*** empty log message ***

ev.3

@@ -133,7 +133,7 @@
 .\" ========================================================================
 .\"
 .IX Title "LIBEV 3"
-.TH LIBEV 3 "2019-06-22" "libev-4.25" "libev - high performance full featured event loop"
+.TH LIBEV 3 "2019-06-23" "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
@@ -703,33 +703,36 @@ This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in th
 .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) event interface
-available in post\-4.18 kernels.
+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.
 .Sp
 If this backend works for you (as of this writing, it was very
-experimental and only supports a subset of file types), it is the best
-event interface available on linux and might be well worth it enabling it
-\&\- if it isn't available in your kernel this will be detected and another
-backend will be chosen.
+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 uses 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.
+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.
 .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 various
-arbitrary limits that can be configured in \fI/proc/sys/fs/aio\-max\-nr\fR
-and \fI/proc/sys/fs/aio\-nr\fR), which could lead to it being skipped during
-initialisation.
+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, like kqueue, it requires
-special support from drivers, and, not surprisingly, not all drivers
-implement it. For example, in linux 4.19, tcp sockets, pipes, event fds,
-files, \fI/dev/null\fR and a few others are supported, but ttys are not, so
-this is not (yet?) a generic event polling interface but is probably still
-be very useful in a web server or similar program.
+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
+(probably because of a bug), so this is not (yet?) a generic event polling
+interface.
+.Sp
+To work around this latter 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.
 .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.

ev.c

@@ -2809,7 +2809,9 @@ ev_recommended_backends (void) EV_NOEXCEPT
 #endif
 
   /* TODO: linuxaio is very experimental */
+#if !EV_RECOMMEND_LINUXAIO
   flags &= ~EVBACKEND_LINUXAIO;
+#endif
 
   return flags;
 }