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Comparing libev/ev.3 (file contents):
Revision 1.111 by root, Sun Jun 23 02:02:24 2019 UTC vs.
Revision 1.113 by root, Mon Jun 24 19:53:47 2019 UTC

… …
131.\} 131.\}
132.rm #[ #] #H #V #F C 132.rm #[ #] #H #V #F C
133.\" ======================================================================== 133.\" ========================================================================
134.\" 134.\"
135.IX Title "LIBEV 3" 135.IX Title "LIBEV 3"
136.TH LIBEV 3 "2019-06-22" "libev-4.25" "libev - high performance full featured event loop" 136.TH LIBEV 3 "2019-06-24" "libev-4.25" "libev - high performance full featured event loop"
137.\" For nroff, turn off justification. Always turn off hyphenation; it makes 137.\" For nroff, turn off justification. Always turn off hyphenation; it makes
138.\" way too many mistakes in technical documents. 138.\" way too many mistakes in technical documents.
139.if n .ad l 139.if n .ad l
140.nh 140.nh
141.SH "NAME" 141.SH "NAME"
… …
701This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in the same way as 701This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in the same way as
702\&\f(CW\*(C`EVBACKEND_POLL\*(C'\fR. 702\&\f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
703.ie n .IP """EVBACKEND_LINUXAIO"" (value 64, Linux)" 4 703.ie n .IP """EVBACKEND_LINUXAIO"" (value 64, Linux)" 4
704.el .IP "\f(CWEVBACKEND_LINUXAIO\fR (value 64, Linux)" 4 704.el .IP "\f(CWEVBACKEND_LINUXAIO\fR (value 64, Linux)" 4
705.IX Item "EVBACKEND_LINUXAIO (value 64, Linux)" 705.IX Item "EVBACKEND_LINUXAIO (value 64, Linux)"
706Use the linux-specific linux aio (\fInot\fR \f(CWaio(7)\fR) event interface 706Use the linux-specific linux aio (\fInot\fR \f(CWaio(7)\fR but \f(CWio_submit(2)\fR) event interface available in post\-4.18 kernels.
707available in post\-4.18 kernels.
708.Sp 707.Sp
709If this backend works for you (as of this writing, it was very 708If this backend works for you (as of this writing, it was very
710experimental and only supports a subset of file types), it is the best 709experimental), it is the best event interface available on linux and might
711event interface available on linux and might be well worth it enabling it 710be well worth enabling it \- if it isn't available in your kernel this will
712\&\- if it isn't available in your kernel this will be detected and another 711be detected and this backend will be skipped.
713backend will be chosen.
714.Sp 712.Sp
715This backend can batch oneshot requests and uses a user-space ring buffer 713This backend can batch oneshot requests and supports a user-space ring
716to receive events. It also doesn't suffer from most of the design problems 714buffer to receive events. It also doesn't suffer from most of the design
717of epoll (such as not being able to remove event sources from the epoll 715problems of epoll (such as not being able to remove event sources from the
718set), and generally sounds too good to be true. Because, this being the 716epoll set), and generally sounds too good to be true. Because, this being
719linux kernel, of course it suffers from a whole new set of limitations. 717the linux kernel, of course it suffers from a whole new set of limitations.
720.Sp 718.Sp
721For one, it is not easily embeddable (but probably could be done using 719For one, it is not easily embeddable (but probably could be done using
722an event fd at some extra overhead). It also is subject to various 720an event fd at some extra overhead). It also is subject to a system wide
723arbitrary limits that can be configured in \fI/proc/sys/fs/aio\-max\-nr\fR 721limit that can be configured in \fI/proc/sys/fs/aio\-max\-nr\fR \- each loop
724and \fI/proc/sys/fs/aio\-nr\fR), which could lead to it being skipped during 722currently requires \f(CW61\fR of this number. If no aio requests are left, this
725initialisation. 723backend will be skipped during initialisation.
726.Sp 724.Sp
727Most problematic in practise, however, is that, like kqueue, it requires 725Most problematic in practise, however, is that not all file descriptors
728special support from drivers, and, not surprisingly, not all drivers
729implement it. For example, in linux 4.19, tcp sockets, pipes, event fds, 726work with it. For example, in linux 5.1, tcp sockets, pipes, event fds,
730files, \fI/dev/null\fR and a few others are supported, but ttys are not, so 727files, \fI/dev/null\fR and a few others are supported, but ttys do not work
731this is not (yet?) a generic event polling interface but is probably still 728properly (a known bug that the kernel developers don't care about, see
732be very useful in a web server or similar program. 729<https://lore.kernel.org/patchwork/patch/1047453/>), so this is not
730(yet?) a generic event polling interface.
731.Sp
732Overall, it seems the linux developers just don't want it to have a
733generic event handling mechanism other than \f(CW\*(C`select\*(C'\fR or \f(CW\*(C`poll\*(C'\fR.
734.Sp
735To work around the fd type problem, the current version of libev uses
736epoll as a fallback for file deescriptor types that do not work. Epoll
737is used in, kind of, slow mode that hopefully avoids most of its design
738problems and requires 1\-3 extra syscalls per active fd every iteration.
733.Sp 739.Sp
734This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in the same way as 740This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in the same way as
735\&\f(CW\*(C`EVBACKEND_POLL\*(C'\fR. 741\&\f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
736.ie n .IP """EVBACKEND_KQUEUE"" (value 8, most \s-1BSD\s0 clones)" 4 742.ie n .IP """EVBACKEND_KQUEUE"" (value 8, most \s-1BSD\s0 clones)" 4
737.el .IP "\f(CWEVBACKEND_KQUEUE\fR (value 8, most \s-1BSD\s0 clones)" 4 743.el .IP "\f(CWEVBACKEND_KQUEUE\fR (value 8, most \s-1BSD\s0 clones)" 4

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