C<ev_now> function is usually faster and also often returns the timestamp
you actually want to know.
+=item ev_sleep (ev_tstamp interval)
+
+Sleep for the given interval: The current thread will be blocked until
+either it is interrupted or the given time interval has passed. Basically
+this is a subsecond-resolution C<sleep ()>.
+
=item int ev_version_major ()
=item int ev_version_minor ()
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
-the fastest backend for a low number of fds.
+using this backend. It doesn't scale too well (O(highest_fd)), but its
+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
-select, but handles sparse fds better and has no artificial limit on the
-number of fds you can use (except it will slow down considerably with a
-lot of inactive fds). It scales similarly to select, i.e. O(total_fds).
+And this is your standard poll(2) backend. It's more complicated
+than select, but handles sparse fds better and has no artificial
+limit on the number of fds you can use (except it will slow down
+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).
-
-While stopping and starting an I/O watcher in the same iteration will
-result in some caching, there is still a syscall per such incident
+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.
+
+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, dup()ed file descriptors might not work very
-well if you register events for both fds.
+best to avoid that. Also, C<dup ()>'ed file descriptors might not work
+very well if you register events for both fds.
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 all BSDs except NetBSD (usually it doesn't work with
-anything but sockets and pipes, except on Darwin, where of course it's
-completely useless). For this reason it's not being "autodetected"
+was broken on all BSDs except NetBSD (usually it doesn't work reliably
+with anything but sockets and pipes, except on Darwin, where of course
+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>).
+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, of
-course). While starting and stopping an I/O watcher does not cause an
-extra syscall as with epoll, it still adds up to four event changes per
-incident, so its best to avoid that.
+course). While stopping, setting and starting an I/O watcher does never
+cause an extra syscall as with C<EVBACKEND_EPOLL>, it still adds up to
+two event changes per incident, support for C<fork ()> is very bad and it
+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 port mechanism. As with everything on Solaris,
+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 ports can result in a lot of spurious
+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
ev_ref (loop);
ev_signal_stop (loop, &exitsig);
+=item ev_set_io_collect_interval (loop, ev_tstamp interval)
+
+=item ev_set_timeout_collect_interval (loop, ev_tstamp interval)
+
+These advanced functions influence the time that libev will spend waiting
+for events. Both are by default C<0>, meaning that libev will try to
+invoke timer/periodic callbacks and I/O callbacks with minimum latency.
+
+Setting these to a higher value (the C<interval> I<must> be >= C<0>)
+allows libev to delay invocation of I/O and timer/periodic callbacks to
+increase efficiency of loop iterations.
+
+The background is that sometimes your program runs just fast enough to
+handle one (or very few) event(s) per loop iteration. While this makes
+the program responsive, it also wastes a lot of CPU time to poll for new
+events, especially with backends like C<select ()> which have a high
+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. 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. 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>,
+as this approsaches the timing granularity of most systems.
+
=back
=head3 The special problem of disappearing file descriptors
-Some backends (e.g kqueue, epoll) need to be told about closing a file
+Some backends (e.g. kqueue, epoll) need to be told about closing a file
descriptor (either by calling C<close> explicitly or by any other means,
such as C<dup>). The reason is that you register interest in some file
descriptor, but when it goes away, the operating system will silently drop
the libev application should not optimise around libev but should leave
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
+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
+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
+useless behaviour. Libev fully supports fork, but needs to be told about
+it in the child.
+
+To support fork in your programs, you either have to call
+C<ev_default_fork ()> or C<ev_loop_fork ()> after a fork in the child,
+enable C<EVFLAG_FORKCHECK>, or resort to C<EVBACKEND_SELECT> or
+C<EVBACKEND_POLL>.
+
=head3 Watcher-Specific Functions
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
-their job. As C<ev_check> watchers are often used to embed other event
-loops those other event loops might be in an unusable state until their
-C<ev_check> watcher ran (always remind yourself to coexist peacefully with
-others).
+supports this, they will be called before other C<ev_check> watchers
+did their job. As C<ev_check> watchers are often used to embed other
+(non-libev) event loops those other event loops might be in an unusable
+state until their C<ev_check> watcher ran (always remind yourself to
+coexist peacefully with others).
=head3 Watcher-Specific Functions and Data Members
(CLOCK_REALTIME, ...)> and will not normally affect correctness. See the
note about libraries in the description of C<EV_USE_MONOTONIC>, though.
+=item EV_USE_NANOSLEEP
+
+If defined to be C<1>, libev will assume that C<nanosleep ()> is available
+and will use it for delays. Otherwise it will use C<select ()>.
+
=item EV_USE_SELECT
If undefined or defined to be C<1>, libev will compile in support for the
Can be used to change the callback member declaration in each watcher,
and the way callbacks are invoked and set. Must expand to a struct member
-definition and a statement, respectively. See the F<ev.c> header file for
+definition and a statement, respectively. See the F<ev.h> header file for
their default definitions. One possible use for overriding these is to
avoid the C<struct ev_loop *> as first argument in all cases, or to use
method calls instead of plain function calls in C++.
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