| … | |
… | |
| 574 | This backend maps C<EV_READ> and C<EV_WRITE> in the same way as |
574 | This backend maps C<EV_READ> and C<EV_WRITE> in the same way as |
| 575 | C<EVBACKEND_POLL>. |
575 | C<EVBACKEND_POLL>. |
| 576 | |
576 | |
| 577 | =item C<EVBACKEND_LINUXAIO> (value 64, Linux) |
577 | =item C<EVBACKEND_LINUXAIO> (value 64, Linux) |
| 578 | |
578 | |
| 579 | Use the linux-specific linux aio (I<not> C<< aio(7) >>) event interface |
579 | Use the linux-specific linux aio (I<not> C<< aio(7) >> but C<< |
| 580 | available in post-4.18 kernels. |
580 | io_submit(2) >>) event interface available in post-4.18 kernels. |
| 581 | |
581 | |
| 582 | If this backend works for you (as of this writing, it was very |
582 | If this backend works for you (as of this writing, it was very |
| 583 | experimental and only supports a subset of file types), it is the best |
583 | experimental), it is the best event interface available on linux and might |
| 584 | event interface available on linux and might be well worth it enabling it |
584 | be well worth enabling it - if it isn't available in your kernel this will |
| 585 | - if it isn't available in your kernel this will be detected and another |
585 | be detected and this backend will be skipped. |
| 586 | backend will be chosen. |
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| 587 | |
586 | |
| 588 | This backend can batch oneshot requests and uses a user-space ring buffer |
587 | This backend can batch oneshot requests and supports a user-space ring |
| 589 | to receive events. It also doesn't suffer from most of the design problems |
588 | buffer to receive events. It also doesn't suffer from most of the design |
| 590 | of epoll (such as not being able to remove event sources from the epoll |
589 | problems of epoll (such as not being able to remove event sources from the |
| 591 | set), and generally sounds too good to be true. Because, this being the |
590 | epoll set), and generally sounds too good to be true. Because, this being |
| 592 | linux kernel, of course it suffers from a whole new set of limitations. |
591 | the linux kernel, of course it suffers from a whole new set of limitations. |
| 593 | |
592 | |
| 594 | For one, it is not easily embeddable (but probably could be done using |
593 | For one, it is not easily embeddable (but probably could be done using |
| 595 | an event fd at some extra overhead). It also is subject to various |
594 | an event fd at some extra overhead). It also is subject to a system wide |
| 596 | arbitrary limits that can be configured in F</proc/sys/fs/aio-max-nr> |
595 | limit that can be configured in F</proc/sys/fs/aio-max-nr> - each loop |
| 597 | and F</proc/sys/fs/aio-nr>), which could lead to it being skipped during |
596 | currently requires C<61> of this number. If no aio requests are left, this |
| 598 | initialisation. |
597 | backend will be skipped during initialisation. |
| 599 | |
598 | |
| 600 | Most problematic in practise, however, is that, like kqueue, it requires |
599 | Most problematic in practise, however, is that not all file descriptors |
| 601 | special support from drivers, and, not surprisingly, not all drivers |
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| 602 | implement it. For example, in linux 4.19, tcp sockets, pipes, event fds, |
600 | work with it. For example, in linux 5.1, tcp sockets, pipes, event fds, |
| 603 | files, F</dev/null> and a few others are supported, but ttys are not, so |
601 | files, F</dev/null> and a few others are supported, but ttys do not work |
| 604 | this is not (yet?) a generic event polling interface but is probably still |
602 | properly (a known bug that the kernel developers don't care about, see |
| 605 | be very useful in a web server or similar program. |
603 | L<https://lore.kernel.org/patchwork/patch/1047453/>), so this is not |
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604 | (yet?) a generic event polling interface. |
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605 | |
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606 | Overall, it seems the linux developers just don't want it to have a |
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607 | generic event handling mechanism other than C<select> or C<poll>. |
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608 | |
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609 | To work around the fd type problem, the current version of libev uses |
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610 | epoll as a fallback for file deescriptor types that do not work. Epoll |
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611 | is used in, kind of, slow mode that hopefully avoids most of its design |
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612 | problems and requires 1-3 extra syscalls per active fd every iteration. |
| 606 | |
613 | |
| 607 | This backend maps C<EV_READ> and C<EV_WRITE> in the same way as |
614 | This backend maps C<EV_READ> and C<EV_WRITE> in the same way as |
| 608 | C<EVBACKEND_POLL>. |
615 | C<EVBACKEND_POLL>. |
| 609 | |
616 | |
| 610 | =item C<EVBACKEND_KQUEUE> (value 8, most BSD clones) |
617 | =item C<EVBACKEND_KQUEUE> (value 8, most BSD clones) |
