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)
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
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
-sockets and pipes, except on Darwin, where of course it's completely
-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"
+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>) 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 stopping, setting and starting an I/O watcher does
-never cause an extra syscall as with 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-detetc cases.
+kernel is more efficient (which says nothing about its actual speed, of
+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)
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
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
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
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.
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
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