--- libev/ev.3 2019/06/24 19:53:47 1.113 +++ libev/ev.3 2019/06/25 06:36:59 1.114 @@ -133,7 +133,7 @@ .\" ======================================================================== .\" .IX Title "LIBEV 3" -.TH LIBEV 3 "2019-06-24" "libev-4.25" "libev - high performance full featured event loop" +.TH LIBEV 3 "2019-06-25" "libev-4.25" "libev - high performance full featured event loop" .\" For nroff, turn off justification. Always turn off hyphenation; it makes .\" way too many mistakes in technical documents. .if n .ad l @@ -640,7 +640,7 @@ .ie n .IP """EVBACKEND_EPOLL"" (value 4, Linux)" 4 .el .IP "\f(CWEVBACKEND_EPOLL\fR (value 4, Linux)" 4 .IX Item "EVBACKEND_EPOLL (value 4, Linux)" -Use the linux-specific \fBepoll\fR\|(7) interface (for both pre\- and post\-2.6.9 +Use the Linux-specific \fBepoll\fR\|(7) interface (for both pre\- and post\-2.6.9 kernels). .Sp For few fds, this backend is a bit little slower than poll and select, but @@ -703,54 +703,58 @@ .ie n .IP """EVBACKEND_LINUXAIO"" (value 64, Linux)" 4 .el .IP "\f(CWEVBACKEND_LINUXAIO\fR (value 64, Linux)" 4 .IX Item "EVBACKEND_LINUXAIO (value 64, Linux)" -Use 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. +Use the Linux-specific Linux \s-1AIO\s0 (\fInot\fR \f(CWaio(7)\fR but \f(CWio_submit(2)\fR) event interface available in post\-4.18 kernels (but libev +only tries to use it in 4.19+). +.Sp +This is another Linux train wreck of an event interface. .Sp If this backend works for you (as of this writing, it was very -experimental), it is the best event interface available on linux and might +experimental), it is the best event interface available on Linux and might be well worth enabling it \- if it isn't available in your kernel this will be detected and this backend will be skipped. .Sp This backend can batch oneshot requests and supports a user-space ring buffer to receive events. It also doesn't suffer from most of the design -problems of epoll (such as not being able to remove event sources from the -epoll set), and generally sounds too good to be true. Because, this being -the linux kernel, of course it suffers from a whole new set of limitations. +problems of epoll (such as not being able to remove event sources from +the epoll set), and generally sounds too good to be true. Because, this +being the Linux kernel, of course it suffers from a whole new set of +limitations, forcing you to fall back to epoll, inheriting all its design +issues. .Sp For one, it is not easily embeddable (but probably could be done using an event fd at some extra overhead). It also is subject to a system wide -limit that can be configured in \fI/proc/sys/fs/aio\-max\-nr\fR \- each loop -currently requires \f(CW61\fR of this number. If no aio requests are left, this -backend will be skipped during initialisation. -.Sp -Most problematic in practise, however, is that not all file descriptors -work with it. For example, in linux 5.1, tcp sockets, pipes, event fds, -files, \fI/dev/null\fR and a few others are supported, but ttys do not work +limit that can be configured in \fI/proc/sys/fs/aio\-max\-nr\fR. If no \s-1AIO\s0 +requests are left, this backend will be skipped during initialisation, and +will switch to epoll when the loop is active. +.Sp +Most problematic in practice, however, is that not all file descriptors +work with it. For example, in Linux 5.1, \s-1TCP\s0 sockets, pipes, event fds, +files, \fI/dev/null\fR and many others are supported, but ttys do not work properly (a known bug that the kernel developers don't care about, see ), so this is not (yet?) a generic event polling interface. .Sp -Overall, it seems the linux developers just don't want it to have a +Overall, it seems the Linux developers just don't want it to have a generic event handling mechanism other than \f(CW\*(C`select\*(C'\fR or \f(CW\*(C`poll\*(C'\fR. .Sp -To work around the fd type problem, the current version of libev uses -epoll as a fallback for file deescriptor types that do not work. Epoll -is used in, kind of, slow mode that hopefully avoids most of its design -problems and requires 1\-3 extra syscalls per active fd every iteration. +To work around all these problem, the current version of libev uses its +epoll backend as a fallback for file descriptor types that do not work. Or +falls back completely to epoll if the kernel acts up. .Sp This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in the same way as \&\f(CW\*(C`EVBACKEND_POLL\*(C'\fR. .ie n .IP """EVBACKEND_KQUEUE"" (value 8, most \s-1BSD\s0 clones)" 4 .el .IP "\f(CWEVBACKEND_KQUEUE\fR (value 8, most \s-1BSD\s0 clones)" 4 .IX Item "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). Unlike epoll, however, whose brokenness -is by design, these kqueue bugs can (and eventually will) be fixed -without \s-1API\s0 changes to existing programs. For this reason it's not being -\&\*(L"auto-detected\*(R" unless you explicitly specify it in the flags (i.e. using -\&\f(CW\*(C`EVBACKEND_KQUEUE\*(C'\fR) or libev was compiled on a known-to-be-good (\-enough) -system like NetBSD. +Kqueue deserves special mention, as at the time this backend was +implemented, 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 be (and mostly have been) +fixed without \s-1API\s0 changes to existing programs. For this reason it's not +being \*(L"auto-detected\*(R" on all platforms unless you explicitly specify it +in the flags (i.e. using \f(CW\*(C`EVBACKEND_KQUEUE\*(C'\fR) or libev was compiled on a +known-to-be-good (\-enough) system like NetBSD. .Sp 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 @@ -761,7 +765,7 @@ course). While stopping, setting and starting an I/O watcher does never cause an extra system call as with \f(CW\*(C`EVBACKEND_EPOLL\*(C'\fR, it still adds up to two event changes per incident. Support for \f(CW\*(C`fork ()\*(C'\fR is very bad (you -might have to leak fd's on fork, but it's more sane than epoll) and it +might have to leak fds on fork, but it's more sane than epoll) and it drops fds silently in similarly hard-to-detect cases. .Sp This backend usually performs well under most conditions.