BDB/BDB.pm

=head1 NAME

BDB - Asynchronous Berkeley DB access

=head1 SYNOPSIS

 use BDB;

=head1 DESCRIPTION

=head2 EXAMPLE

=head1 REQUEST ANATOMY AND LIFETIME

Every request method creates a request. which is a C data structure not
directly visible to Perl.

During their existance, bdb requests travel through the following states,
in order:

=over 4

=item ready

Immediately after a request is created it is put into the ready state,
waiting for a thread to execute it.

=item execute

A thread has accepted the request for processing and is currently
executing it (e.g. blocking in read).

=item pending

The request has been executed and is waiting for result processing.

While request submission and execution is fully asynchronous, result
processing is not and relies on the perl interpreter calling C<poll_cb>
(or another function with the same effect).

=item result

The request results are processed synchronously by C<poll_cb>.

The C<poll_cb> function will process all outstanding aio requests by
calling their callbacks, freeing memory associated with them and managing
any groups they are contained in.

=item done

Request has reached the end of its lifetime and holds no resources anymore
(except possibly for the Perl object, but its connection to the actual
aio request is severed and calling its methods will either do nothing or
result in a runtime error).

=back

=cut

package BDB;

no warnings;
use strict 'vars';

use base 'Exporter';

BEGIN {
   our $VERSION = '0.1';

   our @BDB_REQ = qw(
      db_env_create db_env_open db_env_close
      db_create db_open db_close db_compact db_sync db_put
   );
   our @EXPORT = (@BDB_REQ, qw(dbreq_pri dbreq_nice));
   our @EXPORT_OK = qw(poll_fileno poll_cb poll_wait flush
                       min_parallel max_parallel max_idle
                       nreqs nready npending nthreads
                       max_poll_time max_poll_reqs);

   require XSLoader;
   XSLoader::load ("BDB", $VERSION);
}

=head2 SUPPORT FUNCTIONS

=head3 EVENT PROCESSING AND EVENT LOOP INTEGRATION

=over 4

=item $fileno = BDB::poll_fileno

Return the I<request result pipe file descriptor>. This filehandle must be
polled for reading by some mechanism outside this module (e.g. Event or
select, see below or the SYNOPSIS). If the pipe becomes readable you have
to call C<poll_cb> to check the results.

See C<poll_cb> for an example.

=item BDB::poll_cb

Process some outstanding events on the result pipe. You have to call this
regularly. Returns the number of events processed. Returns immediately
when no events are outstanding. The amount of events processed depends on
the settings of C<BDB::max_poll_req> and C<BDB::max_poll_time>.

If not all requests were processed for whatever reason, the filehandle
will still be ready when C<poll_cb> returns.

Example: Install an Event watcher that automatically calls
BDB::poll_cb with high priority:

   Event->io (fd => BDB::poll_fileno,
              poll => 'r', async => 1,
              cb => \&BDB::poll_cb);

=item BDB::max_poll_reqs $nreqs

=item BDB::max_poll_time $seconds

These set the maximum number of requests (default C<0>, meaning infinity)
that are being processed by C<BDB::poll_cb> in one call, respectively
the maximum amount of time (default C<0>, meaning infinity) spent in
C<BDB::poll_cb> to process requests (more correctly the mininum amount
of time C<poll_cb> is allowed to use).

Setting C<max_poll_time> to a non-zero value creates an overhead of one
syscall per request processed, which is not normally a problem unless your
callbacks are really really fast or your OS is really really slow (I am
not mentioning Solaris here). Using C<max_poll_reqs> incurs no overhead.

Setting these is useful if you want to ensure some level of
interactiveness when perl is not fast enough to process all requests in
time.

For interactive programs, values such as C<0.01> to C<0.1> should be fine.

Example: Install an Event watcher that automatically calls
BDB::poll_cb with low priority, to ensure that other parts of the
program get the CPU sometimes even under high AIO load.

   # try not to spend much more than 0.1s in poll_cb
   BDB::max_poll_time 0.1;

   # use a low priority so other tasks have priority
   Event->io (fd => BDB::poll_fileno,
              poll => 'r', nice => 1,
              cb => &BDB::poll_cb);

=item BDB::poll_wait

If there are any outstanding requests and none of them in the result
phase, wait till the result filehandle becomes ready for reading (simply
does a C<select> on the filehandle. This is useful if you want to
synchronously wait for some requests to finish).

See C<nreqs> for an example.

=item BDB::poll

Waits until some requests have been handled.

Returns the number of requests processed, but is otherwise strictly
equivalent to:

   BDB::poll_wait, BDB::poll_cb

=item BDB::flush

Wait till all outstanding AIO requests have been handled.

Strictly equivalent to:

   BDB::poll_wait, BDB::poll_cb
      while BDB::nreqs;

=head3 CONTROLLING THE NUMBER OF THREADS

=item BDB::min_parallel $nthreads

Set the minimum number of AIO threads to C<$nthreads>. The current
default is C<8>, which means eight asynchronous operations can execute
concurrently at any one time (the number of outstanding requests,
however, is unlimited).

BDB starts threads only on demand, when an AIO request is queued and
no free thread exists. Please note that queueing up a hundred requests can
create demand for a hundred threads, even if it turns out that everything
is in the cache and could have been processed faster by a single thread.

It is recommended to keep the number of threads relatively low, as some
Linux kernel versions will scale negatively with the number of threads
(higher parallelity => MUCH higher latency). With current Linux 2.6
versions, 4-32 threads should be fine.

Under most circumstances you don't need to call this function, as the
module selects a default that is suitable for low to moderate load.

=item BDB::max_parallel $nthreads

Sets the maximum number of AIO threads to C<$nthreads>. If more than the
specified number of threads are currently running, this function kills
them. This function blocks until the limit is reached.

While C<$nthreads> are zero, aio requests get queued but not executed
until the number of threads has been increased again.

This module automatically runs C<max_parallel 0> at program end, to ensure
that all threads are killed and that there are no outstanding requests.

Under normal circumstances you don't need to call this function.

=item BDB::max_idle $nthreads

Limit the number of threads (default: 4) that are allowed to idle (i.e.,
threads that did not get a request to process within 10 seconds). That
means if a thread becomes idle while C<$nthreads> other threads are also
idle, it will free its resources and exit.

This is useful when you allow a large number of threads (e.g. 100 or 1000)
to allow for extremely high load situations, but want to free resources
under normal circumstances (1000 threads can easily consume 30MB of RAM).

The default is probably ok in most situations, especially if thread
creation is fast. If thread creation is very slow on your system you might
want to use larger values.

=item $oldmaxreqs = BDB::max_outstanding $maxreqs

This is a very bad function to use in interactive programs because it
blocks, and a bad way to reduce concurrency because it is inexact: Better
use an C<aio_group> together with a feed callback.

Sets the maximum number of outstanding requests to C<$nreqs>. If you
to queue up more than this number of requests, the next call to the
C<poll_cb> (and C<poll_some> and other functions calling C<poll_cb>)
function will block until the limit is no longer exceeded.

The default value is very large, so there is no practical limit on the
number of outstanding requests.

You can still queue as many requests as you want. Therefore,
C<max_oustsanding> is mainly useful in simple scripts (with low values) or
as a stop gap to shield against fatal memory overflow (with large values).

=item BDB::set_sync_prepare $cb

Sets a callback that is called whenever a request is created without an
explicit callback. It has to return two code references. The first is used
as the request callback, and the second is called to wait until the first
callback has been called. The default implementation works like this:

   sub {
      my $status;
      (
         sub { $status = $! },
         sub { BDB::poll while !defined $status; $! = $status },
      )
   }

=back

=head3 STATISTICAL INFORMATION

=over 4

=item BDB::nreqs

Returns the number of requests currently in the ready, execute or pending
states (i.e. for which their callback has not been invoked yet).

Example: wait till there are no outstanding requests anymore:

   BDB::poll_wait, BDB::poll_cb
      while BDB::nreqs;

=item BDB::nready

Returns the number of requests currently in the ready state (not yet
executed).

=item BDB::npending

Returns the number of requests currently in the pending state (executed,
but not yet processed by poll_cb).

=back

=cut

set_sync_prepare {
   my $status;
   (
      sub {
         $status = $!;
      },
      sub {
         BDB::poll while !defined $status;
         $! = $status;
      },
   )
};

min_parallel 8;

END { flush }

1;

=head2 FORK BEHAVIOUR

This module should do "the right thing" when the process using it forks:

Before the fork, IO::AIO enters a quiescent state where no requests
can be added in other threads and no results will be processed. After
the fork the parent simply leaves the quiescent state and continues
request/result processing, while the child frees the request/result queue
(so that the requests started before the fork will only be handled in the
parent). Threads will be started on demand until the limit set in the
parent process has been reached again.

In short: the parent will, after a short pause, continue as if fork had
not been called, while the child will act as if IO::AIO has not been used
yet.

=head2 MEMORY USAGE

Per-request usage:

Each aio request uses - depending on your architecture - around 100-200
bytes of memory. In addition, stat requests need a stat buffer (possibly
a few hundred bytes), readdir requires a result buffer and so on. Perl
scalars and other data passed into aio requests will also be locked and
will consume memory till the request has entered the done state.

This is now awfully much, so queuing lots of requests is not usually a
problem.

Per-thread usage:

In the execution phase, some aio requests require more memory for
temporary buffers, and each thread requires a stack and other data
structures (usually around 16k-128k, depending on the OS).

=head1 KNOWN BUGS

Known bugs will be fixed in the next release.

=head1 SEE ALSO

L<Coro::AIO>.

=head1 AUTHOR

 Marc Lehmann <schmorp@schmorp.de>
 http://home.schmorp.de/

=cut

Revision:	1.3
Committed:	Mon Feb 5 22:19:07 2007 UTC (17 years, 3 months ago) by root
Branch:	MAIN
Changes since 1.2:	+37 -1 lines
Log Message:	* empty log message *
#	Content
1	=head1 NAME
2
3	BDB - Asynchronous Berkeley DB access
4
5	=head1 SYNOPSIS
6
7	use BDB;
8
9	=head1 DESCRIPTION
10
11	=head2 EXAMPLE
12
13	=head1 REQUEST ANATOMY AND LIFETIME
14
15	Every request method creates a request. which is a C data structure not
16	directly visible to Perl.
17
18	During their existance, bdb requests travel through the following states,
19	in order:
20
21	=over 4
22
23	=item ready
24
25	Immediately after a request is created it is put into the ready state,
26	waiting for a thread to execute it.
27
28	=item execute
29
30	A thread has accepted the request for processing and is currently
31	executing it (e.g. blocking in read).
32
33	=item pending
34
35	The request has been executed and is waiting for result processing.
36
37	While request submission and execution is fully asynchronous, result
38	processing is not and relies on the perl interpreter calling C<poll_cb>
39	(or another function with the same effect).
40
41	=item result
42
43	The request results are processed synchronously by C<poll_cb>.
44
45	The C<poll_cb> function will process all outstanding aio requests by
46	calling their callbacks, freeing memory associated with them and managing
47	any groups they are contained in.
48
49	=item done
50
51	Request has reached the end of its lifetime and holds no resources anymore
52	(except possibly for the Perl object, but its connection to the actual
53	aio request is severed and calling its methods will either do nothing or
54	result in a runtime error).
55
56	=back
57
58	=cut
59
60	package BDB;
61
62	no warnings;
63	use strict 'vars';
64
65	use base 'Exporter';
66
67	BEGIN {
68	our $VERSION = '0.1';
69
70	our @BDB_REQ = qw(
71	db_env_create db_env_open db_env_close
72	db_create db_open db_close db_compact db_sync db_put
73	);
74	our @EXPORT = (@BDB_REQ, qw(dbreq_pri dbreq_nice));
75	our @EXPORT_OK = qw(poll_fileno poll_cb poll_wait flush
76	min_parallel max_parallel max_idle
77	nreqs nready npending nthreads
78	max_poll_time max_poll_reqs);
79
80	require XSLoader;
81	XSLoader::load ("BDB", $VERSION);
82	}
83
84	=head2 SUPPORT FUNCTIONS
85
86	=head3 EVENT PROCESSING AND EVENT LOOP INTEGRATION
87
88	=over 4
89
90	=item $fileno = BDB::poll_fileno
91
92	Return the I<request result pipe file descriptor>. This filehandle must be
93	polled for reading by some mechanism outside this module (e.g. Event or
94	select, see below or the SYNOPSIS). If the pipe becomes readable you have
95	to call C<poll_cb> to check the results.
96
97	See C<poll_cb> for an example.
98
99	=item BDB::poll_cb
100
101	Process some outstanding events on the result pipe. You have to call this
102	regularly. Returns the number of events processed. Returns immediately
103	when no events are outstanding. The amount of events processed depends on
104	the settings of C<BDB::max_poll_req> and C<BDB::max_poll_time>.
105
106	If not all requests were processed for whatever reason, the filehandle
107	will still be ready when C<poll_cb> returns.
108
109	Example: Install an Event watcher that automatically calls
110	BDB::poll_cb with high priority:
111
112	Event->io (fd => BDB::poll_fileno,
113	poll => 'r', async => 1,
114	cb => \&BDB::poll_cb);
115
116	=item BDB::max_poll_reqs $nreqs
117
118	=item BDB::max_poll_time $seconds
119
120	These set the maximum number of requests (default C<0>, meaning infinity)
121	that are being processed by C<BDB::poll_cb> in one call, respectively
122	the maximum amount of time (default C<0>, meaning infinity) spent in
123	C<BDB::poll_cb> to process requests (more correctly the mininum amount
124	of time C<poll_cb> is allowed to use).
125
126	Setting C<max_poll_time> to a non-zero value creates an overhead of one
127	syscall per request processed, which is not normally a problem unless your
128	callbacks are really really fast or your OS is really really slow (I am
129	not mentioning Solaris here). Using C<max_poll_reqs> incurs no overhead.
130
131	Setting these is useful if you want to ensure some level of
132	interactiveness when perl is not fast enough to process all requests in
133	time.
134
135	For interactive programs, values such as C<0.01> to C<0.1> should be fine.
136
137	Example: Install an Event watcher that automatically calls
138	BDB::poll_cb with low priority, to ensure that other parts of the
139	program get the CPU sometimes even under high AIO load.
140
141	# try not to spend much more than 0.1s in poll_cb
142	BDB::max_poll_time 0.1;
143
144	# use a low priority so other tasks have priority
145	Event->io (fd => BDB::poll_fileno,
146	poll => 'r', nice => 1,
147	cb => &BDB::poll_cb);
148
149	=item BDB::poll_wait
150
151	If there are any outstanding requests and none of them in the result
152	phase, wait till the result filehandle becomes ready for reading (simply
153	does a C<select> on the filehandle. This is useful if you want to
154	synchronously wait for some requests to finish).
155
156	See C<nreqs> for an example.
157
158	=item BDB::poll
159
160	Waits until some requests have been handled.
161
162	Returns the number of requests processed, but is otherwise strictly
163	equivalent to:
164
165	BDB::poll_wait, BDB::poll_cb
166
167	=item BDB::flush
168
169	Wait till all outstanding AIO requests have been handled.
170
171	Strictly equivalent to:
172
173	BDB::poll_wait, BDB::poll_cb
174	while BDB::nreqs;
175
176	=head3 CONTROLLING THE NUMBER OF THREADS
177
178	=item BDB::min_parallel $nthreads
179
180	Set the minimum number of AIO threads to C<$nthreads>. The current
181	default is C<8>, which means eight asynchronous operations can execute
182	concurrently at any one time (the number of outstanding requests,
183	however, is unlimited).
184
185	BDB starts threads only on demand, when an AIO request is queued and
186	no free thread exists. Please note that queueing up a hundred requests can
187	create demand for a hundred threads, even if it turns out that everything
188	is in the cache and could have been processed faster by a single thread.
189
190	It is recommended to keep the number of threads relatively low, as some
191	Linux kernel versions will scale negatively with the number of threads
192	(higher parallelity => MUCH higher latency). With current Linux 2.6
193	versions, 4-32 threads should be fine.
194
195	Under most circumstances you don't need to call this function, as the
196	module selects a default that is suitable for low to moderate load.
197
198	=item BDB::max_parallel $nthreads
199
200	Sets the maximum number of AIO threads to C<$nthreads>. If more than the
201	specified number of threads are currently running, this function kills
202	them. This function blocks until the limit is reached.
203
204	While C<$nthreads> are zero, aio requests get queued but not executed
205	until the number of threads has been increased again.
206
207	This module automatically runs C<max_parallel 0> at program end, to ensure
208	that all threads are killed and that there are no outstanding requests.
209
210	Under normal circumstances you don't need to call this function.
211
212	=item BDB::max_idle $nthreads
213
214	Limit the number of threads (default: 4) that are allowed to idle (i.e.,
215	threads that did not get a request to process within 10 seconds). That
216	means if a thread becomes idle while C<$nthreads> other threads are also
217	idle, it will free its resources and exit.
218
219	This is useful when you allow a large number of threads (e.g. 100 or 1000)
220	to allow for extremely high load situations, but want to free resources
221	under normal circumstances (1000 threads can easily consume 30MB of RAM).
222
223	The default is probably ok in most situations, especially if thread
224	creation is fast. If thread creation is very slow on your system you might
225	want to use larger values.
226
227	=item $oldmaxreqs = BDB::max_outstanding $maxreqs
228
229	This is a very bad function to use in interactive programs because it
230	blocks, and a bad way to reduce concurrency because it is inexact: Better
231	use an C<aio_group> together with a feed callback.
232
233	Sets the maximum number of outstanding requests to C<$nreqs>. If you
234	to queue up more than this number of requests, the next call to the
235	C<poll_cb> (and C<poll_some> and other functions calling C<poll_cb>)
236	function will block until the limit is no longer exceeded.
237
238	The default value is very large, so there is no practical limit on the
239	number of outstanding requests.
240
241	You can still queue as many requests as you want. Therefore,
242	C<max_oustsanding> is mainly useful in simple scripts (with low values) or
243	as a stop gap to shield against fatal memory overflow (with large values).
244
245	=item BDB::set_sync_prepare $cb
246
247	Sets a callback that is called whenever a request is created without an
248	explicit callback. It has to return two code references. The first is used
249	as the request callback, and the second is called to wait until the first
250	callback has been called. The default implementation works like this:
251
252	sub {
253	my $status;
254	(
255	sub { $status = $! },
256	sub { BDB::poll while !defined $status; $! = $status },
257	)
258	}
259
260	=back
261
262	=head3 STATISTICAL INFORMATION
263
264	=over 4
265
266	=item BDB::nreqs
267
268	Returns the number of requests currently in the ready, execute or pending
269	states (i.e. for which their callback has not been invoked yet).
270
271	Example: wait till there are no outstanding requests anymore:
272
273	BDB::poll_wait, BDB::poll_cb
274	while BDB::nreqs;
275
276	=item BDB::nready
277
278	Returns the number of requests currently in the ready state (not yet
279	executed).
280
281	=item BDB::npending
282
283	Returns the number of requests currently in the pending state (executed,
284	but not yet processed by poll_cb).
285
286	=back
287
288	=cut
289
290	set_sync_prepare {
291	my $status;
292	(
293	sub {
294	$status = $!;
295	},
296	sub {
297	BDB::poll while !defined $status;
298	$! = $status;
299	},
300	)
301	};
302
303	min_parallel 8;
304
305	END { flush }
306
307	1;
308
309	=head2 FORK BEHAVIOUR
310
311	This module should do "the right thing" when the process using it forks:
312
313	Before the fork, IO::AIO enters a quiescent state where no requests
314	can be added in other threads and no results will be processed. After
315	the fork the parent simply leaves the quiescent state and continues
316	request/result processing, while the child frees the request/result queue
317	(so that the requests started before the fork will only be handled in the
318	parent). Threads will be started on demand until the limit set in the
319	parent process has been reached again.
320
321	In short: the parent will, after a short pause, continue as if fork had
322	not been called, while the child will act as if IO::AIO has not been used
323	yet.
324
325	=head2 MEMORY USAGE
326
327	Per-request usage:
328
329	Each aio request uses - depending on your architecture - around 100-200
330	bytes of memory. In addition, stat requests need a stat buffer (possibly
331	a few hundred bytes), readdir requires a result buffer and so on. Perl
332	scalars and other data passed into aio requests will also be locked and
333	will consume memory till the request has entered the done state.
334
335	This is now awfully much, so queuing lots of requests is not usually a
336	problem.
337
338	Per-thread usage:
339
340	In the execution phase, some aio requests require more memory for
341	temporary buffers, and each thread requires a stack and other data
342	structures (usually around 16k-128k, depending on the OS).
343
344	=head1 KNOWN BUGS
345
346	Known bugs will be fixed in the next release.
347
348	=head1 SEE ALSO
349
350	L<Coro::AIO>.
351
352	=head1 AUTHOR
353
354	Marc Lehmann <schmorp@schmorp.de>
355	http://home.schmorp.de/
356
357	=cut
358