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386 | For few fds, this backend is a bit little slower than poll and select, |
386 | For few fds, this backend is a bit little slower than poll and select, |
387 | but it scales phenomenally better. While poll and select usually scale |
387 | but it scales phenomenally better. While poll and select usually scale |
388 | like O(total_fds) where n is the total number of fds (or the highest fd), |
388 | like O(total_fds) where n is the total number of fds (or the highest fd), |
389 | epoll scales either O(1) or O(active_fds). |
389 | epoll scales either O(1) or O(active_fds). |
390 | |
390 | |
391 | The epoll syscalls are the most misdesigned of the more advanced event |
391 | The epoll mechanism deserves honorable mention as the most misdesigned |
392 | mechanisms: problems include silently dropping fds, requiring a system |
392 | of the more advanced event mechanisms: mere annoyances include silently |
393 | call per change per fd (and unnecessary guessing of parameters), problems |
393 | dropping file descriptors, requiring a system call per change per file |
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394 | descriptor (and unnecessary guessing of parameters), problems with dup and |
394 | with dup and so on. The biggest issue is fork races, however - if a |
395 | so on. The biggest issue is fork races, however - if a program forks then |
395 | program forks then I<both> parent and child process have to recreate the |
396 | I<both> parent and child process have to recreate the epoll set, which can |
396 | epoll set, which can take considerable time (one syscall per fd) and is of |
397 | take considerable time (one syscall per file descriptor) and is of course |
397 | course hard to detect. |
398 | hard to detect. |
398 | |
399 | |
399 | Epoll is also notoriously buggy - embedding epoll fds should work, but |
400 | Epoll is also notoriously buggy - embedding epoll fds I<should> work, but |
400 | of course doesn't, and epoll just loves to report events for totally |
401 | of course I<doesn't>, and epoll just loves to report events for totally |
401 | I<different> file descriptors (even already closed ones, so one cannot |
402 | I<different> file descriptors (even already closed ones, so one cannot |
402 | even remove them from the set) than registered in the set (especially |
403 | even remove them from the set) than registered in the set (especially |
403 | on SMP systems). Libev tries to counter these spurious notifications by |
404 | on SMP systems). Libev tries to counter these spurious notifications by |
404 | employing an additional generation counter and comparing that against the |
405 | employing an additional generation counter and comparing that against the |
405 | events to filter out spurious ones. |
406 | events to filter out spurious ones, recreating the set when required. |
406 | |
407 | |
407 | While stopping, setting and starting an I/O watcher in the same iteration |
408 | While stopping, setting and starting an I/O watcher in the same iteration |
408 | will result in some caching, there is still a system call per such incident |
409 | will result in some caching, there is still a system call per such |
409 | (because the fd could point to a different file description now), so its |
410 | incident (because the same I<file descriptor> could point to a different |
410 | best to avoid that. Also, C<dup ()>'ed file descriptors might not work |
411 | I<file description> now), so its best to avoid that. Also, C<dup ()>'ed |
411 | very well if you register events for both fds. |
412 | file descriptors might not work very well if you register events for both |
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413 | file descriptors. |
412 | |
414 | |
413 | Best performance from this backend is achieved by not unregistering all |
415 | Best performance from this backend is achieved by not unregistering all |
414 | watchers for a file descriptor until it has been closed, if possible, |
416 | watchers for a file descriptor until it has been closed, if possible, |
415 | i.e. keep at least one watcher active per fd at all times. Stopping and |
417 | i.e. keep at least one watcher active per fd at all times. Stopping and |
416 | starting a watcher (without re-setting it) also usually doesn't cause |
418 | starting a watcher (without re-setting it) also usually doesn't cause |
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424 | This backend maps C<EV_READ> and C<EV_WRITE> in the same way as |
426 | This backend maps C<EV_READ> and C<EV_WRITE> in the same way as |
425 | C<EVBACKEND_POLL>. |
427 | C<EVBACKEND_POLL>. |
426 | |
428 | |
427 | =item C<EVBACKEND_KQUEUE> (value 8, most BSD clones) |
429 | =item C<EVBACKEND_KQUEUE> (value 8, most BSD clones) |
428 | |
430 | |
429 | Kqueue deserves special mention, as at the time of this writing, it was |
431 | Kqueue deserves special mention, as at the time of this writing, it |
430 | broken on all BSDs except NetBSD (usually it doesn't work reliably with |
432 | was broken on all BSDs except NetBSD (usually it doesn't work reliably |
431 | anything but sockets and pipes, except on Darwin, where of course it's |
433 | with anything but sockets and pipes, except on Darwin, where of course |
432 | completely useless). For this reason it's not being "auto-detected" unless |
434 | it's completely useless). Unlike epoll, however, whose brokenness |
433 | you explicitly specify it in the flags (i.e. using C<EVBACKEND_KQUEUE>) or |
435 | is by design, these kqueue bugs can (and eventually will) be fixed |
434 | libev was compiled on a known-to-be-good (-enough) system like NetBSD. |
436 | without API changes to existing programs. For this reason it's not being |
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437 | "auto-detected" unless you explicitly specify it in the flags (i.e. using |
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438 | C<EVBACKEND_KQUEUE>) or libev was compiled on a known-to-be-good (-enough) |
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439 | system like NetBSD. |
435 | |
440 | |
436 | You still can embed kqueue into a normal poll or select backend and use it |
441 | You still can embed kqueue into a normal poll or select backend and use it |
437 | only for sockets (after having made sure that sockets work with kqueue on |
442 | only for sockets (after having made sure that sockets work with kqueue on |
438 | the target platform). See C<ev_embed> watchers for more info. |
443 | the target platform). See C<ev_embed> watchers for more info. |
439 | |
444 | |