BDB/BDB.pm

=head1 NAME

BDB - Asynchronous Berkeley DB access

=head1 SYNOPSIS

 use BDB;

=head1 DESCRIPTION

See the eg/ directory in the distribution and the berkeleydb C
documentation. This is inadequate, but the only sources of documentation
known for this module so far.

=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_open db_env_close db_env_txn_checkpoint db_env_lock_detect
      db_env_memp_sync db_env_memp_trickle
      db_open db_close db_compact db_sync db_put db_get db_pget db_del db_key_range
      db_txn_commit db_txn_abort
      db_c_close db_c_count db_c_put db_c_get db_c_pget db_c_del
      db_sequence_open db_sequence_close
      db_sequence_get db_sequence_remove
   );
   our @EXPORT = (@BDB_REQ, qw(dbreq_pri dbreq_nice db_env_create db_create));
   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:	1.7
Committed:	Mon Mar 5 19:47:01 2007 UTC (17 years, 2 months ago) by root
Branch:	MAIN
CVS Tags:	rel-0_1
Changes since 1.6:	+4 -0 lines
Log Message:	* empty log message *
#	User	Rev	Content
1	root	1.1	=head1 NAME
2
3	root	1.2	BDB - Asynchronous Berkeley DB access
4	root	1.1
5			=head1 SYNOPSIS
6
7	root	1.2	use BDB;
8	root	1.1
9			=head1 DESCRIPTION
10
11	root	1.7	See the eg/ directory in the distribution and the berkeleydb C
12			documentation. This is inadequate, but the only sources of documentation
13			known for this module so far.
14
15	root	1.1	=head2 EXAMPLE
16
17			=head1 REQUEST ANATOMY AND LIFETIME
18
19			Every request method creates a request. which is a C data structure not
20			directly visible to Perl.
21
22			During their existance, bdb requests travel through the following states,
23			in order:
24
25			=over 4
26
27			=item ready
28
29			Immediately after a request is created it is put into the ready state,
30			waiting for a thread to execute it.
31
32			=item execute
33
34			A thread has accepted the request for processing and is currently
35			executing it (e.g. blocking in read).
36
37			=item pending
38
39			The request has been executed and is waiting for result processing.
40
41			While request submission and execution is fully asynchronous, result
42			processing is not and relies on the perl interpreter calling C<poll_cb>
43			(or another function with the same effect).
44
45			=item result
46
47			The request results are processed synchronously by C<poll_cb>.
48
49			The C<poll_cb> function will process all outstanding aio requests by
50			calling their callbacks, freeing memory associated with them and managing
51			any groups they are contained in.
52
53			=item done
54
55			Request has reached the end of its lifetime and holds no resources anymore
56			(except possibly for the Perl object, but its connection to the actual
57			aio request is severed and calling its methods will either do nothing or
58			result in a runtime error).
59
60			=back
61
62			=cut
63
64	root	1.2	package BDB;
65	root	1.1
66			no warnings;
67			use strict 'vars';
68
69			use base 'Exporter';
70
71			BEGIN {
72			our $VERSION = '0.1';
73
74	root	1.3	our @BDB_REQ = qw(
75	root	1.6	db_env_open db_env_close db_env_txn_checkpoint db_env_lock_detect
76			db_env_memp_sync db_env_memp_trickle
77			db_open db_close db_compact db_sync db_put db_get db_pget db_del db_key_range
78	root	1.4	db_txn_commit db_txn_abort
79	root	1.5	db_c_close db_c_count db_c_put db_c_get db_c_pget db_c_del
80	root	1.6	db_sequence_open db_sequence_close
81			db_sequence_get db_sequence_remove
82			);
83			our @EXPORT = (@BDB_REQ, qw(dbreq_pri dbreq_nice db_env_create db_create));
84			our @EXPORT_OK = qw(
85			poll_fileno poll_cb poll_wait flush
86			min_parallel max_parallel max_idle
87			nreqs nready npending nthreads
88			max_poll_time max_poll_reqs
89	root	1.3	);
90	root	1.1
91			require XSLoader;
92	root	1.2	XSLoader::load ("BDB", $VERSION);
93	root	1.1	}
94
95			=head2 SUPPORT FUNCTIONS
96
97			=head3 EVENT PROCESSING AND EVENT LOOP INTEGRATION
98
99			=over 4
100
101	root	1.2	=item $fileno = BDB::poll_fileno
102	root	1.1
103			Return the I<request result pipe file descriptor>. This filehandle must be
104			polled for reading by some mechanism outside this module (e.g. Event or
105			select, see below or the SYNOPSIS). If the pipe becomes readable you have
106			to call C<poll_cb> to check the results.
107
108			See C<poll_cb> for an example.
109
110	root	1.2	=item BDB::poll_cb
111	root	1.1
112			Process some outstanding events on the result pipe. You have to call this
113			regularly. Returns the number of events processed. Returns immediately
114			when no events are outstanding. The amount of events processed depends on
115	root	1.2	the settings of C<BDB::max_poll_req> and C<BDB::max_poll_time>.
116	root	1.1
117			If not all requests were processed for whatever reason, the filehandle
118			will still be ready when C<poll_cb> returns.
119
120			Example: Install an Event watcher that automatically calls
121	root	1.2	BDB::poll_cb with high priority:
122	root	1.1
123	root	1.2	Event->io (fd => BDB::poll_fileno,
124	root	1.1	poll => 'r', async => 1,
125	root	1.2	cb => \&BDB::poll_cb);
126	root	1.1
127	root	1.2	=item BDB::max_poll_reqs $nreqs
128	root	1.1
129	root	1.2	=item BDB::max_poll_time $seconds
130	root	1.1
131			These set the maximum number of requests (default C<0>, meaning infinity)
132	root	1.2	that are being processed by C<BDB::poll_cb> in one call, respectively
133	root	1.1	the maximum amount of time (default C<0>, meaning infinity) spent in
134	root	1.2	C<BDB::poll_cb> to process requests (more correctly the mininum amount
135	root	1.1	of time C<poll_cb> is allowed to use).
136
137			Setting C<max_poll_time> to a non-zero value creates an overhead of one
138			syscall per request processed, which is not normally a problem unless your
139			callbacks are really really fast or your OS is really really slow (I am
140			not mentioning Solaris here). Using C<max_poll_reqs> incurs no overhead.
141
142			Setting these is useful if you want to ensure some level of
143			interactiveness when perl is not fast enough to process all requests in
144			time.
145
146			For interactive programs, values such as C<0.01> to C<0.1> should be fine.
147
148			Example: Install an Event watcher that automatically calls
149	root	1.2	BDB::poll_cb with low priority, to ensure that other parts of the
150	root	1.1	program get the CPU sometimes even under high AIO load.
151
152			# try not to spend much more than 0.1s in poll_cb
153	root	1.2	BDB::max_poll_time 0.1;
154	root	1.1
155			# use a low priority so other tasks have priority
156	root	1.2	Event->io (fd => BDB::poll_fileno,
157	root	1.1	poll => 'r', nice => 1,
158	root	1.2	cb => &BDB::poll_cb);
159	root	1.1
160	root	1.2	=item BDB::poll_wait
161	root	1.1
162			If there are any outstanding requests and none of them in the result
163			phase, wait till the result filehandle becomes ready for reading (simply
164			does a C<select> on the filehandle. This is useful if you want to
165			synchronously wait for some requests to finish).
166
167			See C<nreqs> for an example.
168
169	root	1.2	=item BDB::poll
170	root	1.1
171			Waits until some requests have been handled.
172
173			Returns the number of requests processed, but is otherwise strictly
174			equivalent to:
175
176	root	1.2	BDB::poll_wait, BDB::poll_cb
177	root	1.1
178	root	1.2	=item BDB::flush
179	root	1.1
180			Wait till all outstanding AIO requests have been handled.
181
182			Strictly equivalent to:
183
184	root	1.2	BDB::poll_wait, BDB::poll_cb
185			while BDB::nreqs;
186	root	1.1
187			=head3 CONTROLLING THE NUMBER OF THREADS
188
189	root	1.2	=item BDB::min_parallel $nthreads
190	root	1.1
191			Set the minimum number of AIO threads to C<$nthreads>. The current
192			default is C<8>, which means eight asynchronous operations can execute
193			concurrently at any one time (the number of outstanding requests,
194			however, is unlimited).
195
196	root	1.2	BDB starts threads only on demand, when an AIO request is queued and
197	root	1.1	no free thread exists. Please note that queueing up a hundred requests can
198			create demand for a hundred threads, even if it turns out that everything
199			is in the cache and could have been processed faster by a single thread.
200
201			It is recommended to keep the number of threads relatively low, as some
202			Linux kernel versions will scale negatively with the number of threads
203			(higher parallelity => MUCH higher latency). With current Linux 2.6
204			versions, 4-32 threads should be fine.
205
206			Under most circumstances you don't need to call this function, as the
207			module selects a default that is suitable for low to moderate load.
208
209	root	1.2	=item BDB::max_parallel $nthreads
210	root	1.1
211			Sets the maximum number of AIO threads to C<$nthreads>. If more than the
212			specified number of threads are currently running, this function kills
213			them. This function blocks until the limit is reached.
214
215			While C<$nthreads> are zero, aio requests get queued but not executed
216			until the number of threads has been increased again.
217
218			This module automatically runs C<max_parallel 0> at program end, to ensure
219			that all threads are killed and that there are no outstanding requests.
220
221			Under normal circumstances you don't need to call this function.
222
223	root	1.2	=item BDB::max_idle $nthreads
224	root	1.1
225			Limit the number of threads (default: 4) that are allowed to idle (i.e.,
226			threads that did not get a request to process within 10 seconds). That
227			means if a thread becomes idle while C<$nthreads> other threads are also
228			idle, it will free its resources and exit.
229
230			This is useful when you allow a large number of threads (e.g. 100 or 1000)
231			to allow for extremely high load situations, but want to free resources
232			under normal circumstances (1000 threads can easily consume 30MB of RAM).
233
234			The default is probably ok in most situations, especially if thread
235			creation is fast. If thread creation is very slow on your system you might
236			want to use larger values.
237
238	root	1.2	=item $oldmaxreqs = BDB::max_outstanding $maxreqs
239	root	1.1
240			This is a very bad function to use in interactive programs because it
241			blocks, and a bad way to reduce concurrency because it is inexact: Better
242			use an C<aio_group> together with a feed callback.
243
244			Sets the maximum number of outstanding requests to C<$nreqs>. If you
245			to queue up more than this number of requests, the next call to the
246			C<poll_cb> (and C<poll_some> and other functions calling C<poll_cb>)
247			function will block until the limit is no longer exceeded.
248
249			The default value is very large, so there is no practical limit on the
250			number of outstanding requests.
251
252			You can still queue as many requests as you want. Therefore,
253			C<max_oustsanding> is mainly useful in simple scripts (with low values) or
254			as a stop gap to shield against fatal memory overflow (with large values).
255
256	root	1.3	=item BDB::set_sync_prepare $cb
257
258			Sets a callback that is called whenever a request is created without an
259			explicit callback. It has to return two code references. The first is used
260			as the request callback, and the second is called to wait until the first
261			callback has been called. The default implementation works like this:
262
263			sub {
264			my $status;
265			(
266			sub { $status = $! },
267			sub { BDB::poll while !defined $status; $! = $status },
268			)
269			}
270
271			=back
272
273	root	1.1	=head3 STATISTICAL INFORMATION
274
275	root	1.3	=over 4
276
277	root	1.2	=item BDB::nreqs
278	root	1.1
279			Returns the number of requests currently in the ready, execute or pending
280			states (i.e. for which their callback has not been invoked yet).
281
282			Example: wait till there are no outstanding requests anymore:
283
284	root	1.2	BDB::poll_wait, BDB::poll_cb
285			while BDB::nreqs;
286	root	1.1
287	root	1.2	=item BDB::nready
288	root	1.1
289			Returns the number of requests currently in the ready state (not yet
290			executed).
291
292	root	1.2	=item BDB::npending
293	root	1.1
294			Returns the number of requests currently in the pending state (executed,
295			but not yet processed by poll_cb).
296
297			=back
298
299			=cut
300
301	root	1.3	set_sync_prepare {
302			my $status;
303			(
304			sub {
305			$status = $!;
306			},
307			sub {
308			BDB::poll while !defined $status;
309			$! = $status;
310			},
311			)
312			};
313
314	root	1.1	min_parallel 8;
315
316			END { flush }
317
318			1;
319
320			=head2 FORK BEHAVIOUR
321
322			This module should do "the right thing" when the process using it forks:
323
324			Before the fork, IO::AIO enters a quiescent state where no requests
325			can be added in other threads and no results will be processed. After
326			the fork the parent simply leaves the quiescent state and continues
327			request/result processing, while the child frees the request/result queue
328			(so that the requests started before the fork will only be handled in the
329			parent). Threads will be started on demand until the limit set in the
330			parent process has been reached again.
331
332			In short: the parent will, after a short pause, continue as if fork had
333			not been called, while the child will act as if IO::AIO has not been used
334			yet.
335
336			=head2 MEMORY USAGE
337
338			Per-request usage:
339
340			Each aio request uses - depending on your architecture - around 100-200
341			bytes of memory. In addition, stat requests need a stat buffer (possibly
342			a few hundred bytes), readdir requires a result buffer and so on. Perl
343			scalars and other data passed into aio requests will also be locked and
344			will consume memory till the request has entered the done state.
345
346			This is now awfully much, so queuing lots of requests is not usually a
347			problem.
348
349			Per-thread usage:
350
351			In the execution phase, some aio requests require more memory for
352			temporary buffers, and each thread requires a stack and other data
353			structures (usually around 16k-128k, depending on the OS).
354
355			=head1 KNOWN BUGS
356
357			Known bugs will be fixed in the next release.
358
359			=head1 SEE ALSO
360
361			L<Coro::AIO>.
362
363			=head1 AUTHOR
364
365			Marc Lehmann <schmorp@schmorp.de>
366			http://home.schmorp.de/
367
368			=cut
369