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