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Revision 1.63 by root, Fri Apr 25 02:03:38 2008 UTC vs.
Revision 1.95 by root, Sat Apr 26 11:06:45 2008 UTC

80module. 80module.
81 81
82During the first call of any watcher-creation method, the module tries 82During the first call of any watcher-creation method, the module tries
83to detect the currently loaded event loop by probing whether one of the 83to detect the currently loaded event loop by probing whether one of the
84following modules is already loaded: L<Coro::EV>, L<Coro::Event>, L<EV>, 84following modules is already loaded: L<Coro::EV>, L<Coro::Event>, L<EV>,
85L<Event>, L<Glib>, L<Tk>, L<AnyEvent::Impl::Perl>, L<Event::Lib>, L<Qt>, 85L<Event>, L<Glib>, L<AnyEvent::Impl::Perl>, L<Tk>, L<Event::Lib>, L<Qt>,
86L<POE>. The first one found is used. If none are found, the module tries 86L<POE>. The first one found is used. If none are found, the module tries
87to load these modules (excluding Event::Lib, Qt and POE as the pure perl 87to load these modules (excluding Tk, Event::Lib, Qt and POE as the pure perl
88adaptor should always succeed) in the order given. The first one that can 88adaptor should always succeed) in the order given. The first one that can
89be successfully loaded will be used. If, after this, still none could be 89be successfully loaded will be used. If, after this, still none could be
90found, AnyEvent will fall back to a pure-perl event loop, which is not 90found, AnyEvent will fall back to a pure-perl event loop, which is not
91very efficient, but should work everywhere. 91very efficient, but should work everywhere.
92 92
136 136
137Note that C<my $w; $w => combination. This is necessary because in Perl, 137Note that C<my $w; $w => combination. This is necessary because in Perl,
138my variables are only visible after the statement in which they are 138my variables are only visible after the statement in which they are
139declared. 139declared.
140 140
141=head2 IO WATCHERS 141=head2 I/O WATCHERS
142 142
143You can create an I/O watcher by calling the C<< AnyEvent->io >> method 143You can create an I/O watcher by calling the C<< AnyEvent->io >> method
144with the following mandatory key-value pairs as arguments: 144with the following mandatory key-value pairs as arguments:
145 145
146C<fh> the Perl I<file handle> (I<not> file descriptor) to watch for 146C<fh> the Perl I<file handle> (I<not> file descriptor) to watch
147events. C<poll> must be a string that is either C<r> or C<w>, which 147for events. C<poll> must be a string that is either C<r> or C<w>,
148creates a watcher waiting for "r"eadable or "w"ritable events, 148which creates a watcher waiting for "r"eadable or "w"ritable events,
149respectively. C<cb> is the callback to invoke each time the file handle 149respectively. C<cb> is the callback to invoke each time the file handle
150becomes ready. 150becomes ready.
151 151
152As long as the I/O watcher exists it will keep the file descriptor or a 152Although the callback might get passed parameters, their value and
153copy of it alive/open. 153presence is undefined and you cannot rely on them. Portable AnyEvent
154callbacks cannot use arguments passed to I/O watcher callbacks.
154 155
156The I/O watcher might use the underlying file descriptor or a copy of it.
155It is not allowed to close a file handle as long as any watcher is active 157You must not close a file handle as long as any watcher is active on the
156on the underlying file descriptor. 158underlying file descriptor.
157 159
158Some event loops issue spurious readyness notifications, so you should 160Some event loops issue spurious readyness notifications, so you should
159always use non-blocking calls when reading/writing from/to your file 161always use non-blocking calls when reading/writing from/to your file
160handles. 162handles.
161 163
172 174
173You can create a time watcher by calling the C<< AnyEvent->timer >> 175You can create a time watcher by calling the C<< AnyEvent->timer >>
174method with the following mandatory arguments: 176method with the following mandatory arguments:
175 177
176C<after> specifies after how many seconds (fractional values are 178C<after> specifies after how many seconds (fractional values are
177supported) should the timer activate. C<cb> the callback to invoke in that 179supported) the callback should be invoked. C<cb> is the callback to invoke
178case. 180in that case.
181
182Although the callback might get passed parameters, their value and
183presence is undefined and you cannot rely on them. Portable AnyEvent
184callbacks cannot use arguments passed to time watcher callbacks.
179 185
180The timer callback will be invoked at most once: if you want a repeating 186The timer callback will be invoked at most once: if you want a repeating
181timer you have to create a new watcher (this is a limitation by both Tk 187timer you have to create a new watcher (this is a limitation by both Tk
182and Glib). 188and Glib).
183 189
228 234
229You can watch for signals using a signal watcher, C<signal> is the signal 235You can watch for signals using a signal watcher, C<signal> is the signal
230I<name> without any C<SIG> prefix, C<cb> is the Perl callback to 236I<name> without any C<SIG> prefix, C<cb> is the Perl callback to
231be invoked whenever a signal occurs. 237be invoked whenever a signal occurs.
232 238
239Although the callback might get passed parameters, their value and
240presence is undefined and you cannot rely on them. Portable AnyEvent
241callbacks cannot use arguments passed to signal watcher callbacks.
242
233Multiple signal occurances can be clumped together into one callback 243Multiple signal occurances can be clumped together into one callback
234invocation, and callback invocation will be synchronous. synchronous means 244invocation, and callback invocation will be synchronous. synchronous means
235that it might take a while until the signal gets handled by the process, 245that it might take a while until the signal gets handled by the process,
236but it is guarenteed not to interrupt any other callbacks. 246but it is guarenteed not to interrupt any other callbacks.
237 247
251 261
252The child process is specified by the C<pid> argument (if set to C<0>, it 262The child process is specified by the C<pid> argument (if set to C<0>, it
253watches for any child process exit). The watcher will trigger as often 263watches for any child process exit). The watcher will trigger as often
254as status change for the child are received. This works by installing a 264as status change for the child are received. This works by installing a
255signal handler for C<SIGCHLD>. The callback will be called with the pid 265signal handler for C<SIGCHLD>. The callback will be called with the pid
256and exit status (as returned by waitpid). 266and exit status (as returned by waitpid), so unlike other watcher types,
267you I<can> rely on child watcher callback arguments.
257 268
258Example: wait for pid 1333 269There is a slight catch to child watchers, however: you usually start them
270I<after> the child process was created, and this means the process could
271have exited already (and no SIGCHLD will be sent anymore).
272
273Not all event models handle this correctly (POE doesn't), but even for
274event models that I<do> handle this correctly, they usually need to be
275loaded before the process exits (i.e. before you fork in the first place).
276
277This means you cannot create a child watcher as the very first thing in an
278AnyEvent program, you I<have> to create at least one watcher before you
279C<fork> the child (alternatively, you can call C<AnyEvent::detect>).
280
281Example: fork a process and wait for it
282
283 my $done = AnyEvent->condvar;
284
285 AnyEvent::detect; # force event module to be initialised
286
287 my $pid = fork or exit 5;
259 288
260 my $w = AnyEvent->child ( 289 my $w = AnyEvent->child (
261 pid => 1333, 290 pid => $pid,
262 cb => sub { 291 cb => sub {
263 my ($pid, $status) = @_; 292 my ($pid, $status) = @_;
264 warn "pid $pid exited with status $status"; 293 warn "pid $pid exited with status $status";
294 $done->broadcast;
265 }, 295 },
266 ); 296 );
297
298 # do something else, then wait for process exit
299 $done->wait;
267 300
268=head2 CONDITION VARIABLES 301=head2 CONDITION VARIABLES
269 302
270Condition variables can be created by calling the C<< AnyEvent->condvar >> 303Condition variables can be created by calling the C<< AnyEvent->condvar >>
271method without any arguments. 304method without any arguments.
359 AnyEvent::Impl::CoroEV based on Coro::EV, best choice. 392 AnyEvent::Impl::CoroEV based on Coro::EV, best choice.
360 AnyEvent::Impl::CoroEvent based on Coro::Event, second best choice. 393 AnyEvent::Impl::CoroEvent based on Coro::Event, second best choice.
361 AnyEvent::Impl::EV based on EV (an interface to libev, best choice). 394 AnyEvent::Impl::EV based on EV (an interface to libev, best choice).
362 AnyEvent::Impl::Event based on Event, second best choice. 395 AnyEvent::Impl::Event based on Event, second best choice.
363 AnyEvent::Impl::Glib based on Glib, third-best choice. 396 AnyEvent::Impl::Glib based on Glib, third-best choice.
397 AnyEvent::Impl::Perl pure-perl implementation, inefficient but portable.
364 AnyEvent::Impl::Tk based on Tk, very bad choice. 398 AnyEvent::Impl::Tk based on Tk, very bad choice.
365 AnyEvent::Impl::Perl pure-perl implementation, inefficient but portable.
366 AnyEvent::Impl::Qt based on Qt, cannot be autoprobed (see its docs). 399 AnyEvent::Impl::Qt based on Qt, cannot be autoprobed (see its docs).
367 AnyEvent::Impl::EventLib based on Event::Lib, leaks memory and worse. 400 AnyEvent::Impl::EventLib based on Event::Lib, leaks memory and worse.
368 AnyEvent::Impl::POE based on POE, not generic enough for full support. 401 AnyEvent::Impl::POE based on POE, not generic enough for full support.
369 402
370There is no support for WxWidgets, as WxWidgets has no support for 403There is no support for WxWidgets, as WxWidgets has no support for
706 739
707=back 740=back
708 741
709=head1 EXAMPLE PROGRAM 742=head1 EXAMPLE PROGRAM
710 743
711The following program uses an IO watcher to read data from STDIN, a timer 744The following program uses an I/O watcher to read data from STDIN, a timer
712to display a message once per second, and a condition variable to quit the 745to display a message once per second, and a condition variable to quit the
713program when the user enters quit: 746program when the user enters quit:
714 747
715 use AnyEvent; 748 use AnyEvent;
716 749
860 $quit->broadcast; 893 $quit->broadcast;
861 }); 894 });
862 895
863 $quit->wait; 896 $quit->wait;
864 897
898
899=head1 BENCHMARKS
900
901To give you an idea of the performance and overheads that AnyEvent adds
902over the event loops themselves and to give you an impression of the speed
903of various event loops I prepared some benchmarks.
904
905=head2 BENCHMARKING ANYEVENT OVERHEAD
906
907Here is a benchmark of various supported event models used natively and
908through anyevent. The benchmark creates a lot of timers (with a zero
909timeout) and I/O watchers (watching STDOUT, a pty, to become writable,
910which it is), lets them fire exactly once and destroys them again.
911
912Source code for this benchmark is found as F<eg/bench> in the AnyEvent
913distribution.
914
915=head3 Explanation of the columns
916
917I<watcher> is the number of event watchers created/destroyed. Since
918different event models feature vastly different performances, each event
919loop was given a number of watchers so that overall runtime is acceptable
920and similar between tested event loop (and keep them from crashing): Glib
921would probably take thousands of years if asked to process the same number
922of watchers as EV in this benchmark.
923
924I<bytes> is the number of bytes (as measured by the resident set size,
925RSS) consumed by each watcher. This method of measuring captures both C
926and Perl-based overheads.
927
928I<create> is the time, in microseconds (millionths of seconds), that it
929takes to create a single watcher. The callback is a closure shared between
930all watchers, to avoid adding memory overhead. That means closure creation
931and memory usage is not included in the figures.
932
933I<invoke> is the time, in microseconds, used to invoke a simple
934callback. The callback simply counts down a Perl variable and after it was
935invoked "watcher" times, it would C<< ->broadcast >> a condvar once to
936signal the end of this phase.
937
938I<destroy> is the time, in microseconds, that it takes to destroy a single
939watcher.
940
941=head3 Results
942
943 name watchers bytes create invoke destroy comment
944 EV/EV 400000 244 0.56 0.46 0.31 EV native interface
945 EV/Any 100000 244 2.50 0.46 0.29 EV + AnyEvent watchers
946 CoroEV/Any 100000 244 2.49 0.44 0.29 coroutines + Coro::Signal
947 Perl/Any 100000 513 4.92 0.87 1.12 pure perl implementation
948 Event/Event 16000 516 31.88 31.30 0.85 Event native interface
949 Event/Any 16000 936 39.17 33.63 1.43 Event + AnyEvent watchers
950 Glib/Any 16000 1357 98.22 12.41 54.00 quadratic behaviour
951 Tk/Any 2000 1860 26.97 67.98 14.00 SEGV with >> 2000 watchers
952 POE/Event 2000 6644 108.64 736.02 14.73 via POE::Loop::Event
953 POE/Select 2000 6343 94.13 809.12 565.96 via POE::Loop::Select
954
955=head3 Discussion
956
957The benchmark does I<not> measure scalability of the event loop very
958well. For example, a select-based event loop (such as the pure perl one)
959can never compete with an event loop that uses epoll when the number of
960file descriptors grows high. In this benchmark, all events become ready at
961the same time, so select/poll-based implementations get an unnatural speed
962boost.
963
964Also, note that the number of watchers usually has a nonlinear effect on
965overall speed, that is, creating twice as many watchers doesn't take twice
966the time - usually it takes longer. This puts event loops tested with a
967higher number of watchers at a disadvantage.
968
969C<EV> is the sole leader regarding speed and memory use, which are both
970maximal/minimal, respectively. Even when going through AnyEvent, it uses
971far less memory than any other event loop and is still faster than Event
972natively.
973
974The pure perl implementation is hit in a few sweet spots (both the
975constant timeout and the use of a single fd hit optimisations in the perl
976interpreter and the backend itself). Nevertheless this shows that it
977adds very little overhead in itself. Like any select-based backend its
978performance becomes really bad with lots of file descriptors (and few of
979them active), of course, but this was not subject of this benchmark.
980
981The C<Event> module has a relatively high setup and callback invocation
982cost, but overall scores in on the third place.
983
984C<Glib>'s memory usage is quite a bit higher, but it features a
985faster callback invocation and overall ends up in the same class as
986C<Event>. However, Glib scales extremely badly, doubling the number of
987watchers increases the processing time by more than a factor of four,
988making it completely unusable when using larger numbers of watchers
989(note that only a single file descriptor was used in the benchmark, so
990inefficiencies of C<poll> do not account for this).
991
992The C<Tk> adaptor works relatively well. The fact that it crashes with
993more than 2000 watchers is a big setback, however, as correctness takes
994precedence over speed. Nevertheless, its performance is surprising, as the
995file descriptor is dup()ed for each watcher. This shows that the dup()
996employed by some adaptors is not a big performance issue (it does incur a
997hidden memory cost inside the kernel which is not reflected in the figures
998above).
999
1000C<POE>, regardless of underlying event loop (whether using its pure
1001perl select-based backend or the Event module, the POE-EV backend
1002couldn't be tested because it wasn't working) shows abysmal performance
1003and memory usage: Watchers use almost 30 times as much memory as
1004EV watchers, and 10 times as much memory as Event (the high memory
1005requirements are caused by requiring a session for each watcher). Watcher
1006invocation speed is almost 900 times slower than with AnyEvent's pure perl
1007implementation. The design of the POE adaptor class in AnyEvent can not
1008really account for this, as session creation overhead is small compared
1009to execution of the state machine, which is coded pretty optimally within
1010L<AnyEvent::Impl::POE>. POE simply seems to be abysmally slow.
1011
1012=head3 Summary
1013
1014=over 4
1015
1016=item * Using EV through AnyEvent is faster than any other event loop
1017(even when used without AnyEvent), but most event loops have acceptable
1018performance with or without AnyEvent.
1019
1020=item * The overhead AnyEvent adds is usually much smaller than the overhead of
1021the actual event loop, only with extremely fast event loops such as EV
1022adds AnyEvent significant overhead.
1023
1024=item * You should avoid POE like the plague if you want performance or
1025reasonable memory usage.
1026
1027=back
1028
1029=head2 BENCHMARKING THE LARGE SERVER CASE
1030
1031This benchmark atcually benchmarks the event loop itself. It works by
1032creating a number of "servers": each server consists of a socketpair, a
1033timeout watcher that gets reset on activity (but never fires), and an I/O
1034watcher waiting for input on one side of the socket. Each time the socket
1035watcher reads a byte it will write that byte to a random other "server".
1036
1037The effect is that there will be a lot of I/O watchers, only part of which
1038are active at any one point (so there is a constant number of active
1039fds for each loop iterstaion, but which fds these are is random). The
1040timeout is reset each time something is read because that reflects how
1041most timeouts work (and puts extra pressure on the event loops).
1042
1043In this benchmark, we use 10000 socketpairs (20000 sockets), of which 100
1044(1%) are active. This mirrors the activity of large servers with many
1045connections, most of which are idle at any one point in time.
1046
1047Source code for this benchmark is found as F<eg/bench2> in the AnyEvent
1048distribution.
1049
1050=head3 Explanation of the columns
1051
1052I<sockets> is the number of sockets, and twice the number of "servers" (as
1053each server has a read and write socket end).
1054
1055I<create> is the time it takes to create a socketpair (which is
1056nontrivial) and two watchers: an I/O watcher and a timeout watcher.
1057
1058I<request>, the most important value, is the time it takes to handle a
1059single "request", that is, reading the token from the pipe and forwarding
1060it to another server. This includes deleting the old timeout and creating
1061a new one that moves the timeout into the future.
1062
1063=head3 Results
1064
1065 name sockets create request
1066 EV 20000 69.01 11.16
1067 Perl 20000 75.28 112.76
1068 Event 20000 212.62 257.32
1069 Glib 20000 651.16 1896.30
1070 POE 20000 349.67 12317.24 uses POE::Loop::Event
1071
1072=head3 Discussion
1073
1074This benchmark I<does> measure scalability and overall performance of the
1075particular event loop.
1076
1077EV is again fastest. Since it is using epoll on my system, the setup time
1078is relatively high, though.
1079
1080Perl surprisingly comes second. It is much faster than the C-based event
1081loops Event and Glib.
1082
1083Event suffers from high setup time as well (look at its code and you will
1084understand why). Callback invocation also has a high overhead compared to
1085the C<< $_->() for .. >>-style loop that the Perl event loop uses. Event
1086uses select or poll in basically all documented configurations.
1087
1088Glib is hit hard by its quadratic behaviour w.r.t. many watchers. It
1089clearly fails to perform with many filehandles or in busy servers.
1090
1091POE is still completely out of the picture, taking over 1000 times as long
1092as EV, and over 100 times as long as the Perl implementation, even though
1093it uses a C-based event loop in this case.
1094
1095=head3 Summary
1096
1097=over 4
1098
1099=item * The pure perl implementation performs extremely well, considering
1100that it uses select.
1101
1102=item * Avoid Glib or POE in large projects where performance matters.
1103
1104=back
1105
1106=head2 BENCHMARKING SMALL SERVERS
1107
1108While event loops should scale (and select-based ones do not...) even to
1109large servers, most programs we (or I :) actually write have only a few
1110I/O watchers.
1111
1112In this benchmark, I use the same benchmark program as in the large server
1113case, but it uses only eight "servers", of which three are active at any
1114one time. This should reflect performance for a small server relatively
1115well.
1116
1117The columns are identical to the previous table.
1118
1119=head3 Results
1120
1121 name sockets create request
1122 EV 16 20.00 6.54
1123 Event 16 81.27 35.86
1124 Glib 16 32.63 15.48
1125 Perl 16 24.62 162.37
1126 POE 16 261.87 276.28 uses POE::Loop::Event
1127
1128=head3 Discussion
1129
1130The benchmark tries to test the performance of a typical small
1131server. While knowing how various event loops perform is interesting, keep
1132in mind that their overhead in this case is usually not as important, due
1133to the small absolute number of watchers.
1134
1135EV is again fastest.
1136
1137The C-based event loops Event and Glib come in second this time, as the
1138overhead of running an iteration is much smaller in C than in Perl (little
1139code to execute in the inner loop, and perl's function calling overhead is
1140high, and updating all the data structures is costly).
1141
1142The pure perl event loop is much slower, but still competitive.
1143
1144POE also performs much better in this case, but is is stillf ar behind the
1145others.
1146
1147=head3 Summary
1148
1149=over 4
1150
1151=item * C-based event loops perform very well with small number of
1152watchers, as the management overhead dominates.
1153
1154=back
1155
1156
865=head1 FORK 1157=head1 FORK
866 1158
867Most event libraries are not fork-safe. The ones who are usually are 1159Most event libraries are not fork-safe. The ones who are usually are
868because they are so inefficient. Only L<EV> is fully fork-aware. 1160because they are so inefficient. Only L<EV> is fully fork-aware.
869 1161
870If you have to fork, you must either do so I<before> creating your first 1162If you have to fork, you must either do so I<before> creating your first
871watcher OR you must not use AnyEvent at all in the child. 1163watcher OR you must not use AnyEvent at all in the child.
1164
872 1165
873=head1 SECURITY CONSIDERATIONS 1166=head1 SECURITY CONSIDERATIONS
874 1167
875AnyEvent can be forced to load any event model via 1168AnyEvent can be forced to load any event model via
876$ENV{PERL_ANYEVENT_MODEL}. While this cannot (to my knowledge) be used to 1169$ENV{PERL_ANYEVENT_MODEL}. While this cannot (to my knowledge) be used to
884 1177
885 BEGIN { delete $ENV{PERL_ANYEVENT_MODEL} } 1178 BEGIN { delete $ENV{PERL_ANYEVENT_MODEL} }
886 1179
887 use AnyEvent; 1180 use AnyEvent;
888 1181
1182
889=head1 SEE ALSO 1183=head1 SEE ALSO
890 1184
891Event modules: L<Coro::EV>, L<EV>, L<EV::Glib>, L<Glib::EV>, 1185Event modules: L<Coro::EV>, L<EV>, L<EV::Glib>, L<Glib::EV>,
892L<Coro::Event>, L<Event>, L<Glib::Event>, L<Glib>, L<Coro>, L<Tk>, 1186L<Coro::Event>, L<Event>, L<Glib::Event>, L<Glib>, L<Coro>, L<Tk>,
893L<Event::Lib>, L<Qt>, L<POE>. 1187L<Event::Lib>, L<Qt>, L<POE>.
897L<AnyEvent::Impl::Tk>, L<AnyEvent::Impl::Perl>, L<AnyEvent::Impl::EventLib>, 1191L<AnyEvent::Impl::Tk>, L<AnyEvent::Impl::Perl>, L<AnyEvent::Impl::EventLib>,
898L<AnyEvent::Impl::Qt>, L<AnyEvent::Impl::POE>. 1192L<AnyEvent::Impl::Qt>, L<AnyEvent::Impl::POE>.
899 1193
900Nontrivial usage examples: L<Net::FCP>, L<Net::XMPP2>. 1194Nontrivial usage examples: L<Net::FCP>, L<Net::XMPP2>.
901 1195
1196
902=head1 AUTHOR 1197=head1 AUTHOR
903 1198
904 Marc Lehmann <schmorp@schmorp.de> 1199 Marc Lehmann <schmorp@schmorp.de>
905 http://home.schmorp.de/ 1200 http://home.schmorp.de/
906 1201

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