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_del
      db_txn_commit db_txn_abort
      db_c_close db_c_count db_c_put db_c_get db_c_pget db_c_del
   );
   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:	1.5
Committed:	Tue Feb 6 01:09:04 2007 UTC (17 years, 3 months ago) by root
Branch:	MAIN
Changes since 1.4:	+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 db_del
73	db_txn_commit db_txn_abort
74	db_c_close db_c_count db_c_put db_c_get db_c_pget db_c_del
75	);
76	our @EXPORT = (@BDB_REQ, qw(dbreq_pri dbreq_nice));
77	our @EXPORT_OK = qw(poll_fileno poll_cb poll_wait flush
78	min_parallel max_parallel max_idle
79	nreqs nready npending nthreads
80	max_poll_time max_poll_reqs);
81
82	require XSLoader;
83	XSLoader::load ("BDB", $VERSION);
84	}
85
86	=head2 SUPPORT FUNCTIONS
87
88	=head3 EVENT PROCESSING AND EVENT LOOP INTEGRATION
89
90	=over 4
91
92	=item $fileno = BDB::poll_fileno
93
94	Return the I<request result pipe file descriptor>. This filehandle must be
95	polled for reading by some mechanism outside this module (e.g. Event or
96	select, see below or the SYNOPSIS). If the pipe becomes readable you have
97	to call C<poll_cb> to check the results.
98
99	See C<poll_cb> for an example.
100
101	=item BDB::poll_cb
102
103	Process some outstanding events on the result pipe. You have to call this
104	regularly. Returns the number of events processed. Returns immediately
105	when no events are outstanding. The amount of events processed depends on
106	the settings of C<BDB::max_poll_req> and C<BDB::max_poll_time>.
107
108	If not all requests were processed for whatever reason, the filehandle
109	will still be ready when C<poll_cb> returns.
110
111	Example: Install an Event watcher that automatically calls
112	BDB::poll_cb with high priority:
113
114	Event->io (fd => BDB::poll_fileno,
115	poll => 'r', async => 1,
116	cb => \&BDB::poll_cb);
117
118	=item BDB::max_poll_reqs $nreqs
119
120	=item BDB::max_poll_time $seconds
121
122	These set the maximum number of requests (default C<0>, meaning infinity)
123	that are being processed by C<BDB::poll_cb> in one call, respectively
124	the maximum amount of time (default C<0>, meaning infinity) spent in
125	C<BDB::poll_cb> to process requests (more correctly the mininum amount
126	of time C<poll_cb> is allowed to use).
127
128	Setting C<max_poll_time> to a non-zero value creates an overhead of one
129	syscall per request processed, which is not normally a problem unless your
130	callbacks are really really fast or your OS is really really slow (I am
131	not mentioning Solaris here). Using C<max_poll_reqs> incurs no overhead.
132
133	Setting these is useful if you want to ensure some level of
134	interactiveness when perl is not fast enough to process all requests in
135	time.
136
137	For interactive programs, values such as C<0.01> to C<0.1> should be fine.
138
139	Example: Install an Event watcher that automatically calls
140	BDB::poll_cb with low priority, to ensure that other parts of the
141	program get the CPU sometimes even under high AIO load.
142
143	# try not to spend much more than 0.1s in poll_cb
144	BDB::max_poll_time 0.1;
145
146	# use a low priority so other tasks have priority
147	Event->io (fd => BDB::poll_fileno,
148	poll => 'r', nice => 1,
149	cb => &BDB::poll_cb);
150
151	=item BDB::poll_wait
152
153	If there are any outstanding requests and none of them in the result
154	phase, wait till the result filehandle becomes ready for reading (simply
155	does a C<select> on the filehandle. This is useful if you want to
156	synchronously wait for some requests to finish).
157
158	See C<nreqs> for an example.
159
160	=item BDB::poll
161
162	Waits until some requests have been handled.
163
164	Returns the number of requests processed, but is otherwise strictly
165	equivalent to:
166
167	BDB::poll_wait, BDB::poll_cb
168
169	=item BDB::flush
170
171	Wait till all outstanding AIO requests have been handled.
172
173	Strictly equivalent to:
174
175	BDB::poll_wait, BDB::poll_cb
176	while BDB::nreqs;
177
178	=head3 CONTROLLING THE NUMBER OF THREADS
179
180	=item BDB::min_parallel $nthreads
181
182	Set the minimum number of AIO threads to C<$nthreads>. The current
183	default is C<8>, which means eight asynchronous operations can execute
184	concurrently at any one time (the number of outstanding requests,
185	however, is unlimited).
186
187	BDB starts threads only on demand, when an AIO request is queued and
188	no free thread exists. Please note that queueing up a hundred requests can
189	create demand for a hundred threads, even if it turns out that everything
190	is in the cache and could have been processed faster by a single thread.
191
192	It is recommended to keep the number of threads relatively low, as some
193	Linux kernel versions will scale negatively with the number of threads
194	(higher parallelity => MUCH higher latency). With current Linux 2.6
195	versions, 4-32 threads should be fine.
196
197	Under most circumstances you don't need to call this function, as the
198	module selects a default that is suitable for low to moderate load.
199
200	=item BDB::max_parallel $nthreads
201
202	Sets the maximum number of AIO threads to C<$nthreads>. If more than the
203	specified number of threads are currently running, this function kills
204	them. This function blocks until the limit is reached.
205
206	While C<$nthreads> are zero, aio requests get queued but not executed
207	until the number of threads has been increased again.
208
209	This module automatically runs C<max_parallel 0> at program end, to ensure
210	that all threads are killed and that there are no outstanding requests.
211
212	Under normal circumstances you don't need to call this function.
213
214	=item BDB::max_idle $nthreads
215
216	Limit the number of threads (default: 4) that are allowed to idle (i.e.,
217	threads that did not get a request to process within 10 seconds). That
218	means if a thread becomes idle while C<$nthreads> other threads are also
219	idle, it will free its resources and exit.
220
221	This is useful when you allow a large number of threads (e.g. 100 or 1000)
222	to allow for extremely high load situations, but want to free resources
223	under normal circumstances (1000 threads can easily consume 30MB of RAM).
224
225	The default is probably ok in most situations, especially if thread
226	creation is fast. If thread creation is very slow on your system you might
227	want to use larger values.
228
229	=item $oldmaxreqs = BDB::max_outstanding $maxreqs
230
231	This is a very bad function to use in interactive programs because it
232	blocks, and a bad way to reduce concurrency because it is inexact: Better
233	use an C<aio_group> together with a feed callback.
234
235	Sets the maximum number of outstanding requests to C<$nreqs>. If you
236	to queue up more than this number of requests, the next call to the
237	C<poll_cb> (and C<poll_some> and other functions calling C<poll_cb>)
238	function will block until the limit is no longer exceeded.
239
240	The default value is very large, so there is no practical limit on the
241	number of outstanding requests.
242
243	You can still queue as many requests as you want. Therefore,
244	C<max_oustsanding> is mainly useful in simple scripts (with low values) or
245	as a stop gap to shield against fatal memory overflow (with large values).
246
247	=item BDB::set_sync_prepare $cb
248
249	Sets a callback that is called whenever a request is created without an
250	explicit callback. It has to return two code references. The first is used
251	as the request callback, and the second is called to wait until the first
252	callback has been called. The default implementation works like this:
253
254	sub {
255	my $status;
256	(
257	sub { $status = $! },
258	sub { BDB::poll while !defined $status; $! = $status },
259	)
260	}
261
262	=back
263
264	=head3 STATISTICAL INFORMATION
265
266	=over 4
267
268	=item BDB::nreqs
269
270	Returns the number of requests currently in the ready, execute or pending
271	states (i.e. for which their callback has not been invoked yet).
272
273	Example: wait till there are no outstanding requests anymore:
274
275	BDB::poll_wait, BDB::poll_cb
276	while BDB::nreqs;
277
278	=item BDB::nready
279
280	Returns the number of requests currently in the ready state (not yet
281	executed).
282
283	=item BDB::npending
284
285	Returns the number of requests currently in the pending state (executed,
286	but not yet processed by poll_cb).
287
288	=back
289
290	=cut
291
292	set_sync_prepare {
293	my $status;
294	(
295	sub {
296	$status = $!;
297	},
298	sub {
299	BDB::poll while !defined $status;
300	$! = $status;
301	},
302	)
303	};
304
305	min_parallel 8;
306
307	END { flush }
308
309	1;
310
311	=head2 FORK BEHAVIOUR
312
313	This module should do "the right thing" when the process using it forks:
314
315	Before the fork, IO::AIO enters a quiescent state where no requests
316	can be added in other threads and no results will be processed. After
317	the fork the parent simply leaves the quiescent state and continues
318	request/result processing, while the child frees the request/result queue
319	(so that the requests started before the fork will only be handled in the
320	parent). Threads will be started on demand until the limit set in the
321	parent process has been reached again.
322
323	In short: the parent will, after a short pause, continue as if fork had
324	not been called, while the child will act as if IO::AIO has not been used
325	yet.
326
327	=head2 MEMORY USAGE
328
329	Per-request usage:
330
331	Each aio request uses - depending on your architecture - around 100-200
332	bytes of memory. In addition, stat requests need a stat buffer (possibly
333	a few hundred bytes), readdir requires a result buffer and so on. Perl
334	scalars and other data passed into aio requests will also be locked and
335	will consume memory till the request has entered the done state.
336
337	This is now awfully much, so queuing lots of requests is not usually a
338	problem.
339
340	Per-thread usage:
341
342	In the execution phase, some aio requests require more memory for
343	temporary buffers, and each thread requires a stack and other data
344	structures (usually around 16k-128k, depending on the OS).
345
346	=head1 KNOWN BUGS
347
348	Known bugs will be fixed in the next release.
349
350	=head1 SEE ALSO
351
352	L<Coro::AIO>.
353
354	=head1 AUTHOR
355
356	Marc Lehmann <schmorp@schmorp.de>
357	http://home.schmorp.de/
358
359	=cut
360