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Comparing libev/ev.pod (file contents):
Revision 1.449 by root, Sun Jun 23 02:02:30 2019 UTC vs.
Revision 1.452 by root, Tue Jun 25 04:58:36 2019 UTC

… …
575C<EVBACKEND_POLL>. 575C<EVBACKEND_POLL>.
576 576
577=item C<EVBACKEND_LINUXAIO> (value 64, Linux) 577=item C<EVBACKEND_LINUXAIO> (value 64, Linux)
578 578
579Use the linux-specific linux aio (I<not> C<< aio(7) >> but C<< 579Use the linux-specific linux aio (I<not> C<< aio(7) >> but C<<
580io_submit(2) >>) event interface available in post-4.18 kernels. 580io_submit(2) >>) event interface available in post-4.18 kernels (but libev
581only tries to use it in 4.19+).
582
583This is another linux trainwreck of an event interface.
581 584
582If this backend works for you (as of this writing, it was very 585If this backend works for you (as of this writing, it was very
583experimental), it is the best event interface available on linux and might 586experimental), it is the best event interface available on linux and might
584be well worth enabling it - if it isn't available in your kernel this will 587be well worth enabling it - if it isn't available in your kernel this will
585be detected and this backend will be skipped. 588be detected and this backend will be skipped.
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587This backend can batch oneshot requests and supports a user-space ring 590This backend can batch oneshot requests and supports a user-space ring
588buffer to receive events. It also doesn't suffer from most of the design 591buffer to receive events. It also doesn't suffer from most of the design
589problems of epoll (such as not being able to remove event sources from 592problems of epoll (such as not being able to remove event sources from
590the epoll set), and generally sounds too good to be true. Because, this 593the epoll set), and generally sounds too good to be true. Because, this
591being the linux kernel, of course it suffers from a whole new set of 594being the linux kernel, of course it suffers from a whole new set of
592limitations. 595limitations, forcing you to fall back to epoll, inheriting all its design
596issues.
593 597
594For one, it is not easily embeddable (but probably could be done using 598For one, it is not easily embeddable (but probably could be done using
595an event fd at some extra overhead). It also is subject to a system wide 599an event fd at some extra overhead). It also is subject to a system wide
596limit that can be configured in F</proc/sys/fs/aio-max-nr> - each loop 600limit that can be configured in F</proc/sys/fs/aio-max-nr>. If no aio
597currently requires C<61> of this number. If no aio requests are left, this
598backend will be skipped during initialisation. 601requests are left, this backend will be skipped during initialisation, and
602will switch to epoll when the loop is active.
599 603
600Most problematic in practise, however, is that not all file descriptors 604Most problematic in practice, however, is that not all file descriptors
601work with it. For example, in linux 5.1, tcp sockets, pipes, event fds, 605work with it. For example, in linux 5.1, tcp sockets, pipes, event fds,
602files, F</dev/null> and a few others are supported, but ttys do not work 606files, F</dev/null> and a few others are supported, but ttys do not work
603(probably because of a bug), so this is not (yet?) a generic event polling 607properly (a known bug that the kernel developers don't care about, see
604interface. 608L<https://lore.kernel.org/patchwork/patch/1047453/>), so this is not
609(yet?) a generic event polling interface.
605 610
611Overall, it seems the linux developers just don't want it to have a
612generic event handling mechanism other than C<select> or C<poll>.
613
606To work around this latter problem, the current version of libev uses 614To work around all these problem, the current version of libev uses its
607epoll as a fallback for file deescriptor types that do not work. Epoll 615epoll backend as a fallback for file descriptor types that do not work. Or
608is used in, kind of, slow mode that hopefully avoids most of its design 616falls back completely to epoll if the kernel acts up.
609problems and requires 1-3 extra syscalls per active fd every iteration.
610 617
611This backend maps C<EV_READ> and C<EV_WRITE> in the same way as 618This backend maps C<EV_READ> and C<EV_WRITE> in the same way as
612C<EVBACKEND_POLL>. 619C<EVBACKEND_POLL>.
613 620
614=item C<EVBACKEND_KQUEUE> (value 8, most BSD clones) 621=item C<EVBACKEND_KQUEUE> (value 8, most BSD clones)

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