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Revision: 1.16
Committed: Sun Sep 16 18:17:58 2007 UTC (16 years, 8 months ago) by root
Branch: MAIN
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# 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 See the BerkeleyDB documentation (L<http://www.oracle.com/technology/documentation/berkeley-db/db/index.html>).
12 The BDB API is very similar to the C API (the translation has been very faithful).
13
14 See also the example sections in the document below and possibly the eg/
15 subdirectory of the BDB distribution. Last not least see the IO::AIO
16 documentation, as that module uses almost the same asynchronous request
17 model as this module.
18
19 I know this is woefully inadequate documentation. Send a patch!
20
21
22 =head1 REQUEST ANATOMY AND LIFETIME
23
24 Every request method creates a request. which is a C data structure not
25 directly visible to Perl.
26
27 During their existance, bdb requests travel through the following states,
28 in order:
29
30 =over 4
31
32 =item ready
33
34 Immediately after a request is created it is put into the ready state,
35 waiting for a thread to execute it.
36
37 =item execute
38
39 A thread has accepted the request for processing and is currently
40 executing it (e.g. blocking in read).
41
42 =item pending
43
44 The request has been executed and is waiting for result processing.
45
46 While request submission and execution is fully asynchronous, result
47 processing is not and relies on the perl interpreter calling C<poll_cb>
48 (or another function with the same effect).
49
50 =item result
51
52 The request results are processed synchronously by C<poll_cb>.
53
54 The C<poll_cb> function will process all outstanding aio requests by
55 calling their callbacks, freeing memory associated with them and managing
56 any groups they are contained in.
57
58 =item done
59
60 Request has reached the end of its lifetime and holds no resources anymore
61 (except possibly for the Perl object, but its connection to the actual
62 aio request is severed and calling its methods will either do nothing or
63 result in a runtime error).
64
65 =back
66
67 =cut
68
69 package BDB;
70
71 no warnings;
72 use strict 'vars';
73
74 use base 'Exporter';
75
76 BEGIN {
77 our $VERSION = '1.1';
78
79 our @BDB_REQ = qw(
80 db_env_open db_env_close db_env_txn_checkpoint db_env_lock_detect
81 db_env_memp_sync db_env_memp_trickle
82 db_open db_close db_compact db_sync db_put db_get db_pget db_del db_key_range
83 db_txn_commit db_txn_abort db_txn_finish
84 db_c_close db_c_count db_c_put db_c_get db_c_pget db_c_del
85 db_sequence_open db_sequence_close
86 db_sequence_get db_sequence_remove
87 );
88 our @EXPORT = (@BDB_REQ, qw(dbreq_pri dbreq_nice db_env_create db_create));
89 our @EXPORT_OK = qw(
90 poll_fileno poll_cb poll_wait flush
91 min_parallel max_parallel max_idle
92 nreqs nready npending nthreads
93 max_poll_time max_poll_reqs
94 );
95
96 require XSLoader;
97 XSLoader::load ("BDB", $VERSION);
98 }
99
100 =head2 BERKELEYDB FUNCTIONS
101
102 All of these are functions. The create functions simply return a new
103 object and never block. All the remaining functions all take an optional
104 callback as last argument. If it is missing, then the fucntion will be
105 executed synchronously.
106
107 BDB functions that cannot block (mostly functions that manipulate
108 settings) are method calls on the relevant objects, so the rule of thumb
109 is: if its a method, its not blocking, if its a function, it takes a
110 callback as last argument.
111
112 In the following, C<$int> signifies an integer return value,
113 C<octetstring> is a "binary string" (i.e. a perl string with no character
114 indices >255), C<U32> is an unsigned 32 bit integer, C<int> is some
115 integer, C<NV> is a floating point value.
116
117 The C<SV *> types are generic perl scalars (for input and output of data
118 values), and the C<SV *callback> is the optional callback function to call
119 when the request is completed.
120
121 The various C<DB_ENV> etc. arguments are handles return by
122 C<db_env_create>, C<db_create>, C<txn_begin> and so on. If they have an
123 appended C<_ornull> this means they are optional and you can pass C<undef>
124 for them, resulting a NULL pointer on the C level.
125
126 =head3 BDB functions
127
128 Functions in the BDB namespace, exported by default:
129
130 $env = db_env_create (U32 env_flags = 0)
131 flags: RPCCLIENT
132
133 db_env_open (DB_ENV *env, octetstring db_home, U32 open_flags, int mode, SV *callback = &PL_sv_undef)
134 open_flags: INIT_CDB INIT_LOCK INIT_LOG INIT_MPOOL INIT_REP INIT_TXN RECOVER RECOVER_FATAL USE_ENVIRON USE_ENVIRON_ROOT CREATE LOCKDOWN PRIVATE REGISTER SYSTEM_MEM
135 db_env_close (DB_ENV *env, U32 flags = 0, SV *callback = &PL_sv_undef)
136 db_env_txn_checkpoint (DB_ENV *env, U32 kbyte = 0, U32 min = 0, U32 flags = 0, SV *callback = &PL_sv_undef)
137 flags: FORCE
138 db_env_lock_detect (DB_ENV *env, U32 flags = 0, U32 atype = DB_LOCK_DEFAULT, SV *dummy = 0, SV *callback = &PL_sv_undef)
139 atype: LOCK_DEFAULT LOCK_EXPIRE LOCK_MAXLOCKS LOCK_MAXWRITE LOCK_MINLOCKS LOCK_MINWRITE LOCK_OLDEST LOCK_RANDOM LOCK_YOUNGEST
140 db_env_memp_sync (DB_ENV *env, SV *dummy = 0, SV *callback = &PL_sv_undef)
141 db_env_memp_trickle (DB_ENV *env, int percent, SV *dummy = 0, SV *callback = &PL_sv_undef)
142
143 $db = db_create (DB_ENV *env = 0, U32 flags = 0)
144 flags: XA_CREATE
145
146 db_open (DB *db, DB_TXN_ornull *txnid, octetstring file, octetstring database, int type, U32 flags, int mode, SV *callback = &PL_sv_undef)
147 flags: AUTO_COMMIT CREATE EXCL MULTIVERSION NOMMAP RDONLY READ_UNCOMMITTED THREAD TRUNCATE
148 db_close (DB *db, U32 flags = 0, SV *callback = &PL_sv_undef)
149 flags: DB_NOSYNC
150 db_compact (DB *db, DB_TXN_ornull *txn = 0, SV *start = 0, SV *stop = 0, SV *unused1 = 0, U32 flags = DB_FREE_SPACE, SV *unused2 = 0, SV *callback = &PL_sv_undef)
151 flags: FREELIST_ONLY FREE_SPACE
152 db_sync (DB *db, U32 flags = 0, SV *callback = &PL_sv_undef)
153 db_key_range (DB *db, DB_TXN_ornull *txn, SV *key, SV *key_range, U32 flags = 0, SV *callback = &PL_sv_undef)
154 db_put (DB *db, DB_TXN_ornull *txn, SV *key, SV *data, U32 flags = 0, SV *callback = &PL_sv_undef)
155 flags: APPEND NODUPDATA NOOVERWRITE
156 db_get (DB *db, DB_TXN_ornull *txn, SV *key, SV *data, U32 flags = 0, SV *callback = &PL_sv_undef)
157 flags: CONSUME CONSUME_WAIT GET_BOTH SET_RECNO MULTIPLE READ_COMMITTED READ_UNCOMMITTED RMW
158 db_pget (DB *db, DB_TXN_ornull *txn, SV *key, SV *pkey, SV *data, U32 flags = 0, SV *callback = &PL_sv_undef)
159 flags: CONSUME CONSUME_WAIT GET_BOTH SET_RECNO MULTIPLE READ_COMMITTED READ_UNCOMMITTED RMW
160 db_del (DB *db, DB_TXN_ornull *txn, SV *key, U32 flags = 0, SV *callback = &PL_sv_undef)
161 db_txn_commit (DB_TXN *txn, U32 flags = 0, SV *callback = &PL_sv_undef)
162 flags: TXN_NOSYNC TXN_SYNC
163 db_txn_abort (DB_TXN *txn, SV *callback = &PL_sv_undef)
164
165 db_c_close (DBC *dbc, SV *callback = &PL_sv_undef)
166 db_c_count (DBC *dbc, SV *count, U32 flags = 0, SV *callback = &PL_sv_undef)
167 db_c_put (DBC *dbc, SV *key, SV *data, U32 flags = 0, SV *callback = &PL_sv_undef)
168 flags: AFTER BEFORE CURRENT KEYFIRST KEYLAST NODUPDATA
169 db_c_get (DBC *dbc, SV *key, SV *data, U32 flags = 0, SV *callback = &PL_sv_undef)
170 flags: CURRENT FIRST GET_BOTH GET_BOTH_RANGE GET_RECNO JOIN_ITEM LAST NEXT NEXT_DUP NEXT_NODUP PREV PREV_DUP PREV_NODUP SET SET_RANGE SET_RECNO READ_UNCOMMITTED MULTIPLE MULTIPLE_KEY RMW
171 db_c_pget (DBC *dbc, SV *key, SV *pkey, SV *data, U32 flags = 0, SV *callback = &PL_sv_undef)
172 db_c_del (DBC *dbc, U32 flags = 0, SV *callback = &PL_sv_undef)
173
174 db_sequence_open (DB_SEQUENCE *seq, DB_TXN_ornull *txnid, SV *key, U32 flags = 0, SV *callback = &PL_sv_undef)
175 flags: CREATE EXCL
176 db_sequence_close (DB_SEQUENCE *seq, U32 flags = 0, SV *callback = &PL_sv_undef)
177 db_sequence_get (DB_SEQUENCE *seq, DB_TXN_ornull *txnid, int delta, SV *seq_value, U32 flags = DB_TXN_NOSYNC, SV *callback = &PL_sv_undef)
178 flags: TXN_NOSYNC
179 db_sequence_remove (DB_SEQUENCE *seq, DB_TXN_ornull *txnid = 0, U32 flags = 0, SV *callback = &PL_sv_undef)
180 flags: TXN_NOSYNC
181
182 =head4 db_txn_finish (DB_TXN *txn, U32 flags = 0, SV *callback = &PL_sv_undef)
183
184 This is not actually a Berkeley DB function but a BDB module
185 extension. The background for this exytension is: It is very annoying to
186 have to check every single BDB function for error returns and provide a
187 codepath out of your transaction. While the BDB module still makes this
188 possible, it contains the following extensions:
189
190 When a transaction-protected function returns any operating system
191 error (errno > 0), BDB will set the C<TXN_DEADLOCK> flag on the
192 transaction. This flag is also set by Berkeley DB functions themselves
193 when an operation fails with LOCK_DEADLOCK, and it causes all further
194 operations on that transaction (including C<db_txn_commit>) to fail.
195
196 The C<db_txn_finish> request will look at this flag, and, if it is set,
197 will automatically call C<db_txn_abort> (setting errno to C<LOCK_DEADLOCK>
198 if it isn't set to something else yet). If it isn't set, it will call
199 C<db_txn_commit> and return the error normally.
200
201 How to use this? Easy: just write your transaction normally:
202
203 my $txn = $db_env->txn_begin;
204 db_get $db, $txn, "key", my $data;
205 db_put $db, $txn, "key", $data + 1 unless $! == BDB::NOTFOUND;
206 db_txn_finish $txn;
207 die "transaction failed" if $!;
208
209 That is, handle only the expected errors. If something unexpected happens
210 (EIO, LOCK_NOTGRANTED or a deadlock in either db_get or db_put), then the remaining
211 requests (db_put in this case) will simply be skipped (they will fail with
212 LOCK_DEADLOCK) and the transaction will be aborted.
213
214 You can use the C<< $txn->failed >> method to check wether a transaction
215 has failed in this way and abort further processing (excluding
216 C<db_txn_finish>).
217
218 =head3 DB_ENV/database environment methods
219
220 Methods available on DB_ENV/$env handles:
221
222 DESTROY (DB_ENV_ornull *env)
223 CODE:
224 if (env)
225 env->close (env, 0);
226
227 $int = $env->set_data_dir (const char *dir)
228 $int = $env->set_tmp_dir (const char *dir)
229 $int = $env->set_lg_dir (const char *dir)
230 $int = $env->set_shm_key (long shm_key)
231 $int = $env->set_cachesize (U32 gbytes, U32 bytes, int ncache = 0)
232 $int = $env->set_flags (U32 flags, int onoff)
233 $env->set_errfile (FILE *errfile = 0)
234 $env->set_msgfile (FILE *msgfile = 0)
235 $int = $env->set_verbose (U32 which, int onoff = 1)
236 $int = $env->set_encrypt (const char *password, U32 flags = 0)
237 $int = $env->set_timeout (NV timeout_seconds, U32 flags = SET_TXN_TIMEOUT)
238 $int = $env->set_mp_max_openfd (int maxopenfd);
239 $int = $env->set_mp_max_write (int maxwrite, int maxwrite_sleep);
240 $int = $env->set_mp_mmapsize (int mmapsize_mb)
241 $int = $env->set_lk_detect (U32 detect = DB_LOCK_DEFAULT)
242 $int = $env->set_lk_max_lockers (U32 max)
243 $int = $env->set_lk_max_locks (U32 max)
244 $int = $env->set_lk_max_objects (U32 max)
245 $int = $env->set_lg_bsize (U32 max)
246 $int = $env->set_lg_max (U32 max)
247
248 $txn = $env->txn_begin (DB_TXN_ornull *parent = 0, U32 flags = 0)
249 flags: READ_COMMITTED READ_UNCOMMITTED TXN_NOSYNC TXN_NOWAIT TXN_SNAPSHOT TXN_SYNC TXN_WAIT TXN_WRITE_NOSYNC
250
251 =head4 Example:
252
253 use AnyEvent;
254 use BDB;
255
256 our $FH; open $FH, "<&=" . BDB::poll_fileno;
257 our $WATCHER = AnyEvent->io (fh => $FH, poll => 'r', cb => \&BDB::poll_cb);
258
259 BDB::min_parallel 8;
260
261 my $env = db_env_create;
262
263 mkdir "bdtest", 0700;
264 db_env_open
265 $env,
266 "bdtest",
267 BDB::INIT_LOCK | BDB::INIT_LOG | BDB::INIT_MPOOL | BDB::INIT_TXN | BDB::RECOVER | BDB::USE_ENVIRON | BDB::CREATE,
268 0600;
269
270 $env->set_flags (BDB::AUTO_COMMIT | BDB::TXN_NOSYNC, 1);
271
272
273 =head3 DB/database methods
274
275 Methods available on DB/$db handles:
276
277 DESTROY (DB_ornull *db)
278 CODE:
279 if (db)
280 {
281 SV *env = (SV *)db->app_private;
282 db->close (db, 0);
283 SvREFCNT_dec (env);
284 }
285
286 $int = $db->set_cachesize (U32 gbytes, U32 bytes, int ncache = 0)
287 $int = $db->set_flags (U32 flags)
288 flags: CHKSUM ENCRYPT TXN_NOT_DURABLE
289 Btree: DUP DUPSORT RECNUM REVSPLITOFF
290 Hash: DUP DUPSORT
291 Queue: INORDER
292 Recno: RENUMBER SNAPSHOT
293
294 $int = $db->set_encrypt (const char *password, U32 flags)
295 $int = $db->set_lorder (int lorder)
296 $int = $db->set_bt_minkey (U32 minkey)
297 $int = $db->set_re_delim (int delim)
298 $int = $db->set_re_pad (int re_pad)
299 $int = $db->set_re_source (char *source)
300 $int = $db->set_re_len (U32 re_len)
301 $int = $db->set_h_ffactor (U32 h_ffactor)
302 $int = $db->set_h_nelem (U32 h_nelem)
303 $int = $db->set_q_extentsize (U32 extentsize)
304
305 $dbc = $db->cursor (DB_TXN_ornull *txn = 0, U32 flags = 0)
306 flags: READ_COMMITTED READ_UNCOMMITTED WRITECURSOR TXN_SNAPSHOT
307 $seq = $db->sequence (U32 flags = 0)
308
309 =head4 Example:
310
311 my $db = db_create $env;
312 db_open $db, undef, "table", undef, BDB::BTREE, BDB::AUTO_COMMIT | BDB::CREATE | BDB::READ_UNCOMMITTED, 0600;
313
314 for (1..1000) {
315 db_put $db, undef, "key $_", "data $_";
316
317 db_key_range $db, undef, "key $_", my $keyrange;
318 my ($lt, $eq, $gt) = @$keyrange;
319 }
320
321 db_del $db, undef, "key $_" for 1..1000;
322
323 db_sync $db;
324
325
326 =head3 DB_TXN/transaction methods
327
328 Methods available on DB_TXN/$txn handles:
329
330 DESTROY (DB_TXN_ornull *txn)
331 CODE:
332 if (txn)
333 txn->abort (txn);
334
335 $int = $txn->set_timeout (NV timeout_seconds, U32 flags = SET_TXN_TIMEOUT)
336 flags: SET_LOCK_TIMEOUT SET_TXN_TIMEOUT
337
338 $bool = $txn->failed
339 # see db_txn_finish documentation, above
340
341
342 =head3 DBC/cursor methods
343
344 Methods available on DBC/$dbc handles:
345
346 DESTROY (DBC_ornull *dbc)
347 CODE:
348 if (dbc)
349 dbc->c_close (dbc);
350
351 =head4 Example:
352
353 my $c = $db->cursor;
354
355 for (;;) {
356 db_c_get $c, my $key, my $data, BDB::NEXT;
357 warn "<$!,$key,$data>";
358 last if $!;
359 }
360
361 db_c_close $c;
362
363
364 =head3 DB_SEQUENCE/sequence methods
365
366 Methods available on DB_SEQUENCE/$seq handles:
367
368 DESTROY (DB_SEQUENCE_ornull *seq)
369 CODE:
370 if (seq)
371 seq->close (seq, 0);
372
373 $int = $seq->initial_value (db_seq_t value)
374 $int = $seq->set_cachesize (U32 size)
375 $int = $seq->set_flags (U32 flags)
376 flags: SEQ_DEC SEQ_INC SEQ_WRAP
377 $int = $seq->set_range (db_seq_t min, db_seq_t max)
378
379 =head4 Example:
380
381 my $seq = $db->sequence;
382
383 db_sequence_open $seq, undef, "seq", BDB::CREATE;
384 db_sequence_get $seq, undef, 1, my $value;
385
386
387 =head2 SUPPORT FUNCTIONS
388
389 =head3 EVENT PROCESSING AND EVENT LOOP INTEGRATION
390
391 =over 4
392
393 =item $fileno = BDB::poll_fileno
394
395 Return the I<request result pipe file descriptor>. This filehandle must be
396 polled for reading by some mechanism outside this module (e.g. Event or
397 select, see below or the SYNOPSIS). If the pipe becomes readable you have
398 to call C<poll_cb> to check the results.
399
400 See C<poll_cb> for an example.
401
402 =item BDB::poll_cb
403
404 Process some outstanding events on the result pipe. You have to call this
405 regularly. Returns the number of events processed. Returns immediately
406 when no events are outstanding. The amount of events processed depends on
407 the settings of C<BDB::max_poll_req> and C<BDB::max_poll_time>.
408
409 If not all requests were processed for whatever reason, the filehandle
410 will still be ready when C<poll_cb> returns.
411
412 Example: Install an Event watcher that automatically calls
413 BDB::poll_cb with high priority:
414
415 Event->io (fd => BDB::poll_fileno,
416 poll => 'r', async => 1,
417 cb => \&BDB::poll_cb);
418
419 =item BDB::max_poll_reqs $nreqs
420
421 =item BDB::max_poll_time $seconds
422
423 These set the maximum number of requests (default C<0>, meaning infinity)
424 that are being processed by C<BDB::poll_cb> in one call, respectively
425 the maximum amount of time (default C<0>, meaning infinity) spent in
426 C<BDB::poll_cb> to process requests (more correctly the mininum amount
427 of time C<poll_cb> is allowed to use).
428
429 Setting C<max_poll_time> to a non-zero value creates an overhead of one
430 syscall per request processed, which is not normally a problem unless your
431 callbacks are really really fast or your OS is really really slow (I am
432 not mentioning Solaris here). Using C<max_poll_reqs> incurs no overhead.
433
434 Setting these is useful if you want to ensure some level of
435 interactiveness when perl is not fast enough to process all requests in
436 time.
437
438 For interactive programs, values such as C<0.01> to C<0.1> should be fine.
439
440 Example: Install an Event watcher that automatically calls
441 BDB::poll_cb with low priority, to ensure that other parts of the
442 program get the CPU sometimes even under high AIO load.
443
444 # try not to spend much more than 0.1s in poll_cb
445 BDB::max_poll_time 0.1;
446
447 # use a low priority so other tasks have priority
448 Event->io (fd => BDB::poll_fileno,
449 poll => 'r', nice => 1,
450 cb => &BDB::poll_cb);
451
452 =item BDB::poll_wait
453
454 If there are any outstanding requests and none of them in the result
455 phase, wait till the result filehandle becomes ready for reading (simply
456 does a C<select> on the filehandle. This is useful if you want to
457 synchronously wait for some requests to finish).
458
459 See C<nreqs> for an example.
460
461 =item BDB::poll
462
463 Waits until some requests have been handled.
464
465 Returns the number of requests processed, but is otherwise strictly
466 equivalent to:
467
468 BDB::poll_wait, BDB::poll_cb
469
470 =item BDB::flush
471
472 Wait till all outstanding AIO requests have been handled.
473
474 Strictly equivalent to:
475
476 BDB::poll_wait, BDB::poll_cb
477 while BDB::nreqs;
478
479 =back
480
481 =head3 CONTROLLING THE NUMBER OF THREADS
482
483 =over 4
484
485 =item BDB::min_parallel $nthreads
486
487 Set the minimum number of AIO threads to C<$nthreads>. The current
488 default is C<8>, which means eight asynchronous operations can execute
489 concurrently at any one time (the number of outstanding requests,
490 however, is unlimited).
491
492 BDB starts threads only on demand, when an AIO request is queued and
493 no free thread exists. Please note that queueing up a hundred requests can
494 create demand for a hundred threads, even if it turns out that everything
495 is in the cache and could have been processed faster by a single thread.
496
497 It is recommended to keep the number of threads relatively low, as some
498 Linux kernel versions will scale negatively with the number of threads
499 (higher parallelity => MUCH higher latency). With current Linux 2.6
500 versions, 4-32 threads should be fine.
501
502 Under most circumstances you don't need to call this function, as the
503 module selects a default that is suitable for low to moderate load.
504
505 =item BDB::max_parallel $nthreads
506
507 Sets the maximum number of AIO threads to C<$nthreads>. If more than the
508 specified number of threads are currently running, this function kills
509 them. This function blocks until the limit is reached.
510
511 While C<$nthreads> are zero, aio requests get queued but not executed
512 until the number of threads has been increased again.
513
514 This module automatically runs C<max_parallel 0> at program end, to ensure
515 that all threads are killed and that there are no outstanding requests.
516
517 Under normal circumstances you don't need to call this function.
518
519 =item BDB::max_idle $nthreads
520
521 Limit the number of threads (default: 4) that are allowed to idle (i.e.,
522 threads that did not get a request to process within 10 seconds). That
523 means if a thread becomes idle while C<$nthreads> other threads are also
524 idle, it will free its resources and exit.
525
526 This is useful when you allow a large number of threads (e.g. 100 or 1000)
527 to allow for extremely high load situations, but want to free resources
528 under normal circumstances (1000 threads can easily consume 30MB of RAM).
529
530 The default is probably ok in most situations, especially if thread
531 creation is fast. If thread creation is very slow on your system you might
532 want to use larger values.
533
534 =item $oldmaxreqs = BDB::max_outstanding $maxreqs
535
536 This is a very bad function to use in interactive programs because it
537 blocks, and a bad way to reduce concurrency because it is inexact: Better
538 use an C<aio_group> together with a feed callback.
539
540 Sets the maximum number of outstanding requests to C<$nreqs>. If you
541 to queue up more than this number of requests, the next call to the
542 C<poll_cb> (and C<poll_some> and other functions calling C<poll_cb>)
543 function will block until the limit is no longer exceeded.
544
545 The default value is very large, so there is no practical limit on the
546 number of outstanding requests.
547
548 You can still queue as many requests as you want. Therefore,
549 C<max_oustsanding> is mainly useful in simple scripts (with low values) or
550 as a stop gap to shield against fatal memory overflow (with large values).
551
552 =item BDB::set_sync_prepare $cb
553
554 Sets a callback that is called whenever a request is created without an
555 explicit callback. It has to return two code references. The first is used
556 as the request callback, and the second is called to wait until the first
557 callback has been called. The default implementation works like this:
558
559 sub {
560 my $status;
561 (
562 sub { $status = $! },
563 sub { BDB::poll while !defined $status; $! = $status },
564 )
565 }
566
567 =back
568
569 =head3 STATISTICAL INFORMATION
570
571 =over 4
572
573 =item BDB::nreqs
574
575 Returns the number of requests currently in the ready, execute or pending
576 states (i.e. for which their callback has not been invoked yet).
577
578 Example: wait till there are no outstanding requests anymore:
579
580 BDB::poll_wait, BDB::poll_cb
581 while BDB::nreqs;
582
583 =item BDB::nready
584
585 Returns the number of requests currently in the ready state (not yet
586 executed).
587
588 =item BDB::npending
589
590 Returns the number of requests currently in the pending state (executed,
591 but not yet processed by poll_cb).
592
593 =back
594
595 =cut
596
597 set_sync_prepare {
598 my $status;
599 (
600 sub {
601 $status = $!;
602 },
603 sub {
604 BDB::poll while !defined $status;
605 $! = $status;
606 },
607 )
608 };
609
610 min_parallel 8;
611
612 END { flush }
613
614 1;
615
616 =head2 FORK BEHAVIOUR
617
618 This module should do "the right thing" when the process using it forks:
619
620 Before the fork, IO::AIO enters a quiescent state where no requests
621 can be added in other threads and no results will be processed. After
622 the fork the parent simply leaves the quiescent state and continues
623 request/result processing, while the child frees the request/result queue
624 (so that the requests started before the fork will only be handled in the
625 parent). Threads will be started on demand until the limit set in the
626 parent process has been reached again.
627
628 In short: the parent will, after a short pause, continue as if fork had
629 not been called, while the child will act as if IO::AIO has not been used
630 yet.
631
632 =head2 MEMORY USAGE
633
634 Per-request usage:
635
636 Each aio request uses - depending on your architecture - around 100-200
637 bytes of memory. In addition, stat requests need a stat buffer (possibly
638 a few hundred bytes), readdir requires a result buffer and so on. Perl
639 scalars and other data passed into aio requests will also be locked and
640 will consume memory till the request has entered the done state.
641
642 This is not awfully much, so queuing lots of requests is not usually a
643 problem.
644
645 Per-thread usage:
646
647 In the execution phase, some aio requests require more memory for
648 temporary buffers, and each thread requires a stack and other data
649 structures (usually around 16k-128k, depending on the OS).
650
651 =head1 KNOWN BUGS
652
653 Known bugs will be fixed in the next release, except:
654
655 If you use a transaction in any request, and the request returns
656 with an operating system error or DB_LOCK_NOTGRANTED, the internal
657 TXN_DEADLOCK flag will be set on the transaction. See C<db_txn_finish>,
658 above.
659
660 =head1 SEE ALSO
661
662 L<Coro::AIO>.
663
664 =head1 AUTHOR
665
666 Marc Lehmann <schmorp@schmorp.de>
667 http://home.schmorp.de/
668
669 =cut
670