[ViewVC] Diff of: cvs/libev/ev.pod

Comparing libev/ev.pod (file contents):
Revision 1.98 by root, Sat Dec 22 06:10:25 2007 UTC vs.
Revision 1.108 by root, Mon Dec 24 10:39:21 2007 UTC

 =head1 SYNOPSIS
   #include <ev.h>
-=head1 EXAMPLE PROGRAM
+=head2 EXAMPLE PROGRAM
   #include <ev.h>
   ev_io stdin_watcher;
   ev_timer timeout_watcher;
 You register interest in certain events by registering so-called I<event
 watchers>, which are relatively small C structures you initialise with the
 details of the event, and then hand it over to libev by I<starting> the
 watcher.
-=head1 FEATURES
+=head2 FEATURES
 Libev supports C<select>, C<poll>, the Linux-specific C<epoll>, the
 BSD-specific C<kqueue> and the Solaris-specific event port mechanisms
 for file descriptor events (C<ev_io>), the Linux C<inotify> interface
 (for C<ev_stat>), relative timers (C<ev_timer>), absolute timers
 It also is quite fast (see this
 L<benchmark|http://libev.schmorp.de/bench.html> comparing it to libevent
 for example).
-=head1 CONVENTIONS
+=head2 CONVENTIONS
 Libev is very configurable. In this manual the default configuration will
 be described, which supports multiple event loops. For more info about
 various configuration options please have a look at B<EMBED> section in
 this manual. If libev was configured without support for multiple event
 loops, then all functions taking an initial argument of name C<loop>
 (which is always of type C<struct ev_loop *>) will not have this argument.
-=head1 TIME REPRESENTATION
+=head2 TIME REPRESENTATION
 Libev represents time as a single floating point number, representing the
 (fractional) number of seconds since the (POSIX) epoch (somewhere near
 the beginning of 1970, details are complicated, don't ask). This type is
 called C<ev_tstamp>, which is what you should use too. It usually aliases
 =item C<EVBACKEND_SELECT>  (value 1, portable select backend)
 This is your standard select(2) backend. Not I<completely> standard, as
 libev tries to roll its own fd_set with no limits on the number of fds,
 but if that fails, expect a fairly low limit on the number of fds when
-using this backend. It doesn't scale too well (O(highest_fd)), but its usually
+using this backend. It doesn't scale too well (O(highest_fd)), but its
-the fastest backend for a low number of fds.
+usually the fastest backend for a low number of (low-numbered :) fds.
+To get good performance out of this backend you need a high amount of
+parallelity (most of the file descriptors should be busy). If you are
+writing a server, you should C<accept ()> in a loop to accept as many
+connections as possible during one iteration. You might also want to have
+a look at C<ev_set_io_collect_interval ()> to increase the amount of
+readyness notifications you get per iteration.
 =item C<EVBACKEND_POLL>    (value 2, poll backend, available everywhere except on windows)
-And this is your standard poll(2) backend. It's more complicated than
+And this is your standard poll(2) backend. It's more complicated
-select, but handles sparse fds better and has no artificial limit on the
+than select, but handles sparse fds better and has no artificial
-number of fds you can use (except it will slow down considerably with a
+limit on the number of fds you can use (except it will slow down
-lot of inactive fds). It scales similarly to select, i.e. O(total_fds).
+considerably with a lot of inactive fds). It scales similarly to select,
+i.e. O(total_fds). See the entry for C<EVBACKEND_SELECT>, above, for
+performance tips.
 =item C<EVBACKEND_EPOLL>   (value 4, Linux)
 For few fds, this backend is a bit little slower than poll and select,
 but it scales phenomenally better. While poll and select usually scale
 like O(total_fds) where n is the total number of fds (or the highest fd),
 epoll scales either O(1) or O(active_fds). The epoll design has a number
 of shortcomings, such as silently dropping events in some hard-to-detect
 cases and rewiring a syscall per fd change, no fork support and bad
-support for dup:
+support for dup.
 While stopping, setting and starting an I/O watcher in the same iteration
 will result in some caching, there is still a syscall per such incident
 (because the fd could point to a different file description now), so its
 best to avoid that. Also, C<dup ()>'ed file descriptors might not work
 Please note that epoll sometimes generates spurious notifications, so you
 need to use non-blocking I/O or other means to avoid blocking when no data
 (or space) is available.
+Best performance from this backend is achieved by not unregistering all
+watchers for a file descriptor until it has been closed, if possible, i.e.
+keep at least one watcher active per fd at all times.
+While nominally embeddeble in other event loops, this feature is broken in
+all kernel versions tested so far.
 =item C<EVBACKEND_KQUEUE>  (value 8, most BSD clones)
 Kqueue deserves special mention, as at the time of this writing, it
-was broken on I<all> BSDs (usually it doesn't work with anything but
+was broken on all BSDs except NetBSD (usually it doesn't work reliably
-sockets and pipes, except on Darwin, where of course it's completely
+with anything but sockets and pipes, except on Darwin, where of course
-useless. On NetBSD, it seems to work for all the FD types I tested, so it
-is used by default there). For this reason it's not being "autodetected"
+it's completely useless). For this reason it's not being "autodetected"
 unless you explicitly specify it explicitly in the flags (i.e. using
 C<EVBACKEND_KQUEUE>) or libev was compiled on a known-to-be-good (-enough)
 system like NetBSD.
+You still can embed kqueue into a normal poll or select backend and use it
+only for sockets (after having made sure that sockets work with kqueue on
+the target platform). See C<ev_embed> watchers for more info.
 It scales in the same way as the epoll backend, but the interface to the
-kernel is more efficient (which says nothing about its actual speed,
+kernel is more efficient (which says nothing about its actual speed, of
-of course). While stopping, setting and starting an I/O watcher does
+course). While stopping, setting and starting an I/O watcher does never
-never cause an extra syscall as with epoll, it still adds up to two event
+cause an extra syscall as with C<EVBACKEND_EPOLL>, it still adds up to
-changes per incident, support for C<fork ()> is very bad and it drops fds
+two event changes per incident, support for C<fork ()> is very bad and it
-silently in similarly hard-to-detetc cases.
+drops fds silently in similarly hard-to-detect cases.
+This backend usually performs well under most conditions.
+While nominally embeddable in other event loops, this doesn't work
+everywhere, so you might need to test for this. And since it is broken
+almost everywhere, you should only use it when you have a lot of sockets
+(for which it usually works), by embedding it into another event loop
+(e.g. C<EVBACKEND_SELECT> or C<EVBACKEND_POLL>) and using it only for
+sockets.
 =item C<EVBACKEND_DEVPOLL> (value 16, Solaris 8)
-This is not implemented yet (and might never be).
+This is not implemented yet (and might never be, unless you send me an
+implementation). According to reports, C</dev/poll> only supports sockets
+and is not embeddable, which would limit the usefulness of this backend
+immensely.
 =item C<EVBACKEND_PORT>    (value 32, Solaris 10)
 This uses the Solaris 10 event port mechanism. As with everything on Solaris,
 it's really slow, but it still scales very well (O(active_fds)).
 Please note that solaris event ports can deliver a lot of spurious
 notifications, so you need to use non-blocking I/O or other means to avoid
 blocking when no data (or space) is available.
+While this backend scales well, it requires one system call per active
+file descriptor per loop iteration. For small and medium numbers of file
+descriptors a "slow" C<EVBACKEND_SELECT> or C<EVBACKEND_POLL> backend
+might perform better.
 =item C<EVBACKEND_ALL>
 Try all backends (even potentially broken ones that wouldn't be tried
 with C<EVFLAG_AUTO>). Since this is a mask, you can do stuff such as
 C<EVBACKEND_ALL & ~EVBACKEND_KQUEUE>.
+It is definitely not recommended to use this flag.
 =back
 If one or more of these are ored into the flags value, then only these
 backends will be tried (in the reverse order as given here). If none are
 overhead for the actual polling but can deliver many events at once.
 By setting a higher I<io collect interval> you allow libev to spend more
 time collecting I/O events, so you can handle more events per iteration,
 at the cost of increasing latency. Timeouts (both C<ev_periodic> and
-C<ev_timer>) will be not affected.
+C<ev_timer>) will be not affected. Setting this to a non-null value will
+introduce an additional C<ev_sleep ()> call into most loop iterations.
 Likewise, by setting a higher I<timeout collect interval> you allow libev
 to spend more time collecting timeouts, at the expense of increased
 latency (the watcher callback will be called later). C<ev_io> watchers
-will not be affected.
+will not be affected. Setting this to a non-null value will not introduce
+any overhead in libev.
 Many (busy) programs can usually benefit by setting the io collect
 interval to a value near C<0.1> or so, which is often enough for
 interactive servers (of course not for games), likewise for timeouts. It
 usually doesn't make much sense to set it to a lower value than C<0.01>,
 optimisations to libev.
 =head3 The special problem of dup'ed file descriptors
 Some backends (e.g. epoll), cannot register events for file descriptors,
-but only events for the underlying file descriptions. That menas when you
+but only events for the underlying file descriptions. That means when you
 have C<dup ()>'ed file descriptors and register events for them, only one
 file descriptor might actually receive events.
-There is no workaorund possible except not registering events
+There is no workaround possible except not registering events
-for potentially C<dup ()>'ed file descriptors or to resort to
+for potentially C<dup ()>'ed file descriptors, or to resort to
 C<EVBACKEND_SELECT> or C<EVBACKEND_POLL>.
 =head3 The special problem of fork
 Some backends (epoll, kqueue) do not support C<fork ()> at all or exhibit
 semantics of C<ev_stat> watchers, which means that libev sometimes needs
 to fall back to regular polling again even with inotify, but changes are
 usually detected immediately, and if the file exists there will be no
 polling.
+=head3 Inotify
+When C<inotify (7)> support has been compiled into libev (generally only
+available on Linux) and present at runtime, it will be used to speed up
+change detection where possible. The inotify descriptor will be created lazily
+when the first C<ev_stat> watcher is being started.
+Inotify presense does not change the semantics of C<ev_stat> watchers
+except that changes might be detected earlier, and in some cases, to avoid
+making regular C<stat> calls. Even in the presense of inotify support
+there are many cases where libev has to resort to regular C<stat> polling.
+(There is no support for kqueue, as apparently it cannot be used to
+implement this functionality, due to the requirement of having a file
+descriptor open on the object at all times).
+=head3 The special problem of stat time resolution
+The C<stat ()> syscall only supports full-second resolution portably, and
+even on systems where the resolution is higher, many filesystems still
+only support whole seconds.
+That means that, if the time is the only thing that changes, you might
+miss updates: on the first update, C<ev_stat> detects a change and calls
+your callback, which does something. When there is another update within
+the same second, C<ev_stat> will be unable to detect it.
+The solution to this is to delay acting on a change for a second (or till
+the next second boundary), using a roughly one-second delay C<ev_timer>
+(C<ev_timer_set (w, 0., 1.01); ev_timer_again (loop, w)>). The C<.01>
+is added to work around small timing inconsistencies of some operating
+systems.
 =head3 Watcher-Specific Functions and Data Members
 =over 4
 =item ev_stat_init (ev_stat *, callback, const char *path, ev_tstamp interval)
 =item const char *path [read-only]
 The filesystem path that is being watched.
 =back
+=head3 Examples
 Example: Watch C</etc/passwd> for attribute changes.
   static void
   passwd_cb (struct ev_loop *loop, ev_stat *w, int revents)
   }
   ...
   ev_stat passwd;
-  ev_stat_init (&passwd, passwd_cb, "/etc/passwd");
+  ev_stat_init (&passwd, passwd_cb, "/etc/passwd", 0.);
   ev_stat_start (loop, &passwd);
+Example: Like above, but additionally use a one-second delay so we do not
+miss updates (however, frequent updates will delay processing, too, so
+one might do the work both on C<ev_stat> callback invocation I<and> on
+C<ev_timer> callback invocation).
+  static ev_stat passwd;
+  static ev_timer timer;
+  static void
+  timer_cb (EV_P_ ev_timer *w, int revents)
+  {
+    ev_timer_stop (EV_A_ w);
+    /* now it's one second after the most recent passwd change */
+  }
+  static void
+  stat_cb (EV_P_ ev_stat *w, int revents)
+  {
+    /* reset the one-second timer */
+    ev_timer_again (EV_A_ &timer);
+  }
+  ...
+  ev_stat_init (&passwd, stat_cb, "/etc/passwd", 0.);
+  ev_stat_start (loop, &passwd);
+  ev_timer_init (&timer, timer_cb, 0., 1.01);
 =head2 C<ev_idle> - when you've got nothing better to do...
 Idle watchers trigger events when no other events of the same or higher
 It is recommended to give C<ev_check> watchers highest (C<EV_MAXPRI>)
 priority, to ensure that they are being run before any other watchers
 after the poll. Also, C<ev_check> watchers (and C<ev_prepare> watchers,
 too) should not activate ("feed") events into libev. While libev fully
-supports this, they will be called before other C<ev_check> watchers did
+supports this, they will be called before other C<ev_check> watchers
-their job. As C<ev_check> watchers are often used to embed other event
+did their job. As C<ev_check> watchers are often used to embed other
-loops those other event loops might be in an unusable state until their
+(non-libev) event loops those other event loops might be in an unusable
-C<ev_check> watcher ran (always remind yourself to coexist peacefully with
+state until their C<ev_check> watcher ran (always remind yourself to
-others).
+coexist peacefully with others).
 =head3 Watcher-Specific Functions and Data Members
 =over 4
 =head2 C<ev_embed> - when one backend isn't enough...
 This is a rather advanced watcher type that lets you embed one event loop
 into another (currently only C<ev_io> events are supported in the embedded
 loop, other types of watchers might be handled in a delayed or incorrect
-fashion and must not be used). (See portability notes, below).
+fashion and must not be used).
 There are primarily two reasons you would want that: work around bugs and
 prioritise I/O.
 As an example for a bug workaround, the kqueue backend might only support
       ev_embed_start (loop_hi, &embed);
     }
   else
     loop_lo = loop_hi;
-=head2 Portability notes
-Kqueue is nominally embeddable, but this is broken on all BSDs that I
-tried, in various ways. Usually the embedded event loop will simply never
-receive events, sometimes it will only trigger a few times, sometimes in a
-loop. Epoll is also nominally embeddable, but many Linux kernel versions
-will always eport the epoll fd as ready, even when no events are pending.
-While libev allows embedding these backends (they are contained in
-C<ev_embeddable_backends ()>), take extreme care that it will actually
-work.
-When in doubt, create a dynamic event loop forced to use sockets (this
-usually works) and possibly another thread and a pipe or so to report to
-your main event loop.
 =head3 Watcher-Specific Functions and Data Members
 =over 4
 =item ev_embed_init (ev_embed *, callback, struct ev_loop *embedded_loop)
 than enough. If you need to manage thousands of children you might want to
 increase this value (I<must> be a power of two).
 =item EV_INOTIFY_HASHSIZE
-C<ev_staz> watchers use a small hash table to distribute workload by
+C<ev_stat> watchers use a small hash table to distribute workload by
 inotify watch id. The default size is C<16> (or C<1> with C<EV_MINIMAL>),
 usually more than enough. If you need to manage thousands of C<ev_stat>
 watchers you might want to increase this value (I<must> be a power of
 two).
 =item Starting and stopping timer/periodic watchers: O(log skipped_other_timers)
 This means that, when you have a watcher that triggers in one hour and
 there are 100 watchers that would trigger before that then inserting will
-have to skip those 100 watchers.
+have to skip roughly seven (C<ld 100>) of these watchers.
-=item Changing timer/periodic watchers (by autorepeat, again): O(log skipped_other_timers)
+=item Changing timer/periodic watchers (by autorepeat or calling again): O(log skipped_other_timers)
-That means that for changing a timer costs less than removing/adding them
+That means that changing a timer costs less than removing/adding them
 as only the relative motion in the event queue has to be paid for.
 =item Starting io/check/prepare/idle/signal/child watchers: O(1)
 These just add the watcher into an array or at the head of a list.
 =item Stopping check/prepare/idle watchers: O(1)
 =item Stopping an io/signal/child watcher: O(number_of_watchers_for_this_(fd/signal/pid % EV_PID_HASHSIZE))
 These watchers are stored in lists then need to be walked to find the
 correct watcher to remove. The lists are usually short (you don't usually
 have many watchers waiting for the same fd or signal).
-=item Finding the next timer per loop iteration: O(1)
+=item Finding the next timer in each loop iteration: O(1)
+By virtue of using a binary heap, the next timer is always found at the
+beginning of the storage array.
 =item Each change on a file descriptor per loop iteration: O(number_of_watchers_for_this_fd)
 A change means an I/O watcher gets started or stopped, which requires
-libev to recalculate its status (and possibly tell the kernel).
+libev to recalculate its status (and possibly tell the kernel, depending
+on backend and wether C<ev_io_set> was used).
-=item Activating one watcher: O(1)
+=item Activating one watcher (putting it into the pending state): O(1)
 =item Priority handling: O(number_of_priorities)
 Priorities are implemented by allocating some space for each
 priority. When doing priority-based operations, libev usually has to
-linearly search all the priorities.
+linearly search all the priorities, but starting/stopping and activating
+watchers becomes O(1) w.r.t. prioritiy handling.
 =back
 =head1 AUTHOR

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Comparing libev/ev.pod (file contents): Revision 1.98 by root, Sat Dec 22 06:10:25 2007 UTC vs. Revision 1.108 by root, Mon Dec 24 10:39:21 2007 UTC

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Comparing libev/ev.pod (file contents):
Revision 1.98 by root, Sat Dec 22 06:10:25 2007 UTC vs.
Revision 1.108 by root, Mon Dec 24 10:39:21 2007 UTC