libeio/eio.pod

=head1 NAME

libeio - truly asynchronous POSIX I/O

=head1 SYNOPSIS

  #include <eio.h>

=head1 DESCRIPTION

The newest version of this document is also available as an html-formatted
web page you might find easier to navigate when reading it for the first
time: L<http://pod.tst.eu/http://cvs.schmorp.de/libeio/eio.pod>.

Note that this library is a by-product of the C<IO::AIO> perl
module, and many of the subtler points regarding requets lifetime
and so on are only documented in its documentation at the
moment: L<http://pod.tst.eu/http://cvs.schmorp.de/IO-AIO/AIO.pm>.

=head2 FEATURES

This library provides fully asynchronous versions of most POSIX functions
dealign with I/O. Unlike most asynchronous libraries, this not only
includes C<read> and C<write>, but also C<open>, C<stat>, C<unlink> and
similar functions, as well as less rarely ones such as C<mknod>, C<futime>
or C<readlink>.

It also offers wrappers around C<sendfile> (Solaris, Linux, HP-UX and
FreeBSD, with emulation on other platforms) and C<readahead> (Linux, with
emulation elsewhere>).

The goal is to enbale you to write fully non-blocking programs. For
example, in a game server, you would not want to freeze for a few seconds
just because the server is running a backup and you happen to call
C<readdir>.

=head2 TIME REPRESENTATION

Libeio represents time as a single floating point number, representing the
(fractional) number of seconds since the (POSIX) epoch (somewhere near
the beginning of 1970, details are complicated, don't ask). This type is
called C<eio_tstamp>, but it is guarenteed to be of type C<double> (or
better), so you can freely use C<double> yourself.

Unlike the name component C<stamp> might indicate, it is also used for
time differences throughout libeio.

=head2 FORK SUPPORT

Calling C<fork ()> is fully supported by this module. It is implemented in these steps:

   1. wait till all requests in "execute" state have been handled
      (basically requests that are already handed over to the kernel).
   2. fork
   3. in the parent, continue business as usual, done
   4. in the child, destroy all ready and pending requests and free the
      memory used by the worker threads. This gives you a fully empty
      libeio queue.

=head1 INITIALISATION/INTEGRATION

Before you can call any eio functions you first have to initialise the
library. The library integrates into any event loop, but can also be used
without one, including in polling mode.

You have to provide the necessary glue yourself, however.

=over 4

=item int eio_init (void (*want_poll)(void), void (*done_poll)(void))

This function initialises the library. On success it returns C<0>, on
failure it returns C<-1> and sets C<errno> appropriately.

It accepts two function pointers specifying callbacks as argument, both of
which can be C<0>, in which case the callback isn't called.

=item want_poll callback

The C<want_poll> callback is invoked whenever libeio wants attention (i.e.
it wants to be polled by calling C<eio_poll>). It is "edge-triggered",
that is, it will only be called once when eio wants attention, until all
pending requests have been handled.

This callback is called while locks are being held, so I<you must
not call any libeio functions inside this callback>. That includes
C<eio_poll>. What you should do is notify some other thread, or wake up
your event loop, and then call C<eio_poll>.

=item done_poll callback

This callback is invoked when libeio detects that all pending requests
have been handled. It is "edge-triggered", that is, it will only be
called once after C<want_poll>. To put it differently, C<want_poll> and
C<done_poll> are invoked in pairs: after C<want_poll> you have to call
C<eio_poll ()> until either C<eio_poll> indicates that everything has been
handled or C<done_poll> has been called, which signals the same.

Note that C<eio_poll> might return after C<done_poll> and C<want_poll>
have been called again, so watch out for races in your code.

As with C<want_poll>, this callback is called while lcoks are being held,
so you I<must not call any libeio functions form within this callback>.

=item int eio_poll ()

This function has to be called whenever there are pending requests that
need finishing. You usually call this after C<want_poll> has indicated
that you should do so, but you can also call this function regularly to
poll for new results.

If any request invocation returns a non-zero value, then C<eio_poll ()>
immediately returns with that value as return value.

Otherwise, if all requests could be handled, it returns C<0>. If for some
reason not all requests have been handled, i.e. some are still pending, it
returns C<-1>.

=back

For libev, you would typically use an C<ev_async> watcher: the
C<want_poll> callback would invoke C<ev_async_send> to wake up the event
loop. Inside the callback set for the watcher, one would call C<eio_poll
()> (followed by C<ev_async_send> again if C<eio_poll> indicates that not
all requests have been handled yet). The race is taken care of because
libev resets/rearms the async watcher before calling your callback,
and therefore, before calling C<eio_poll>. This might result in (some)
spurious wake-ups, but is generally harmless.

For most other event loops, you would typically use a pipe - the event
loop should be told to wait for read readyness on the read end. In
C<want_poll> you would write a single byte, in C<done_poll> you would try
to read that byte, and in the callback for the read end, you would call
C<eio_poll>. The race is avoided here because the event loop should invoke
your callback again and again until the byte has been read (as the pipe
read callback does not read it, only C<done_poll>).

=head2 CONFIGURATION

The functions in this section can sometimes be useful, but the default
configuration will do in most case, so you should skip this section on
first reading.

=over 4

=item eio_set_max_poll_time (eio_tstamp nseconds)

This causes C<eio_poll ()> to return after it has detected that it was
running for C<nsecond> seconds or longer (this number can be fractional).

This can be used to limit the amount of time spent handling eio requests,
for example, in interactive programs, you might want to limit this time to
C<0.01> seconds or so.

Note that:

a) libeio doesn't know how long your request callbacks take, so the time
spent in C<eio_poll> is up to one callback invocation longer then this
interval.

b) this is implemented by calling C<gettimeofday> after each request,
which can be costly.

c) at least one request will be handled.

=item eio_set_max_poll_reqs (unsigned int nreqs)

When C<nreqs> is non-zero, then C<eio_poll> will not handle more than
C<nreqs> requests per invocation. This is a less costly way to limit the
amount of work done by C<eio_poll> then setting a time limit.

If you know your callbacks are generally fast, you could use this to
encourage interactiveness in your programs by setting it to C<10>, C<100>
or even C<1000>.

=item eio_set_min_parallel (unsigned int nthreads)

Make sure libeio can handle at least this many requests in parallel. It
might be able handle more.

=item eio_set_max_parallel (unsigned int nthreads)

Set the maximum number of threads that libeio will spawn.

=item eio_set_max_idle (unsigned int nthreads)

Libeio uses threads internally to handle most requests, and will start and stop threads on demand.

This call can be used to limit the number of idle threads (threads without
work to do): libeio will keep some threads idle in preperation for more
requests, but never longer than C<nthreads> threads.

In addition to this, libeio will also stop threads when they are idle for
a few seconds, regardless of this setting.

=item unsigned int eio_nthreads ()

Return the number of worker threads currently running.

=item unsigned int eio_nreqs ()

Return the number of requests currently handled by libeio. This is the
total number of requests that have been submitted to libeio, but not yet
destroyed.

=item unsigned int eio_nready ()

Returns the number of ready requests, i.e. requests that have been
submitted but have not yet entered the execution phase.

=item unsigned int eio_npending ()

Returns the number of pending requests, i.e. requests that have been
executed and have results, but have not been finished yet by a call to
C<eio_poll>).

=back


=head1 ANATOMY OF AN EIO REQUEST

#TODO


=head1 HIGH LEVEL REQUEST API

#TODO

=back


=head1 LOW LEVEL REQUEST API

#TODO

=head1 EMBEDDING

Libeio can be embedded directly into programs. This functionality is not
documented and not (yet) officially supported.

Note that, when including C<libeio.m4>, you are responsible for defining
the compilation environment (C<_LARGEFILE_SOURCE>, C<_GNU_SOURCE> etc.).

If you need to know how, check the C<IO::AIO> perl module, which does
exactly that.


=head1 COMPILETIME CONFIGURATION

These symbols, if used, must be defined when compiling F<eio.c>.

=over 4

=item EIO_STACKSIZE

This symbol governs the stack size for each eio thread. Libeio itself
was written to use very little stackspace, but when using C<EIO_CUSTOM>
requests, you might want to increase this.

If this symbol is undefined (the default) then libeio will use its default
stack size (C<sizeof (long) * 4096> currently).  If it is defined, but
C<0>, then the default operating system stack size will be used. In all
other cases, the value must be an expression that evaluates to the desired
stack size.

=back


=head1 PORTABILITY REQUIREMENTS

In addition to a working ISO-C implementation, libeio relies on a few
additional extensions:

=over 4

=item POSIX threads

To be portable, this module uses threads, specifically, the POSIX threads
library must be available (and working, which partially excludes many xBSD
systems, where C<fork ()> is buggy).

=item POSIX-compatible filesystem API

This is actually a harder portability requirement: The libeio API is quite
demanding regarding POSIX API calls (symlinks, user/group management
etc.).

=item C<double> must hold a time value in seconds with enough accuracy

The type C<double> is used to represent timestamps. It is required to
have at least 51 bits of mantissa (and 9 bits of exponent), which is good
enough for at least into the year 4000. This requirement is fulfilled by
implementations implementing IEEE 754 (basically all existing ones).

=back

If you know of other additional requirements drop me a note.


=head1 AUTHOR

Marc Lehmann <libeio@schmorp.de>.

Revision:	1.4
Committed:	Wed Apr 22 11:04:49 2009 UTC (16 years, 9 months ago) by root
Branch:	MAIN
CVS Tags:	rel-3_31, rel-3_3, rel-3_2, rel-3_261, rel-3_26, rel-3_25, rel-3_22, rel-3_23, rel-3_21
Changes since 1.3:	+21 -0 lines
Log Message:	* empty log message *
#	Content
1	=head1 NAME
2
3	libeio - truly asynchronous POSIX I/O
4
5	=head1 SYNOPSIS
6
7	#include <eio.h>
8
9	=head1 DESCRIPTION
10
11	The newest version of this document is also available as an html-formatted
12	web page you might find easier to navigate when reading it for the first
13	time: L<http://pod.tst.eu/http://cvs.schmorp.de/libeio/eio.pod>.
14
15	Note that this library is a by-product of the C<IO::AIO> perl
16	module, and many of the subtler points regarding requets lifetime
17	and so on are only documented in its documentation at the
18	moment: L<http://pod.tst.eu/http://cvs.schmorp.de/IO-AIO/AIO.pm>.
19
20	=head2 FEATURES
21
22	This library provides fully asynchronous versions of most POSIX functions
23	dealign with I/O. Unlike most asynchronous libraries, this not only
24	includes C<read> and C<write>, but also C<open>, C<stat>, C<unlink> and
25	similar functions, as well as less rarely ones such as C<mknod>, C<futime>
26	or C<readlink>.
27
28	It also offers wrappers around C<sendfile> (Solaris, Linux, HP-UX and
29	FreeBSD, with emulation on other platforms) and C<readahead> (Linux, with
30	emulation elsewhere>).
31
32	The goal is to enbale you to write fully non-blocking programs. For
33	example, in a game server, you would not want to freeze for a few seconds
34	just because the server is running a backup and you happen to call
35	C<readdir>.
36
37	=head2 TIME REPRESENTATION
38
39	Libeio represents time as a single floating point number, representing the
40	(fractional) number of seconds since the (POSIX) epoch (somewhere near
41	the beginning of 1970, details are complicated, don't ask). This type is
42	called C<eio_tstamp>, but it is guarenteed to be of type C<double> (or
43	better), so you can freely use C<double> yourself.
44
45	Unlike the name component C<stamp> might indicate, it is also used for
46	time differences throughout libeio.
47
48	=head2 FORK SUPPORT
49
50	Calling C<fork ()> is fully supported by this module. It is implemented in these steps:
51
52	1. wait till all requests in "execute" state have been handled
53	(basically requests that are already handed over to the kernel).
54	2. fork
55	3. in the parent, continue business as usual, done
56	4. in the child, destroy all ready and pending requests and free the
57	memory used by the worker threads. This gives you a fully empty
58	libeio queue.
59
60	=head1 INITIALISATION/INTEGRATION
61
62	Before you can call any eio functions you first have to initialise the
63	library. The library integrates into any event loop, but can also be used
64	without one, including in polling mode.
65
66	You have to provide the necessary glue yourself, however.
67
68	=over 4
69
70	=item int eio_init (void (want_poll)(void), void (done_poll)(void))
71
72	This function initialises the library. On success it returns C<0>, on
73	failure it returns C<-1> and sets C<errno> appropriately.
74
75	It accepts two function pointers specifying callbacks as argument, both of
76	which can be C<0>, in which case the callback isn't called.
77
78	=item want_poll callback
79
80	The C<want_poll> callback is invoked whenever libeio wants attention (i.e.
81	it wants to be polled by calling C<eio_poll>). It is "edge-triggered",
82	that is, it will only be called once when eio wants attention, until all
83	pending requests have been handled.
84
85	This callback is called while locks are being held, so I<you must
86	not call any libeio functions inside this callback>. That includes
87	C<eio_poll>. What you should do is notify some other thread, or wake up
88	your event loop, and then call C<eio_poll>.
89
90	=item done_poll callback
91
92	This callback is invoked when libeio detects that all pending requests
93	have been handled. It is "edge-triggered", that is, it will only be
94	called once after C<want_poll>. To put it differently, C<want_poll> and
95	C<done_poll> are invoked in pairs: after C<want_poll> you have to call
96	C<eio_poll ()> until either C<eio_poll> indicates that everything has been
97	handled or C<done_poll> has been called, which signals the same.
98
99	Note that C<eio_poll> might return after C<done_poll> and C<want_poll>
100	have been called again, so watch out for races in your code.
101
102	As with C<want_poll>, this callback is called while lcoks are being held,
103	so you I<must not call any libeio functions form within this callback>.
104
105	=item int eio_poll ()
106
107	This function has to be called whenever there are pending requests that
108	need finishing. You usually call this after C<want_poll> has indicated
109	that you should do so, but you can also call this function regularly to
110	poll for new results.
111
112	If any request invocation returns a non-zero value, then C<eio_poll ()>
113	immediately returns with that value as return value.
114
115	Otherwise, if all requests could be handled, it returns C<0>. If for some
116	reason not all requests have been handled, i.e. some are still pending, it
117	returns C<-1>.
118
119	=back
120
121	For libev, you would typically use an C<ev_async> watcher: the
122	C<want_poll> callback would invoke C<ev_async_send> to wake up the event
123	loop. Inside the callback set for the watcher, one would call C<eio_poll
124	()> (followed by C<ev_async_send> again if C<eio_poll> indicates that not
125	all requests have been handled yet). The race is taken care of because
126	libev resets/rearms the async watcher before calling your callback,
127	and therefore, before calling C<eio_poll>. This might result in (some)
128	spurious wake-ups, but is generally harmless.
129
130	For most other event loops, you would typically use a pipe - the event
131	loop should be told to wait for read readyness on the read end. In
132	C<want_poll> you would write a single byte, in C<done_poll> you would try
133	to read that byte, and in the callback for the read end, you would call
134	C<eio_poll>. The race is avoided here because the event loop should invoke
135	your callback again and again until the byte has been read (as the pipe
136	read callback does not read it, only C<done_poll>).
137
138	=head2 CONFIGURATION
139
140	The functions in this section can sometimes be useful, but the default
141	configuration will do in most case, so you should skip this section on
142	first reading.
143
144	=over 4
145
146	=item eio_set_max_poll_time (eio_tstamp nseconds)
147
148	This causes C<eio_poll ()> to return after it has detected that it was
149	running for C<nsecond> seconds or longer (this number can be fractional).
150
151	This can be used to limit the amount of time spent handling eio requests,
152	for example, in interactive programs, you might want to limit this time to
153	C<0.01> seconds or so.
154
155	Note that:
156
157	a) libeio doesn't know how long your request callbacks take, so the time
158	spent in C<eio_poll> is up to one callback invocation longer then this
159	interval.
160
161	b) this is implemented by calling C<gettimeofday> after each request,
162	which can be costly.
163
164	c) at least one request will be handled.
165
166	=item eio_set_max_poll_reqs (unsigned int nreqs)
167
168	When C<nreqs> is non-zero, then C<eio_poll> will not handle more than
169	C<nreqs> requests per invocation. This is a less costly way to limit the
170	amount of work done by C<eio_poll> then setting a time limit.
171
172	If you know your callbacks are generally fast, you could use this to
173	encourage interactiveness in your programs by setting it to C<10>, C<100>
174	or even C<1000>.
175
176	=item eio_set_min_parallel (unsigned int nthreads)
177
178	Make sure libeio can handle at least this many requests in parallel. It
179	might be able handle more.
180
181	=item eio_set_max_parallel (unsigned int nthreads)
182
183	Set the maximum number of threads that libeio will spawn.
184
185	=item eio_set_max_idle (unsigned int nthreads)
186
187	Libeio uses threads internally to handle most requests, and will start and stop threads on demand.
188
189	This call can be used to limit the number of idle threads (threads without
190	work to do): libeio will keep some threads idle in preperation for more
191	requests, but never longer than C<nthreads> threads.
192
193	In addition to this, libeio will also stop threads when they are idle for
194	a few seconds, regardless of this setting.
195
196	=item unsigned int eio_nthreads ()
197
198	Return the number of worker threads currently running.
199
200	=item unsigned int eio_nreqs ()
201
202	Return the number of requests currently handled by libeio. This is the
203	total number of requests that have been submitted to libeio, but not yet
204	destroyed.
205
206	=item unsigned int eio_nready ()
207
208	Returns the number of ready requests, i.e. requests that have been
209	submitted but have not yet entered the execution phase.
210
211	=item unsigned int eio_npending ()
212
213	Returns the number of pending requests, i.e. requests that have been
214	executed and have results, but have not been finished yet by a call to
215	C<eio_poll>).
216
217	=back
218
219
220	=head1 ANATOMY OF AN EIO REQUEST
221
222	#TODO
223
224
225	=head1 HIGH LEVEL REQUEST API
226
227	#TODO
228
229	=back
230
231
232	=head1 LOW LEVEL REQUEST API
233
234	#TODO
235
236	=head1 EMBEDDING
237
238	Libeio can be embedded directly into programs. This functionality is not
239	documented and not (yet) officially supported.
240
241	Note that, when including C<libeio.m4>, you are responsible for defining
242	the compilation environment (C<_LARGEFILE_SOURCE>, C<_GNU_SOURCE> etc.).
243
244	If you need to know how, check the C<IO::AIO> perl module, which does
245	exactly that.
246
247
248	=head1 COMPILETIME CONFIGURATION
249
250	These symbols, if used, must be defined when compiling F<eio.c>.
251
252	=over 4
253
254	=item EIO_STACKSIZE
255
256	This symbol governs the stack size for each eio thread. Libeio itself
257	was written to use very little stackspace, but when using C<EIO_CUSTOM>
258	requests, you might want to increase this.
259
260	If this symbol is undefined (the default) then libeio will use its default
261	stack size (C<sizeof (long) * 4096> currently). If it is defined, but
262	C<0>, then the default operating system stack size will be used. In all
263	other cases, the value must be an expression that evaluates to the desired
264	stack size.
265
266	=back
267
268
269	=head1 PORTABILITY REQUIREMENTS
270
271	In addition to a working ISO-C implementation, libeio relies on a few
272	additional extensions:
273
274	=over 4
275
276	=item POSIX threads
277
278	To be portable, this module uses threads, specifically, the POSIX threads
279	library must be available (and working, which partially excludes many xBSD
280	systems, where C<fork ()> is buggy).
281
282	=item POSIX-compatible filesystem API
283
284	This is actually a harder portability requirement: The libeio API is quite
285	demanding regarding POSIX API calls (symlinks, user/group management
286	etc.).
287
288	=item C<double> must hold a time value in seconds with enough accuracy
289
290	The type C<double> is used to represent timestamps. It is required to
291	have at least 51 bits of mantissa (and 9 bits of exponent), which is good
292	enough for at least into the year 4000. This requirement is fulfilled by
293	implementations implementing IEEE 754 (basically all existing ones).
294
295	=back
296
297	If you know of other additional requirements drop me a note.
298
299
300	=head1 AUTHOR
301
302	Marc Lehmann <libeio@schmorp.de>.
303