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Revision: 1.15
Committed: Thu Sep 13 21:34:00 2007 UTC (16 years, 8 months ago) by root
Branch: MAIN
Changes since 1.14: +48 -5 lines
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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 1.10 See the BerkeleyDB documentation (L<http://www.oracle.com/technology/documentation/berkeley-db/db/index.html>).
12 root 1.12 The BDB API is very similar to the C API (the translation has been very faithful).
13 root 1.10
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 root 1.7
21 root 1.1
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 root 1.2 package BDB;
70 root 1.1
71     no warnings;
72     use strict 'vars';
73    
74     use base 'Exporter';
75    
76     BEGIN {
77 root 1.15 our $VERSION = '1.1';
78 root 1.1
79 root 1.3 our @BDB_REQ = qw(
80 root 1.6 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 root 1.15 db_txn_commit db_txn_abort db_txn_finish
84 root 1.5 db_c_close db_c_count db_c_put db_c_get db_c_pget db_c_del
85 root 1.6 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 root 1.3 );
95 root 1.1
96     require XSLoader;
97 root 1.2 XSLoader::load ("BDB", $VERSION);
98 root 1.1 }
99    
100 root 1.10 =head2 BERKELEYDB FUNCTIONS
101    
102     All of these are functions. The create functions simply return a new
103     object and never block. All the remaining functions all take an optional
104     callback as last argument. If it is missing, then the fucntion will be
105     executed synchronously.
106    
107     BDB functions that cannot block (mostly functions that manipulate
108     settings) are method calls on the relevant objects, so the rule of thumb
109     is: if its a method, its not blocking, if its a function, it takes a
110     callback as last argument.
111    
112     In the following, C<$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 root 1.11 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 root 1.10
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 root 1.14 flags: RPCCLIENT
132 root 1.10
133     db_env_open (DB_ENV *env, octetstring db_home, U32 open_flags, int mode, SV *callback = &PL_sv_undef)
134 root 1.14 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 root 1.10 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 root 1.14 flags: FORCE
138 root 1.10 db_env_lock_detect (DB_ENV *env, U32 flags = 0, U32 atype = DB_LOCK_DEFAULT, SV *dummy = 0, SV *callback = &PL_sv_undef)
139 root 1.14 atype: LOCK_DEFAULT LOCK_EXPIRE LOCK_MAXLOCKS LOCK_MAXWRITE LOCK_MINLOCKS LOCK_MINWRITE LOCK_OLDEST LOCK_RANDOM LOCK_YOUNGEST
140 root 1.10 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 root 1.14 flags: XA_CREATE
145 root 1.10
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 root 1.14 flags: AUTO_COMMIT CREATE EXCL MULTIVERSION NOMMAP RDONLY READ_UNCOMMITTED THREAD TRUNCATE
148 root 1.10 db_close (DB *db, U32 flags = 0, SV *callback = &PL_sv_undef)
149 root 1.14 flags: DB_NOSYNC
150 root 1.12 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 root 1.14 flags: FREELIST_ONLY FREE_SPACE
152 root 1.10 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 root 1.14 flags: APPEND NODUPDATA NOOVERWRITE
156 root 1.10 db_get (DB *db, DB_TXN_ornull *txn, SV *key, SV *data, U32 flags = 0, SV *callback = &PL_sv_undef)
157 root 1.14 flags: CONSUME CONSUME_WAIT GET_BOTH SET_RECNO MULTIPLE READ_COMMITTED READ_UNCOMMITTED RMW
158 root 1.10 db_pget (DB *db, DB_TXN_ornull *txn, SV *key, SV *pkey, SV *data, U32 flags = 0, SV *callback = &PL_sv_undef)
159 root 1.14 flags: CONSUME CONSUME_WAIT GET_BOTH SET_RECNO MULTIPLE READ_COMMITTED READ_UNCOMMITTED RMW
160 root 1.10 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 root 1.14 flags: TXN_NOSYNC TXN_SYNC
163 root 1.10 db_txn_abort (DB_TXN *txn, SV *callback = &PL_sv_undef)
164 root 1.14
165 root 1.10 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 root 1.14 flags: AFTER BEFORE CURRENT KEYFIRST KEYLAST NODUPDATA
169 root 1.10 db_c_get (DBC *dbc, SV *key, SV *data, U32 flags = 0, SV *callback = &PL_sv_undef)
170 root 1.14 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 root 1.10 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 root 1.14 flags: CREATE EXCL
176 root 1.10 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 root 1.14 flags: TXN_NOSYNC
179 root 1.10 db_sequence_remove (DB_SEQUENCE *seq, DB_TXN_ornull *txnid = 0, U32 flags = 0, SV *callback = &PL_sv_undef)
180 root 1.14 flags: TXN_NOSYNC
181 root 1.10
182 root 1.15 =head4 db_txn_finish (DB_TXN *txn, U32 flags = 0, SV *callback = &PL_sv_undef)
183    
184     This is not a Berkeley DB function but a BDB module extension. It is very
185     annoying to have to check every single BDB function for error returns and
186     provide a codepath out of your transaction. While the BDB module still
187     makes this possible, it contains the following extensions:
188    
189     When a transaction-protected function returns any operating system
190     error (errno > 0), BDB will set the C<TXN_DEADLOCK> flag on the
191     transaction. This flag is also set by Berkeley DB functions externally
192     when an operation fails with LOCK_DEADLOCK, and it causes all further
193     operations on that transaction (including C<db_txn_commit>) to fail.
194    
195     The C<db_txn_finish> request will look at this flag, and, if it is set,
196     will automatically call C<db_txn_abort> (setting errno to C<LOCK_DEADLOCK>
197     if it isn't set). If it isn't set, it will call C<db_txn_commit> and
198     return the error normally.
199    
200     How to use this? Easy: just write your transaction normally:
201    
202     my $txn = $db_env->txn_begin;
203     db_get $db, $txn, "key", my $data;
204     db_put $db, $txn, "key", $data + 1 unless $! == BDB::NOTFOUND;
205     db_txn_finish $txn;
206     die "transaction failed" if $!;
207    
208     That is, handle only the expected errors. If something unexpected happens
209     (EIO, LOCK_NOTGRANTED or a deadlock in either db_get or db_put), then the remaining
210     requests (db_put in this case) will simply be skipped (they will fail with
211     LOCK_DEADLOCK) and the transaction will be aborted.
212    
213     You cna use the C<< $txn->failed >> method to check wether a transaction
214     has failed in this way and abort further processing (excluding
215     C<db_txn_finish>).
216    
217 root 1.10 =head3 DB_ENV/database environment methods
218    
219     Methods available on DB_ENV/$env handles:
220    
221     DESTROY (DB_ENV_ornull *env)
222     CODE:
223     if (env)
224     env->close (env, 0);
225    
226     $int = $env->set_data_dir (const char *dir)
227     $int = $env->set_tmp_dir (const char *dir)
228     $int = $env->set_lg_dir (const char *dir)
229     $int = $env->set_shm_key (long shm_key)
230     $int = $env->set_cachesize (U32 gbytes, U32 bytes, int ncache = 0)
231     $int = $env->set_flags (U32 flags, int onoff)
232     $env->set_errfile (FILE *errfile = 0)
233     $env->set_msgfile (FILE *msgfile = 0)
234     $int = $env->set_verbose (U32 which, int onoff = 1)
235     $int = $env->set_encrypt (const char *password, U32 flags = 0)
236 root 1.15 $int = $env->set_timeout (NV timeout_seconds, U32 flags = SET_TXN_TIMEOUT)
237 root 1.10 $int = $env->set_mp_max_openfd (int maxopenfd);
238     $int = $env->set_mp_max_write (int maxwrite, int maxwrite_sleep);
239     $int = $env->set_mp_mmapsize (int mmapsize_mb)
240     $int = $env->set_lk_detect (U32 detect = DB_LOCK_DEFAULT)
241     $int = $env->set_lk_max_lockers (U32 max)
242     $int = $env->set_lk_max_locks (U32 max)
243     $int = $env->set_lk_max_objects (U32 max)
244     $int = $env->set_lg_bsize (U32 max)
245     $int = $env->set_lg_max (U32 max)
246    
247     $txn = $env->txn_begin (DB_TXN_ornull *parent = 0, U32 flags = 0)
248 root 1.14 flags: READ_COMMITTED READ_UNCOMMITTED TXN_NOSYNC TXN_NOWAIT TXN_SNAPSHOT TXN_SYNC TXN_WAIT TXN_WRITE_NOSYNC
249 root 1.10
250 root 1.12 =head4 Example:
251 root 1.10
252     use AnyEvent;
253     use BDB;
254    
255     our $FH; open $FH, "<&=" . BDB::poll_fileno;
256     our $WATCHER = AnyEvent->io (fh => $FH, poll => 'r', cb => \&BDB::poll_cb);
257    
258     BDB::min_parallel 8;
259    
260     my $env = db_env_create;
261    
262     mkdir "bdtest", 0700;
263     db_env_open
264     $env,
265     "bdtest",
266     BDB::INIT_LOCK | BDB::INIT_LOG | BDB::INIT_MPOOL | BDB::INIT_TXN | BDB::RECOVER | BDB::USE_ENVIRON | BDB::CREATE,
267     0600;
268    
269     $env->set_flags (BDB::AUTO_COMMIT | BDB::TXN_NOSYNC, 1);
270    
271    
272     =head3 DB/database methods
273    
274     Methods available on DB/$db handles:
275    
276     DESTROY (DB_ornull *db)
277     CODE:
278     if (db)
279     {
280     SV *env = (SV *)db->app_private;
281     db->close (db, 0);
282     SvREFCNT_dec (env);
283     }
284    
285     $int = $db->set_cachesize (U32 gbytes, U32 bytes, int ncache = 0)
286     $int = $db->set_flags (U32 flags)
287 root 1.14 flags: CHKSUM ENCRYPT TXN_NOT_DURABLE
288     Btree: DUP DUPSORT RECNUM REVSPLITOFF
289     Hash: DUP DUPSORT
290     Queue: INORDER
291     Recno: RENUMBER SNAPSHOT
292    
293 root 1.10 $int = $db->set_encrypt (const char *password, U32 flags)
294     $int = $db->set_lorder (int lorder)
295     $int = $db->set_bt_minkey (U32 minkey)
296     $int = $db->set_re_delim (int delim)
297     $int = $db->set_re_pad (int re_pad)
298     $int = $db->set_re_source (char *source)
299     $int = $db->set_re_len (U32 re_len)
300     $int = $db->set_h_ffactor (U32 h_ffactor)
301     $int = $db->set_h_nelem (U32 h_nelem)
302     $int = $db->set_q_extentsize (U32 extentsize)
303    
304     $dbc = $db->cursor (DB_TXN_ornull *txn = 0, U32 flags = 0)
305 root 1.14 flags: READ_COMMITTED READ_UNCOMMITTED WRITECURSOR TXN_SNAPSHOT
306 root 1.10 $seq = $db->sequence (U32 flags = 0)
307    
308 root 1.12 =head4 Example:
309 root 1.10
310     my $db = db_create $env;
311     db_open $db, undef, "table", undef, BDB::BTREE, BDB::AUTO_COMMIT | BDB::CREATE | BDB::READ_UNCOMMITTED, 0600;
312    
313     for (1..1000) {
314     db_put $db, undef, "key $_", "data $_";
315    
316     db_key_range $db, undef, "key $_", my $keyrange;
317     my ($lt, $eq, $gt) = @$keyrange;
318     }
319    
320     db_del $db, undef, "key $_" for 1..1000;
321    
322     db_sync $db;
323    
324    
325     =head3 DB_TXN/transaction methods
326    
327     Methods available on DB_TXN/$txn handles:
328    
329     DESTROY (DB_TXN_ornull *txn)
330     CODE:
331     if (txn)
332     txn->abort (txn);
333    
334 root 1.15 $int = $txn->set_timeout (NV timeout_seconds, U32 flags = SET_TXN_TIMEOUT)
335 root 1.14 flags: SET_LOCK_TIMEOUT SET_TXN_TIMEOUT
336 root 1.10
337 root 1.15 $bool = $txn->failed
338     # see db_txn_finish documentation, above
339    
340 root 1.10
341     =head3 DBC/cursor methods
342    
343     Methods available on DBC/$dbc handles:
344    
345     DESTROY (DBC_ornull *dbc)
346     CODE:
347     if (dbc)
348     dbc->c_close (dbc);
349    
350 root 1.12 =head4 Example:
351 root 1.10
352     my $c = $db->cursor;
353    
354     for (;;) {
355     db_c_get $c, my $key, my $data, BDB::NEXT;
356     warn "<$!,$key,$data>";
357     last if $!;
358     }
359    
360     db_c_close $c;
361    
362 root 1.12
363 root 1.10 =head3 DB_SEQUENCE/sequence methods
364    
365     Methods available on DB_SEQUENCE/$seq handles:
366    
367     DESTROY (DB_SEQUENCE_ornull *seq)
368     CODE:
369     if (seq)
370     seq->close (seq, 0);
371    
372     $int = $seq->initial_value (db_seq_t value)
373     $int = $seq->set_cachesize (U32 size)
374     $int = $seq->set_flags (U32 flags)
375 root 1.14 flags: SEQ_DEC SEQ_INC SEQ_WRAP
376 root 1.10 $int = $seq->set_range (db_seq_t min, db_seq_t max)
377    
378 root 1.12 =head4 Example:
379 root 1.10
380     my $seq = $db->sequence;
381    
382     db_sequence_open $seq, undef, "seq", BDB::CREATE;
383     db_sequence_get $seq, undef, 1, my $value;
384    
385    
386 root 1.1 =head2 SUPPORT FUNCTIONS
387    
388     =head3 EVENT PROCESSING AND EVENT LOOP INTEGRATION
389    
390     =over 4
391    
392 root 1.2 =item $fileno = BDB::poll_fileno
393 root 1.1
394     Return the I<request result pipe file descriptor>. This filehandle must be
395     polled for reading by some mechanism outside this module (e.g. Event or
396     select, see below or the SYNOPSIS). If the pipe becomes readable you have
397     to call C<poll_cb> to check the results.
398    
399     See C<poll_cb> for an example.
400    
401 root 1.2 =item BDB::poll_cb
402 root 1.1
403     Process some outstanding events on the result pipe. You have to call this
404     regularly. Returns the number of events processed. Returns immediately
405     when no events are outstanding. The amount of events processed depends on
406 root 1.2 the settings of C<BDB::max_poll_req> and C<BDB::max_poll_time>.
407 root 1.1
408     If not all requests were processed for whatever reason, the filehandle
409     will still be ready when C<poll_cb> returns.
410    
411     Example: Install an Event watcher that automatically calls
412 root 1.2 BDB::poll_cb with high priority:
413 root 1.1
414 root 1.2 Event->io (fd => BDB::poll_fileno,
415 root 1.1 poll => 'r', async => 1,
416 root 1.2 cb => \&BDB::poll_cb);
417 root 1.1
418 root 1.2 =item BDB::max_poll_reqs $nreqs
419 root 1.1
420 root 1.2 =item BDB::max_poll_time $seconds
421 root 1.1
422     These set the maximum number of requests (default C<0>, meaning infinity)
423 root 1.2 that are being processed by C<BDB::poll_cb> in one call, respectively
424 root 1.1 the maximum amount of time (default C<0>, meaning infinity) spent in
425 root 1.2 C<BDB::poll_cb> to process requests (more correctly the mininum amount
426 root 1.1 of time C<poll_cb> is allowed to use).
427    
428     Setting C<max_poll_time> to a non-zero value creates an overhead of one
429     syscall per request processed, which is not normally a problem unless your
430     callbacks are really really fast or your OS is really really slow (I am
431     not mentioning Solaris here). Using C<max_poll_reqs> incurs no overhead.
432    
433     Setting these is useful if you want to ensure some level of
434     interactiveness when perl is not fast enough to process all requests in
435     time.
436    
437     For interactive programs, values such as C<0.01> to C<0.1> should be fine.
438    
439     Example: Install an Event watcher that automatically calls
440 root 1.2 BDB::poll_cb with low priority, to ensure that other parts of the
441 root 1.1 program get the CPU sometimes even under high AIO load.
442    
443     # try not to spend much more than 0.1s in poll_cb
444 root 1.2 BDB::max_poll_time 0.1;
445 root 1.1
446     # use a low priority so other tasks have priority
447 root 1.2 Event->io (fd => BDB::poll_fileno,
448 root 1.1 poll => 'r', nice => 1,
449 root 1.2 cb => &BDB::poll_cb);
450 root 1.1
451 root 1.2 =item BDB::poll_wait
452 root 1.1
453     If there are any outstanding requests and none of them in the result
454     phase, wait till the result filehandle becomes ready for reading (simply
455     does a C<select> on the filehandle. This is useful if you want to
456     synchronously wait for some requests to finish).
457    
458     See C<nreqs> for an example.
459    
460 root 1.2 =item BDB::poll
461 root 1.1
462     Waits until some requests have been handled.
463    
464     Returns the number of requests processed, but is otherwise strictly
465     equivalent to:
466    
467 root 1.2 BDB::poll_wait, BDB::poll_cb
468 root 1.1
469 root 1.2 =item BDB::flush
470 root 1.1
471     Wait till all outstanding AIO requests have been handled.
472    
473     Strictly equivalent to:
474    
475 root 1.2 BDB::poll_wait, BDB::poll_cb
476     while BDB::nreqs;
477 root 1.1
478 root 1.8 =back
479    
480 root 1.1 =head3 CONTROLLING THE NUMBER OF THREADS
481    
482 root 1.8 =over 4
483    
484 root 1.2 =item BDB::min_parallel $nthreads
485 root 1.1
486     Set the minimum number of AIO threads to C<$nthreads>. The current
487     default is C<8>, which means eight asynchronous operations can execute
488     concurrently at any one time (the number of outstanding requests,
489     however, is unlimited).
490    
491 root 1.2 BDB starts threads only on demand, when an AIO request is queued and
492 root 1.1 no free thread exists. Please note that queueing up a hundred requests can
493     create demand for a hundred threads, even if it turns out that everything
494     is in the cache and could have been processed faster by a single thread.
495    
496     It is recommended to keep the number of threads relatively low, as some
497     Linux kernel versions will scale negatively with the number of threads
498     (higher parallelity => MUCH higher latency). With current Linux 2.6
499     versions, 4-32 threads should be fine.
500    
501     Under most circumstances you don't need to call this function, as the
502     module selects a default that is suitable for low to moderate load.
503    
504 root 1.2 =item BDB::max_parallel $nthreads
505 root 1.1
506     Sets the maximum number of AIO threads to C<$nthreads>. If more than the
507     specified number of threads are currently running, this function kills
508     them. This function blocks until the limit is reached.
509    
510     While C<$nthreads> are zero, aio requests get queued but not executed
511     until the number of threads has been increased again.
512    
513     This module automatically runs C<max_parallel 0> at program end, to ensure
514     that all threads are killed and that there are no outstanding requests.
515    
516     Under normal circumstances you don't need to call this function.
517    
518 root 1.2 =item BDB::max_idle $nthreads
519 root 1.1
520     Limit the number of threads (default: 4) that are allowed to idle (i.e.,
521     threads that did not get a request to process within 10 seconds). That
522     means if a thread becomes idle while C<$nthreads> other threads are also
523     idle, it will free its resources and exit.
524    
525     This is useful when you allow a large number of threads (e.g. 100 or 1000)
526     to allow for extremely high load situations, but want to free resources
527     under normal circumstances (1000 threads can easily consume 30MB of RAM).
528    
529     The default is probably ok in most situations, especially if thread
530     creation is fast. If thread creation is very slow on your system you might
531     want to use larger values.
532    
533 root 1.2 =item $oldmaxreqs = BDB::max_outstanding $maxreqs
534 root 1.1
535     This is a very bad function to use in interactive programs because it
536     blocks, and a bad way to reduce concurrency because it is inexact: Better
537     use an C<aio_group> together with a feed callback.
538    
539     Sets the maximum number of outstanding requests to C<$nreqs>. If you
540     to queue up more than this number of requests, the next call to the
541     C<poll_cb> (and C<poll_some> and other functions calling C<poll_cb>)
542     function will block until the limit is no longer exceeded.
543    
544     The default value is very large, so there is no practical limit on the
545     number of outstanding requests.
546    
547     You can still queue as many requests as you want. Therefore,
548     C<max_oustsanding> is mainly useful in simple scripts (with low values) or
549     as a stop gap to shield against fatal memory overflow (with large values).
550    
551 root 1.3 =item BDB::set_sync_prepare $cb
552    
553     Sets a callback that is called whenever a request is created without an
554     explicit callback. It has to return two code references. The first is used
555     as the request callback, and the second is called to wait until the first
556     callback has been called. The default implementation works like this:
557    
558     sub {
559     my $status;
560     (
561     sub { $status = $! },
562     sub { BDB::poll while !defined $status; $! = $status },
563     )
564     }
565    
566     =back
567    
568 root 1.1 =head3 STATISTICAL INFORMATION
569    
570 root 1.3 =over 4
571    
572 root 1.2 =item BDB::nreqs
573 root 1.1
574     Returns the number of requests currently in the ready, execute or pending
575     states (i.e. for which their callback has not been invoked yet).
576    
577     Example: wait till there are no outstanding requests anymore:
578    
579 root 1.2 BDB::poll_wait, BDB::poll_cb
580     while BDB::nreqs;
581 root 1.1
582 root 1.2 =item BDB::nready
583 root 1.1
584     Returns the number of requests currently in the ready state (not yet
585     executed).
586    
587 root 1.2 =item BDB::npending
588 root 1.1
589     Returns the number of requests currently in the pending state (executed,
590     but not yet processed by poll_cb).
591    
592     =back
593    
594     =cut
595    
596 root 1.3 set_sync_prepare {
597     my $status;
598     (
599     sub {
600     $status = $!;
601     },
602     sub {
603     BDB::poll while !defined $status;
604     $! = $status;
605     },
606     )
607     };
608    
609 root 1.1 min_parallel 8;
610    
611     END { flush }
612    
613     1;
614    
615     =head2 FORK BEHAVIOUR
616    
617     This module should do "the right thing" when the process using it forks:
618    
619     Before the fork, IO::AIO enters a quiescent state where no requests
620     can be added in other threads and no results will be processed. After
621     the fork the parent simply leaves the quiescent state and continues
622     request/result processing, while the child frees the request/result queue
623     (so that the requests started before the fork will only be handled in the
624     parent). Threads will be started on demand until the limit set in the
625     parent process has been reached again.
626    
627     In short: the parent will, after a short pause, continue as if fork had
628     not been called, while the child will act as if IO::AIO has not been used
629     yet.
630    
631     =head2 MEMORY USAGE
632    
633     Per-request usage:
634    
635     Each aio request uses - depending on your architecture - around 100-200
636     bytes of memory. In addition, stat requests need a stat buffer (possibly
637     a few hundred bytes), readdir requires a result buffer and so on. Perl
638     scalars and other data passed into aio requests will also be locked and
639     will consume memory till the request has entered the done state.
640    
641 root 1.13 This is not awfully much, so queuing lots of requests is not usually a
642 root 1.1 problem.
643    
644     Per-thread usage:
645    
646     In the execution phase, some aio requests require more memory for
647     temporary buffers, and each thread requires a stack and other data
648     structures (usually around 16k-128k, depending on the OS).
649    
650     =head1 KNOWN BUGS
651    
652 root 1.15 Known bugs will be fixed in the next release, except:
653    
654     If you use a transaction in any request, and the request returns
655     with an operating system error or DB_LOCK_NOTGRANTED, the internal
656     TXN_DEADLOCK flag will be set on the transaction. See C<db_txn_finish>,
657     above.
658 root 1.1
659     =head1 SEE ALSO
660    
661     L<Coro::AIO>.
662    
663     =head1 AUTHOR
664    
665     Marc Lehmann <schmorp@schmorp.de>
666     http://home.schmorp.de/
667    
668     =cut
669