diff options
Diffstat (limited to 'ev.pod')
| -rw-r--r-- | ev.pod | 47 |
1 files changed, 26 insertions, 21 deletions
@@ -388,27 +388,29 @@ 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 syscalls are the most misdesigned of the more advanced event -mechanisms: problems include silently dropping fds, requiring a system -call per change per fd (and unnecessary guessing of parameters), problems -with dup and so on. The biggest issue is fork races, however - if a -program forks then I<both> parent and child process have to recreate the -epoll set, which can take considerable time (one syscall per fd) and is of -course hard to detect. - -Epoll is also notoriously buggy - embedding epoll fds should work, but -of course doesn't, and epoll just loves to report events for totally +The epoll mechanism deserves honorable mention as the most misdesigned +of the more advanced event mechanisms: mere annoyances include silently +dropping file descriptors, requiring a system call per change per file +descriptor (and unnecessary guessing of parameters), problems with dup and +so on. The biggest issue is fork races, however - if a program forks then +I<both> parent and child process have to recreate the epoll set, which can +take considerable time (one syscall per file descriptor) and is of course +hard to detect. + +Epoll is also notoriously buggy - embedding epoll fds I<should> work, but +of course I<doesn't>, and epoll just loves to report events for totally I<different> file descriptors (even already closed ones, so one cannot even remove them from the set) than registered in the set (especially on SMP systems). Libev tries to counter these spurious notifications by employing an additional generation counter and comparing that against the -events to filter out spurious ones. +events to filter out spurious ones, recreating the set when required. While stopping, setting and starting an I/O watcher in the same iteration -will result in some caching, there is still a system call 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 -very well if you register events for both fds. +will result in some caching, there is still a system call per such +incident (because the same I<file descriptor> could point to a different +I<file description> now), so its best to avoid that. Also, C<dup ()>'ed +file descriptors might not work very well if you register events for both +file descriptors. Best performance from this backend is achieved by not unregistering all watchers for a file descriptor until it has been closed, if possible, @@ -426,12 +428,15 @@ C<EVBACKEND_POLL>. =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 reliably with -anything but sockets and pipes, except on Darwin, where of course it's -completely useless). For this reason it's not being "auto-detected" unless -you explicitly specify it in the flags (i.e. using C<EVBACKEND_KQUEUE>) or -libev was compiled on a known-to-be-good (-enough) system like NetBSD. +Kqueue deserves special mention, as at the time of this writing, it +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). Unlike epoll, however, whose brokenness +is by design, these kqueue bugs can (and eventually will) be fixed +without API changes to existing programs. For this reason it's not being +"auto-detected" unless you explicitly specify it 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 |