[ViewVC] Diff of: cvs/libev/ev.pod

Comparing libev/ev.pod (file contents):
Revision 1.443 by root, Thu Aug 30 21:51:15 2018 UTC vs.
Revision 1.454 by root, Tue Jun 25 05:17:50 2019 UTC

 details of the event, and then hand it over to libev by I<starting> the
 watcher.
 =head2 FEATURES
-Libev supports C<select>, C<poll>, the Linux-specific C<epoll>, the
+Libev supports C<select>, C<poll>, the Linux-specific aio and C<epoll>
-BSD-specific C<kqueue> and the Solaris-specific event port mechanisms
+interfaces, the BSD-specific C<kqueue> and the Solaris-specific event port
-for file descriptor events (C<ev_io>), the Linux C<inotify> interface
+mechanisms for file descriptor events (C<ev_io>), the Linux C<inotify>
-(for C<ev_stat>), Linux eventfd/signalfd (for faster and cleaner
+interface (for C<ev_stat>), Linux eventfd/signalfd (for faster and cleaner
 inter-thread wakeup (C<ev_async>)/signal handling (C<ev_signal>)) relative
 timers (C<ev_timer>), absolute timers with customised rescheduling
 (C<ev_periodic>), synchronous signals (C<ev_signal>), process status
 change events (C<ev_child>), and event watchers dealing with the event
 loop mechanism itself (C<ev_idle>, C<ev_embed>, C<ev_prepare> and
 You could override this function in high-availability programs to, say,
 free some memory if it cannot allocate memory, to use a special allocator,
 or even to sleep a while and retry until some memory is available.
+Example: The following is the C<realloc> function that libev itself uses
+which should work with C<realloc> and C<free> functions of all kinds and
+is probably a good basis for your own implementation.
+   static void *
+   ev_realloc_emul (void *ptr, long size) EV_NOEXCEPT
+   {
+     if (size)
+       return realloc (ptr, size);
+     free (ptr);
+     return 0;
+   }
 Example: Replace the libev allocator with one that waits a bit and then
-retries (example requires a standards-compliant C<realloc>).
+retries.
    static void *
    persistent_realloc (void *ptr, size_t size)
    {
+     if (!size)
+       {
+         free (ptr);
+         return 0;
+       }
      for (;;)
        {
          void *newptr = realloc (ptr, size);
          if (newptr)
 This backend maps C<EV_READ> to C<POLLIN | POLLERR | POLLHUP>, and
 C<EV_WRITE> to C<POLLOUT | POLLERR | POLLHUP>.
 =item C<EVBACKEND_EPOLL>   (value 4, Linux)
-Use the linux-specific epoll(7) interface (for both pre- and post-2.6.9
+Use the Linux-specific epoll(7) interface (for both pre- and post-2.6.9
 kernels).
 For few fds, this backend is a bit little slower than poll and select, but
 it scales phenomenally better. While poll and select usually scale like
 O(total_fds) where total_fds is the total number of fds (or the highest
 All this means that, in practice, C<EVBACKEND_SELECT> can be as fast or
 faster than epoll for maybe up to a hundred file descriptors, depending on
 the usage. So sad.
 While nominally embeddable in other event loops, this feature is broken in
-all kernel versions tested so far.
+a lot of kernel revisions, but probably(!) works in current versions.
 This backend maps C<EV_READ> and C<EV_WRITE> in the same way as
 C<EVBACKEND_POLL>.
+=item C<EVBACKEND_LINUXAIO>   (value 64, Linux)
+Use the Linux-specific Linux AIO (I<not> C<< aio(7) >> but C<<
+io_submit(2) >>) event interface available in post-4.18 kernels (but libev
+only tries to use it in 4.19+).
+This is another Linux train wreck of an event interface.
+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
+be well worth enabling it - if it isn't available in your kernel this will
+be detected and this backend will be skipped.
+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, forcing you to fall back to epoll, inheriting all its design
+issues.
+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 F</proc/sys/fs/aio-max-nr>. If no AIO
+requests are left, this backend will be skipped during initialisation, and
+will switch to epoll when the loop is active.
+Most problematic in practice, however, is that not all file descriptors
+work with it. For example, in Linux 5.1, TCP sockets, pipes, event fds,
+files, F</dev/null> and many others are supported, but ttys do not work
+properly (a known bug that the kernel developers don't care about, see
+L<https://lore.kernel.org/patchwork/patch/1047453/>), so this is not
+(yet?) a generic event polling interface.
+Overall, it seems the Linux developers just don't want it to have a
+generic event handling mechanism other than C<select> or C<poll>.
+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.
+This backend maps C<EV_READ> and C<EV_WRITE> in the same way as
+C<EVBACKEND_POLL>.
 =item C<EVBACKEND_KQUEUE>  (value 8, most BSD clones)
-Kqueue deserves special mention, as at the time of this writing, it
+Kqueue deserves special mention, as at the time this backend was
-was broken on all BSDs except NetBSD (usually it doesn't work reliably
+implemented, it was broken on all BSDs except NetBSD (usually it doesn't
-with anything but sockets and pipes, except on Darwin, where of course
+work reliably with anything but sockets and pipes, except on Darwin,
-it's completely useless). Unlike epoll, however, whose brokenness
+where of course it's completely useless). Unlike epoll, however, whose
-is by design, these kqueue bugs can (and eventually will) be fixed
+brokenness is by design, these kqueue bugs can be (and mostly have been)
-without API changes to existing programs. For this reason it's not being
+fixed without API changes to existing programs. For this reason it's not
-"auto-detected" unless you explicitly specify it in the flags (i.e. using
+being "auto-detected" on all platforms unless you explicitly specify it
-C<EVBACKEND_KQUEUE>) or libev was compiled on a known-to-be-good (-enough)
+in the flags (i.e. using C<EVBACKEND_KQUEUE>) or libev was compiled on a
-system like NetBSD.
+known-to-be-good (-enough) system like NetBSD.
 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
 the target platform). See C<ev_embed> watchers for more info.
 It scales in the same way as the epoll backend, but the interface to the
 kernel is more efficient (which says nothing about its actual speed, of
 course). While stopping, setting and starting an I/O watcher does never
 cause an extra system call as with C<EVBACKEND_EPOLL>, it still adds up to
 two event changes per incident. Support for C<fork ()> 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.
 This backend usually performs well under most conditions.
 While nominally embeddable in other event loops, this doesn't work
 Example: Use whatever libev has to offer, but make sure that kqueue is
 used if available.
    struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_KQUEUE);
+Example: Similarly, on linux, you mgiht want to take advantage of the
+linux aio backend if possible, but fall back to something else if that
+isn't available.
+   struct ev_loop *loop = ev_loop_new (ev_recommended_backends () | EVBACKEND_LINUXAIO);
 =item ev_loop_destroy (loop)
 Destroys an event loop object (frees all memory and kernel state
 etc.). None of the active event watchers will be stopped in the normal
 But really, best use non-blocking mode.
 =head3 The special problem of disappearing file descriptors
-Some backends (e.g. kqueue, epoll) need to be told about closing a file
+Some backends (e.g. kqueue, epoll, linuxaio) need to be told about closing
-descriptor (either due to calling C<close> explicitly or any other means,
+a file descriptor (either due to calling C<close> explicitly or any other
-such as C<dup2>). The reason is that you register interest in some file
+means, such as C<dup2>). The reason is that you register interest in some
-descriptor, but when it goes away, the operating system will silently drop
+file descriptor, but when it goes away, the operating system will silently
-this interest. If another file descriptor with the same number then is
+drop this interest. If another file descriptor with the same number then
-registered with libev, there is no efficient way to see that this is, in
+is registered with libev, there is no efficient way to see that this is,
-fact, a different file descriptor.
+in fact, a different file descriptor.
 To avoid having to explicitly tell libev about such cases, libev follows
 the following policy:  Each time C<ev_io_set> is being called, libev
 will assume that this is potentially a new file descriptor, otherwise
 it is assumed that the file descriptor stays the same. That means that
 when you rarely read from a file instead of from a socket, and want to
 reuse the same code path.
 =head3 The special problem of fork
-Some backends (epoll, kqueue) do not support C<fork ()> at all or exhibit
+Some backends (epoll, kqueue, probably linuxaio) do not support C<fork ()>
-useless behaviour. Libev fully supports fork, but needs to be told about
+at all or exhibit useless behaviour. Libev fully supports fork, but needs
-it in the child if you want to continue to use it in the child.
+to be told about it in the child if you want to continue to use it in the
+child.
 To support fork in your child processes, you have to call C<ev_loop_fork
 ()> after a fork in the child, enable C<EVFLAG_FORKCHECK>, or resort to
 C<EVBACKEND_SELECT> or C<EVBACKEND_POLL>.
 C<ev_timer>, which would still trigger roughly 10 seconds after starting
 it, as it uses a relative timeout).
 C<ev_periodic> watchers can also be used to implement vastly more complex
 timers, such as triggering an event on each "midnight, local time", or
-other complicated rules. This cannot be done with C<ev_timer> watchers, as
+other complicated rules. This cannot easily be done with C<ev_timer>
-those cannot react to time jumps.
+watchers, as those cannot react to time jumps.
 As with timers, the callback is guaranteed to be invoked only when the
 point in time where it is supposed to trigger has passed. If multiple
 timers become ready during the same loop iteration then the ones with
 earlier time-out values are invoked before ones with later time-out values
 NOTE: I<< This callback must always return a time that is higher than or
 equal to the passed C<now> value >>.
 This can be used to create very complex timers, such as a timer that
-triggers on "next midnight, local time". To do this, you would calculate the
+triggers on "next midnight, local time". To do this, you would calculate
-next midnight after C<now> and return the timestamp value for this. How
+the next midnight after C<now> and return the timestamp value for
-you do this is, again, up to you (but it is not trivial, which is the main
+this. Here is a (completely untested, no error checking) example on how to
-reason I omitted it as an example).
+do this:
+   #include <time.h>
+   static ev_tstamp
+   my_rescheduler (ev_periodic *w, ev_tstamp now)
+   {
+     time_t tnow = (time_t)now;
+     struct tm tm;
+     localtime_r (&tnow, &tm);
+     tm.tm_sec = tm.tm_min = tm.tm_hour = 0; // midnight current day
+     ++tm.tm_mday; // midnight next day
+     return mktime (&tm);
+   }
+Note: this code might run into trouble on days that have more then two
+midnights (beginning and end).
 =back
 =item ev_periodic_again (loop, ev_periodic *)
 The normal C API should work fine when used from C++: both ev.h and the
 libev sources can be compiled as C++. Therefore, code that uses the C API
 will work fine.
 Proper exception specifications might have to be added to callbacks passed
-to libev: exceptions may be thrown only from watcher callbacks, all
+to libev: exceptions may be thrown only from watcher callbacks, all other
-other callbacks (allocator, syserr, loop acquire/release and periodic
+callbacks (allocator, syserr, loop acquire/release and periodic reschedule
-reschedule callbacks) must not throw exceptions, and might need a C<throw
+callbacks) must not throw exceptions, and might need a C<noexcept>
-()> specification. If you have code that needs to be compiled as both C
+specification. If you have code that needs to be compiled as both C and
-and C++ you can use the C<EV_THROW> macro for this:
+C++ you can use the C<EV_NOEXCEPT> macro for this:
    static void
-   fatal_error (const char *msg) EV_THROW
+   fatal_error (const char *msg) EV_NOEXCEPT
    {
      perror (msg);
      abort ();
    }
    ev_win32.c      required on win32 platforms only
    ev_select.c     only when select backend is enabled
    ev_poll.c       only when poll backend is enabled
    ev_epoll.c      only when the epoll backend is enabled
+   ev_linuxaio.c   only when the linux aio backend is enabled
    ev_kqueue.c     only when the kqueue backend is enabled
    ev_port.c       only when the solaris port backend is enabled
 F<ev.c> includes the backend files directly when enabled, so you only need
 to compile this single file.
 If defined to be C<1>, libev will compile in support for the Linux
 C<epoll>(7) backend. Its availability will be detected at runtime,
 otherwise another method will be used as fallback. This is the preferred
 backend for GNU/Linux systems. If undefined, it will be enabled if the
 headers indicate GNU/Linux + Glibc 2.4 or newer, otherwise disabled.
+=item EV_USE_LINUXAIO
+If defined to be C<1>, libev will compile in support for the Linux
+aio backend. Due to it's currenbt limitations it has to be requested
+explicitly.  If undefined, it will be enabled on linux, otherwise
+disabled.
 =item EV_USE_KQUEUE
 If defined to be C<1>, libev will compile in support for the BSD style
 C<kqueue>(2) backend. Its actual availability will be detected at runtime,

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Comparing libev/ev.pod (file contents): Revision 1.443 by root, Thu Aug 30 21:51:15 2018 UTC vs. Revision 1.454 by root, Tue Jun 25 05:17:50 2019 UTC

Diff Legend

Comparing libev/ev.pod (file contents):
Revision 1.443 by root, Thu Aug 30 21:51:15 2018 UTC vs.
Revision 1.454 by root, Tue Jun 25 05:17:50 2019 UTC