diff options
| -rw-r--r-- | ev.3 | 53 |
1 files changed, 44 insertions, 9 deletions
@@ -453,15 +453,24 @@ environment variable. This is your standard \fIselect\fR\|(2) backend. Not \fIcompletely\fR 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. +.Sp +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 \f(CW\*(C`accept ()\*(C'\fR in a loop to accept as many +connections as possible during one iteration. You might also want to have +a look at \f(CW\*(C`ev_set_io_collect_interval ()\*(C'\fR to increase the amount of +readyness notifications you get per iteration. .ie n .IP """EVBACKEND_POLL"" (value 2, poll backend, available everywhere except on windows)" 4 .el .IP "\f(CWEVBACKEND_POLL\fR (value 2, poll backend, available everywhere except on windows)" 4 .IX Item "EVBACKEND_POLL (value 2, poll backend, available everywhere except on windows)" -And this is your standard \fIpoll\fR\|(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 \fIpoll\fR\|(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 \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR, above, for +performance tips. .ie n .IP """EVBACKEND_EPOLL"" (value 4, Linux)" 4 .el .IP "\f(CWEVBACKEND_EPOLL\fR (value 4, Linux)" 4 .IX Item "EVBACKEND_EPOLL (value 4, Linux)" @@ -471,7 +480,7 @@ 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. .Sp 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 @@ -482,6 +491,13 @@ 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. +.Sp +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. +.Sp +While nominally embeddeble in other event loops, this feature is broken in +all kernel versions tested so far. .ie n .IP """EVBACKEND_KQUEUE"" (value 8, most \s-1BSD\s0 clones)" 4 .el .IP "\f(CWEVBACKEND_KQUEUE\fR (value 8, most \s-1BSD\s0 clones)" 4 .IX Item "EVBACKEND_KQUEUE (value 8, most BSD clones)" @@ -503,10 +519,22 @@ course). While stopping, setting and starting an I/O watcher does never cause an extra syscall as with \f(CW\*(C`EVBACKEND_EPOLL\*(C'\fR, it still adds up to two event changes per incident, support for \f(CW\*(C`fork ()\*(C'\fR is very bad and it drops fds silently in similarly hard-to-detect cases. +.Sp +This backend usually performs well under most conditions. +.Sp +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. \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or \f(CW\*(C`EVBACKEND_POLL\*(C'\fR) and using it only for +sockets. .ie n .IP """EVBACKEND_DEVPOLL"" (value 16, Solaris 8)" 4 .el .IP "\f(CWEVBACKEND_DEVPOLL\fR (value 16, Solaris 8)" 4 .IX Item "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, \f(CW\*(C`/dev/poll\*(C'\fR only supports sockets +and is not embeddable, which would limit the usefulness of this backend +immensely. .ie n .IP """EVBACKEND_PORT"" (value 32, Solaris 10)" 4 .el .IP "\f(CWEVBACKEND_PORT\fR (value 32, Solaris 10)" 4 .IX Item "EVBACKEND_PORT (value 32, Solaris 10)" @@ -516,12 +544,19 @@ 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. +.Sp +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 \*(L"slow\*(R" \f(CW\*(C`EVBACKEND_SELECT\*(C'\fR or \f(CW\*(C`EVBACKEND_POLL\*(C'\fR backend +might perform better. .ie n .IP """EVBACKEND_ALL""" 4 .el .IP "\f(CWEVBACKEND_ALL\fR" 4 .IX Item "EVBACKEND_ALL" Try all backends (even potentially broken ones that wouldn't be tried with \f(CW\*(C`EVFLAG_AUTO\*(C'\fR). Since this is a mask, you can do stuff such as \&\f(CW\*(C`EVBACKEND_ALL & ~EVBACKEND_KQUEUE\*(C'\fR. +.Sp +It is definitely not recommended to use this flag. .RE .RS 4 .Sp @@ -757,7 +792,7 @@ overhead for the actual polling but can deliver many events at once. By setting a higher \fIio collect interval\fR 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 \f(CW\*(C`ev_periodic\*(C'\fR and -\&\f(CW\*(C`ev_timer\*(C'\fR) will be not affected. Setting this to a non-null bvalue will +\&\f(CW\*(C`ev_timer\*(C'\fR) will be not affected. Setting this to a non-null value will introduce an additional \f(CW\*(C`ev_sleep ()\*(C'\fR call into most loop iterations. .Sp Likewise, by setting a higher \fItimeout collect interval\fR you allow libev |