $fh_or_path, $callback->($status)
		345	aio_lstat $fh, $callback->($status)
		346	Works like perl's "stat" or "lstat" in void context. The callback
		347	will be called after the stat and the results will be available
		348	using "stat _" or "-s _" etc...
		349
		350	The pathname passed to "aio_stat" must be absolute. See API NOTES,
		351	above, for an explanation.
		352
		353	Currently, the stats are always 64-bit-stats, i.e. instead of
		354	returning an error when stat'ing a large file, the results will be
		355	silently truncated unless perl itself is compiled with large file
		356	support.
		357
		358	Example: Print the length of /etc/passwd:
		359
		360	aio_stat "/etc/passwd", sub {
		361	$_[0] and die "stat failed: $!";
		362	print "size is ", -s _, "\n";
		363	};
		364
		365	aio_utime $fh_or_path, $atime, $mtime, $callback->($status)
		366	Works like perl's "utime" function (including the special case of
		367	$atime and $mtime being undef). Fractional times are supported if
		368	the underlying syscalls support them.
		369
		370	When called with a pathname, uses utimes(2) if available, otherwise
		371	utime(2). If called on a file descriptor, uses futimes(2) if
		372	available, otherwise returns ENOSYS, so this is not portable.
		373
		374	Examples:
		375
		376	# set atime and mtime to current time (basically touch(1)):
		377	aio_utime "path", undef, undef;
		378	# set atime to current time and mtime to beginning of the epoch:
		379	aio_utime "path", time, undef; # undef==0
		380
		381	aio_chown $fh_or_path, $uid, $gid, $callback->($status)
		382	Works like perl's "chown" function, except that "undef" for either
		383	$uid or $gid is being interpreted as "do not change" (but -1 can
		384	also be used).
		385
		386	Examples:
		387
		388	# same as "chown root path" in the shell:
		389	aio_chown "path", 0, -1;
		390	# same as above:
		391	aio_chown "path", 0, undef;
		392
		393	aio_truncate $fh_or_path, $offset, $callback->($status)
		394	Works like truncate(2) or ftruncate(2).
		395
		396	aio_chmod $fh_or_path, $mode, $callback->($status)
		397	Works like perl's "chmod" function.
		398
		399	aio_unlink $pathname, $callback->($status)
		400	Asynchronously unlink (delete) a file and call the callback with the
		401	result code.
		402
		403	aio_mknod $path, $mode, $dev, $callback->($status)
		404	[EXPERIMENTAL]
		405
		406	Asynchronously create a device node (or fifo). See mknod(2).
		407
		408	The only (POSIX-) portable way of calling this function is:
		409
		410	aio_mknod $path, IO::AIO::S_IFIFO | $mode, 0, sub { ...
		411
		412	aio_link $srcpath, $dstpath, $callback->($status)
		413	Asynchronously create a new link to the existing object at $srcpath
		414	at the path $dstpath and call the callback with the result code.
		415
		416	aio_symlink $srcpath, $dstpath, $callback->($status)
		417	Asynchronously create a new symbolic link to the existing object at
		418	$srcpath at the path $dstpath and call the callback with the result
		419	code.
		420
		421	aio_readlink $path, $callback->($link)
		422	Asynchronously read the symlink specified by $path and pass it to
		423	the callback. If an error occurs, nothing or undef gets passed to
		424	the callback.
		425
		426	aio_rename $srcpath, $dstpath, $callback->($status)
		427	Asynchronously rename the object at $srcpath to $dstpath, just as
		428	rename(2) and call the callback with the result code.
		429
		430	aio_mkdir $pathname, $mode, $callback->($status)
		431	Asynchronously mkdir (create) a directory and call the callback with
		432	the result code. $mode will be modified by the umask at the time the
		433	request is executed, so do not change your umask.
		434
		435	aio_rmdir $pathname, $callback->($status)
		436	Asynchronously rmdir (delete) a directory and call the callback with
		437	the result code.
		438
		439	aio_readdir $pathname, $callback->($entries)
		440	Unlike the POSIX call of the same name, "aio_readdir" reads an
		441	entire directory (i.e. opendir + readdir + closedir). The entries
		442	will not be sorted, and will NOT include the "." and ".." entries.
		443
		444	The callback a single argument which is either "undef" or an
		445	array-ref with the filenames.
		446
		447	aio_load $path, $data, $callback->($status)
		448	This is a composite request that tries to fully load the given file
		449	into memory. Status is the same as with aio_read.
		450
		451	aio_copy $srcpath, $dstpath, $callback->($status)
		452	Try to copy the *file* (directories not supported as either source
		453	or destination) from $srcpath to $dstpath and call the callback with
		454	the 0 (error) or -1 ok.
		455
		456	This is a composite request that creates the destination file with
		457	mode 0200 and copies the contents of the source file into it using
		458	"aio_sendfile", followed by restoring atime, mtime, access mode and
		459	uid/gid, in that order.
		460
		461	If an error occurs, the partial destination file will be unlinked,
		462	if possible, except when setting atime, mtime, access mode and
		463	uid/gid, where errors are being ignored.
		464
		465	aio_move $srcpath, $dstpath, $callback->($status)
		466	Try to move the *file* (directories not supported as either source
		467	or destination) from $srcpath to $dstpath and call the callback with
		468	the 0 (error) or -1 ok.
		469
		470	This is a composite request that tries to rename(2) the file first;
		471	if rename fails with "EXDEV", it copies the file with "aio_copy"
		472	and, if that is successful, unlinks the $srcpath.
		473
		474	aio_scandir $path, $maxreq, $callback->($dirs, $nondirs)
		475	Scans a directory (similar to "aio_readdir") but additionally tries
		476	to efficiently separate the entries of directory $path into two sets
		477	of names, directories you can recurse into (directories), and ones
		478	you cannot recurse into (everything else, including symlinks to
		479	directories).
		480
		481	"aio_scandir" is a composite request that creates of many sub
		482	requests_ $maxreq specifies the maximum number of outstanding aio
		483	requests that this function generates. If it is "<= 0", then a
		484	suitable default will be chosen (currently 4).
		485
		486	On error, the callback is called without arguments, otherwise it
		487	receives two array-refs with path-relative entry names.
		488
		489	Example:
		490
		491	aio_scandir $dir, 0, sub {
		492	my ($dirs, $nondirs) = @_;
		493	print "real directories: @$dirs\n";
		494	print "everything else: @$nondirs\n";
		495	};
		496
		497	Implementation notes.
		498
		499	The "aio_readdir" cannot be avoided, but "stat()"'ing every entry
		500	can.
		501
		502	After reading the directory, the modification time, size etc. of the
		503	directory before and after the readdir is checked, and if they match
		504	(and isn't the current time), the link count will be used to decide
		505	how many entries are directories (if >= 2). Otherwise, no knowledge
		506	of the number of subdirectories will be assumed.
		507
		508	Then entries will be sorted into likely directories (everything
		509	without a non-initial dot currently) and likely non-directories
		510	(everything else). Then every entry plus an appended "/." will be
		511	"stat"'ed, likely directories first. If that succeeds, it assumes
		512	that the entry is a directory or a symlink to directory (which will
		513	be checked seperately). This is often faster than stat'ing the entry
		514	itself because filesystems might detect the type of the entry
		515	without reading the inode data (e.g. ext2fs filetype feature).
		516
		517	If the known number of directories (link count - 2) has been
		518	reached, the rest of the entries is assumed to be non-directories.
		519
		520	This only works with certainty on POSIX (= UNIX) filesystems, which
		521	fortunately are the vast majority of filesystems around.
		522
		523	It will also likely work on non-POSIX filesystems with reduced
		524	efficiency as those tend to return 0 or 1 as link counts, which
		525	disables the directory counting heuristic.
		526
		527	aio_rmtree $path, $callback->($status)
		528	Delete a directory tree starting (and including) $path, return the
		529	status of the final "rmdir" only. This is a composite request that
		530	uses "aio_scandir" to recurse into and rmdir directories, and unlink
		531	everything else.
		532
		533	aio_sync $callback->($status)
		534	Asynchronously call sync and call the callback when finished.
		535
		536	aio_fsync $fh, $callback->($status)
		537	Asynchronously call fsync on the given filehandle and call the
		538	callback with the fsync result code.
		539
		540	aio_fdatasync $fh, $callback->($status)
		541	Asynchronously call fdatasync on the given filehandle and call the
		542	callback with the fdatasync result code.
		543
		544	If this call isn't available because your OS lacks it or it couldn't
		545	be detected, it will be emulated by calling "fsync" instead.
		546
		547	aio_sync_file_range $fh, $offset, $nbytes, $flags, $callback->($status)
		548	Sync the data portion of the file specified by $offset and $length
		549	to disk (but NOT the metadata), by calling the Linux-specific
		550	sync_file_range call. If sync_file_range is not available or it
		551	returns ENOSYS, then fdatasync or fsync is being substituted.
		552
		553	$flags can be a combination of
		554	"IO::AIO::SYNC_FILE_RANGE_WAIT_BEFORE",
		555	"IO::AIO::SYNC_FILE_RANGE_WRITE" and
		556	"IO::AIO::SYNC_FILE_RANGE_WAIT_AFTER": refer to the sync_file_range
		557	manpage for details.
		558
		559	aio_pathsync $path, $callback->($status)
		560	This request tries to open, fsync and close the given path. This is
		561	a composite request intended to sync directories after directory
		562	operations (E.g. rename). This might not work on all operating
		563	systems or have any specific effect, but usually it makes sure that
		564	directory changes get written to disc. It works for anything that
		565	can be opened for read-only, not just directories.
		566
		567	Passes 0 when everything went ok, and -1 on error.
		568
		569	aio_group $callback->(...)
		570	This is a very special aio request: Instead of doing something, it
		571	is a container for other aio requests, which is useful if you want
		572	to bundle many requests into a single, composite, request with a
		573	definite callback and the ability to cancel the whole request with
		574	its subrequests.
		575
		576	Returns an object of class IO::AIO::GRP. See its documentation below
		577	for more info.
		578
		579	Example:
		580
		581	my $grp = aio_group sub {
		582	print "all stats done\n";
		583	};
		584
		585	add $grp
		586	(aio_stat ...),
		587	(aio_stat ...),
		588	...;
		589
		590	aio_nop $callback->()
		591	This is a special request - it does nothing in itself and is only
		592	used for side effects, such as when you want to add a dummy request
		593	to a group so that finishing the requests in the group depends on
		594	executing the given code.
		595
		596	While this request does nothing, it still goes through the execution
		597	phase and still requires a worker thread. Thus, the callback will
		598	not be executed immediately but only after other requests in the
		599	queue have entered their execution phase. This can be used to
		600	measure request latency.
		601
		602	IO::AIO::aio_busy $fractional_seconds, $callback->() *NOT EXPORTED*
		603	Mainly used for debugging and benchmarking, this aio request puts
		604	one of the request workers to sleep for the given time.
		605
		606	While it is theoretically handy to have simple I/O scheduling
		607	requests like sleep and file handle readable/writable, the overhead
		608	this creates is immense (it blocks a thread for a long time) so do
		609	not use this function except to put your application under
		610	artificial I/O pressure.
		611
		612	IO::AIO::REQ CLASS
		613	All non-aggregate "aio_*" functions return an object of this class when
		614	called in non-void context.
		615
		616	cancel $req
		617	Cancels the request, if possible. Has the effect of skipping
		618	execution when entering the execute state and skipping calling the
		619	callback when entering the the result state, but will leave the
		620	request otherwise untouched. That means that requests that currently
		621	execute will not be stopped and resources held by the request will
		622	not be freed prematurely.
		623
		624	cb $req $callback->(...)
		625	Replace (or simply set) the callback registered to the request.
		626
		627	IO::AIO::GRP CLASS
		628	This class is a subclass of IO::AIO::REQ, so all its methods apply to
		629	objects of this class, too.
		630
		631	A IO::AIO::GRP object is a special request that can contain multiple
		632	other aio requests.
		633
		634	You create one by calling the "aio_group" constructing function with a
		635	callback that will be called when all contained requests have entered
		636	the "done" state:
		637
		638	my $grp = aio_group sub {
		639	print "all requests are done\n";
		640	};
		641
		642	You add requests by calling the "add" method with one or more
		643	"IO::AIO::REQ" objects:
		644
		645	$grp->add (aio_unlink "...");
		646
		647	add $grp aio_stat "...", sub {
		648	$_[0] or return $grp->result ("error");
		649
		650	# add another request dynamically, if first succeeded
		651	add $grp aio_open "...", sub {
		652	$grp->result ("ok");
		653	};
		654	};
		655
493	This makes it very easy to create composite requests (see the source	656	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	657	"aio_move" for an application) that work and feel like simple requests.
		658
		659	* The IO::AIO::GRP objects will be cleaned up during calls to
		660	"IO::AIO::poll_cb", just like any other request.
		661
		662	* They can be canceled like any other request. Canceling will cancel
		663	not only the request itself, but also all requests it contains.
		664
		665	* They can also can also be added to other IO::AIO::GRP objects.
		666
		667	* You must not add requests to a group from within the group callback
		668	(or any later time).
		669
		670	Their lifetime, simplified, looks like this: when they are empty, they
		671	will finish very quickly. If they contain only requests that are in the
		672	"done" state, they will also finish. Otherwise they will continue to
		673	exist.
		674
		675	That means after creating a group you have some time to add requests
		676	(precisely before the callback has been invoked, which is only done
		677	within the "poll_cb"). And in the callbacks of those requests, you can
		678	add further requests to the group. And only when all those requests have
		679	finished will the the group itself finish.
		680
		681	add $grp ...
		682	$grp->add (...)
		683	Add one or more requests to the group. Any type of IO::AIO::REQ can
		684	be added, including other groups, as long as you do not create
		685	circular dependencies.
		686
		687	Returns all its arguments.
		688
		689	$grp->cancel_subs
		690	Cancel all subrequests and clears any feeder, but not the group
		691	request itself. Useful when you queued a lot of events but got a
		692	result early.
		693
		694	$grp->result (...)
		695	Set the result value(s) that will be passed to the group callback
		696	when all subrequests have finished and set the groups errno to the
		697	current value of errno (just like calling "errno" without an error
		698	number). By default, no argument will be passed and errno is zero.
		699
		700	$grp->errno ([$errno])
		701	Sets the group errno value to $errno, or the current value of errno
		702	when the argument is missing.
		703
		704	Every aio request has an associated errno value that is restored
		705	when the callback is invoked. This method lets you change this value
		706	from its default (0).
		707
		708	Calling "result" will also set errno, so make sure you either set $!
		709	before the call to "result", or call c<errno> after it.
		710
		711	feed $grp $callback->($grp)
		712	Sets a feeder/generator on this group: every group can have an
		713	attached generator that generates requests if idle. The idea behind
		714	this is that, although you could just queue as many requests as you
		715	want in a group, this might starve other requests for a potentially
		716	long time. For example, "aio_scandir" might generate hundreds of
		717	thousands "aio_stat" requests, delaying any later requests for a
		718	long time.
		719
		720	To avoid this, and allow incremental generation of requests, you can
		721	instead a group and set a feeder on it that generates those
		722	requests. The feed callback will be called whenever there are few
		723	enough (see "limit", below) requests active in the group itself and
		724	is expected to queue more requests.
		725
		726	The feed callback can queue as many requests as it likes (i.e. "add"
		727	does not impose any limits).
		728
		729	If the feed does not queue more requests when called, it will be
		730	automatically removed from the group.
		731
		732	If the feed limit is 0 when this method is called, it will be set to
		733	2 automatically.
		734
		735	Example:
		736
		737	# stat all files in @files, but only ever use four aio requests concurrently:
		738
		739	my $grp = aio_group sub { print "finished\n" };
		740	limit $grp 4;
		741	feed $grp sub {
		742	my $file = pop @files
		743	or return;
		744
		745	add $grp aio_stat $file, sub { ... };
		746	};
		747
		748	limit $grp $num
		749	Sets the feeder limit for the group: The feeder will be called
		750	whenever the group contains less than this many requests.
		751
		752	Setting the limit to 0 will pause the feeding process.
		753
		754	The default value for the limit is 0, but note that setting a feeder
		755	automatically bumps it up to 2.
		756
		757	SUPPORT FUNCTIONS
		758	EVENT PROCESSING AND EVENT LOOP INTEGRATION
		759	$fileno = IO::AIO::poll_fileno
		760	Return the *request result pipe file descriptor*. This filehandle
		761	must be polled for reading by some mechanism outside this module
		762	(e.g. Event or select, see below or the SYNOPSIS). If the pipe
		763	becomes readable you have to call "poll_cb" to check the results.
		764
		765	See "poll_cb" for an example.
		766
		767	IO::AIO::poll_cb
		768	Process some outstanding events on the result pipe. You have to call
		769	this regularly. Returns 0 if all events could be processed, or -1 if
		770	it returned earlier for whatever reason. Returns immediately when no
		771	events are outstanding. The amount of events processed depends on
		772	the settings of "IO::AIO::max_poll_req" and
		773	"IO::AIO::max_poll_time".
		774
		775	If not all requests were processed for whatever reason, the
		776	filehandle will still be ready when "poll_cb" returns, so normally
		777	you don't have to do anything special to have it called later.
		778
		779	Example: Install an Event watcher that automatically calls
		780	IO::AIO::poll_cb with high priority:
		781
		782	Event->io (fd => IO::AIO::poll_fileno,
		783	poll => 'r', async => 1,
		784	cb => \&IO::AIO::poll_cb);
		785
		786	IO::AIO::max_poll_reqs $nreqs
		787	IO::AIO::max_poll_time $seconds
		788	These set the maximum number of requests (default 0, meaning
		789	infinity) that are being processed by "IO::AIO::poll_cb" in one
		790	call, respectively the maximum amount of time (default 0, meaning
		791	infinity) spent in "IO::AIO::poll_cb" to process requests (more
		792	correctly the mininum amount of time "poll_cb" is allowed to use).
		793
		794	Setting "max_poll_time" to a non-zero value creates an overhead of
		795	one syscall per request processed, which is not normally a problem
		796	unless your callbacks are really really fast or your OS is really
		797	really slow (I am not mentioning Solaris here). Using
		798	"max_poll_reqs" incurs no overhead.
		799
		800	Setting these is useful if you want to ensure some level of
		801	interactiveness when perl is not fast enough to process all requests
		802	in time.
		803
		804	For interactive programs, values such as 0.01 to 0.1 should be fine.
		805
		806	Example: Install an Event watcher that automatically calls
		807	IO::AIO::poll_cb with low priority, to ensure that other parts of
		808	the program get the CPU sometimes even under high AIO load.
		809
		810	# try not to spend much more than 0.1s in poll_cb
		811	IO::AIO::max_poll_time 0.1;
		812
		813	# use a low priority so other tasks have priority
		814	Event->io (fd => IO::AIO::poll_fileno,
		815	poll => 'r', nice => 1,
		816	cb => &IO::AIO::poll_cb);
		817
		818	IO::AIO::poll_wait
		819	If there are any outstanding requests and none of them in the result
		820	phase, wait till the result filehandle becomes ready for reading
		821	(simply does a "select" on the filehandle. This is useful if you
		822	want to synchronously wait for some requests to finish).
		823
		824	See "nreqs" for an example.
		825
		826	IO::AIO::poll
		827	Waits until some requests have been handled.
		828
		829	Returns the number of requests processed, but is otherwise strictly
		830	equivalent to:
		831
		832	IO::AIO::poll_wait, IO::AIO::poll_cb
		833
		834	IO::AIO::flush
		835	Wait till all outstanding AIO requests have been handled.
		836
		837	Strictly equivalent to:
		838
		839	IO::AIO::poll_wait, IO::AIO::poll_cb
		840	while IO::AIO::nreqs;
		841
		842	CONTROLLING THE NUMBER OF THREADS
		843	IO::AIO::min_parallel $nthreads
		844	Set the minimum number of AIO threads to $nthreads. The current
		845	default is 8, which means eight asynchronous operations can execute
		846	concurrently at any one time (the number of outstanding requests,
		847	however, is unlimited).
		848
		849	IO::AIO starts threads only on demand, when an AIO request is queued
		850	and no free thread exists. Please note that queueing up a hundred
		851	requests can create demand for a hundred threads, even if it turns
		852	out that everything is in the cache and could have been processed
		853	faster by a single thread.
		854
		855	It is recommended to keep the number of threads relatively low, as
		856	some Linux kernel versions will scale negatively with the number of
		857	threads (higher parallelity => MUCH higher latency). With current
		858	Linux 2.6 versions, 4-32 threads should be fine.
		859
		860	Under most circumstances you don't need to call this function, as
		861	the module selects a default that is suitable for low to moderate
		862	load.
		863
		864	IO::AIO::max_parallel $nthreads
		865	Sets the maximum number of AIO threads to $nthreads. If more than
		866	the specified number of threads are currently running, this function
		867	kills them. This function blocks until the limit is reached.
		868
		869	While $nthreads are zero, aio requests get queued but not executed
		870	until the number of threads has been increased again.
		871
		872	This module automatically runs "max_parallel 0" at program end, to
		873	ensure that all threads are killed and that there are no outstanding
495	requests.	874	requests.
496		875
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.	876	Under normal circumstances you don't need to call this function.
701		877
		878	IO::AIO::max_idle $nthreads
		879	Limit the number of threads (default: 4) that are allowed to idle
		880	(i.e., threads that did not get a request to process within 10
		881	seconds). That means if a thread becomes idle while $nthreads other
		882	threads are also idle, it will free its resources and exit.
		883
		884	This is useful when you allow a large number of threads (e.g. 100 or
		885	1000) to allow for extremely high load situations, but want to free
		886	resources under normal circumstances (1000 threads can easily
		887	consume 30MB of RAM).
		888
		889	The default is probably ok in most situations, especially if thread
		890	creation is fast. If thread creation is very slow on your system you
		891	might want to use larger values.
		892
702	$oldmaxreqs = IO::AIO::max_outstanding $maxreqs	893	IO::AIO::max_outstanding $maxreqs
703	This is a very bad function to use in interactive programs	894	This is a very bad function to use in interactive programs because
704	because it blocks, and a bad way to reduce concurrency because	895	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	896	inexact: Better use an "aio_group" together with a feed callback.
706	callback.
707		897
708	Sets the maximum number of outstanding requests to $nreqs. If	898	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	899	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	900	"poll_cb" (and "poll_some" and other functions calling "poll_cb")
711	"poll_cb") function will block until the limit is no longer	901	function will block until the limit is no longer exceeded.
712	exceeded.
713		902
714	The default value is very large, so there is no practical limit	903	The default value is very large, so there is no practical limit on
715	on the number of outstanding requests.	904	the number of outstanding requests.
716		905
717	You can still queue as many requests as you want. Therefore,	906	You can still queue as many requests as you want. Therefore,
718	"max_oustsanding" is mainly useful in simple scripts (with low	907	"max_outstanding" is mainly useful in simple scripts (with low
719	values) or as a stop gap to shield against fatal memory overflow	908	values) or as a stop gap to shield against fatal memory overflow
720	(with large values).	909	(with large values).
		910
		911	STATISTICAL INFORMATION
		912	IO::AIO::nreqs
		913	Returns the number of requests currently in the ready, execute or
		914	pending states (i.e. for which their callback has not been invoked
		915	yet).
		916
		917	Example: wait till there are no outstanding requests anymore:
		918
		919	IO::AIO::poll_wait, IO::AIO::poll_cb
		920	while IO::AIO::nreqs;
		921
		922	IO::AIO::nready
		923	Returns the number of requests currently in the ready state (not yet
		924	executed).
		925
		926	IO::AIO::npending
		927	Returns the number of requests currently in the pending state
		928	(executed, but not yet processed by poll_cb).
721		929
722	FORK BEHAVIOUR	930	FORK BEHAVIOUR
723	This module should do "the right thing" when the process using it	931	This module should do "the right thing" when the process using it forks:
724	forks:
725		932
726	Before the fork, IO::AIO enters a quiescent state where no requests	933	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.	934	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	935	fork the parent simply leaves the quiescent state and continues
729	continues request/result processing, while the child frees the	936	request/result processing, while the child frees the request/result
730	request/result queue (so that the requests started before the fork	937	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	938	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	939	the parent process has been reached again.
733	again.
734		940
735	In short: the parent will, after a short pause, continue as if fork	941	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	942	not been called, while the child will act as if IO::AIO has not been
737	been used yet.	943	used yet.
738		944
739	MEMORY USAGE	945	MEMORY USAGE
740	Per-request usage:	946	Per-request usage:
741		947
742	Each aio request uses - depending on your architecture - around	948	Each aio request uses - depending on your architecture - around 100-200
743	100-200 bytes of memory. In addition, stat requests need a stat	949	bytes of memory. In addition, stat requests need a stat buffer (possibly
744	buffer (possibly a few hundred bytes), readdir requires a result	950	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	951	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	952	will consume memory till the request has entered the done state.
747	request has entered the done state.
748		953
749	This is now awfully much, so queuing lots of requests is not usually	954	This is not awfully much, so queuing lots of requests is not usually a
750	a problem.	955	problem.
751		956
752	Per-thread usage:	957	Per-thread usage:
753		958
754	In the execution phase, some aio requests require more memory for	959	In the execution phase, some aio requests require more memory for
755	temporary buffers, and each thread requires a stack and other data	960	temporary buffers, and each thread requires a stack and other data
756	structures (usually around 16k-128k, depending on the OS).	961	structures (usually around 16k-128k, depending on the OS).
757		962
758	KNOWN BUGS	963	KNOWN BUGS
759	Known bugs will be fixed in the next release.	964	Known bugs will be fixed in the next release.
760		965
761	SEE ALSO	966	SEE ALSO
762	Coro::AIO.	967	AnyEvent::AIO for easy integration into event loops, Coro::AIO for a
		968	more natural syntax.
763		969
764	AUTHOR	970	AUTHOR
765	Marc Lehmann <schmorp@schmorp.de>	971	Marc Lehmann <schmorp@schmorp.de>
766	http://home.schmorp.de/	972	http://home.schmorp.de/
767		973

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