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

Comparing libev/ev.pod (file contents):
Revision 1.204 by root, Mon Oct 27 11:08:29 2008 UTC vs.
Revision 1.216 by root, Thu Nov 13 15:55:38 2008 UTC

 If you don't know what event loop to use, use the one returned from this
 function.
 Note that this function is I<not> thread-safe, so if you want to use it
 from multiple threads, you have to lock (note also that this is unlikely,
-as loops cannot bes hared easily between threads anyway).
+as loops cannot be shared easily between threads anyway).
 The default loop is the only loop that can handle C<ev_signal> and
 C<ev_child> watchers, and to do this, it always registers a handler
 for C<SIGCHLD>. If this is a problem for your application you can either
 create a dynamic loop with C<ev_loop_new> that doesn't do that, or you
 =item C<EVBACKEND_EPOLL>   (value 4, Linux)
 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 n is the total number of fds (or the highest fd),
-epoll scales either O(1) or O(active_fds). The epoll design has a number
+epoll scales either O(1) or O(active_fds).
-of shortcomings, such as silently dropping events in some hard-to-detect
-cases and requiring a system call per fd change, no fork support and bad
-support for dup.
+The epoll mechanism deserves honorable mention as the most misdesigned
+of the more advanced event mechanisms: mere annoyances include silently
+dropping file descriptors, requiring a system call per change per file
+descriptor (and unnecessary guessing of parameters), problems with dup and
+so on. The biggest issue is fork races, however - if a program forks then
+I<both> parent and child process have to recreate the epoll set, which can
+take considerable time (one syscall per file descriptor) and is of course
+hard to detect.
-Epoll is also notoriously buggy - embedding epoll fds should work, but
+Epoll is also notoriously buggy - embedding epoll fds I<should> work, but
-of course doesn't, and epoll just loves to report events for totally
+of course I<doesn't>, and epoll just loves to report events for totally
-I<different> file descriptors (even already closed ones) than registered
+I<different> file descriptors (even already closed ones, so one cannot
-in the set (especially on SMP systems). Libev tries to counter these
+even remove them from the set) than registered in the set (especially
-spurious notifications by employing an additional generation counter and
+on SMP systems). Libev tries to counter these spurious notifications by
-comparing that against the events to filter out spurious ones.
+employing an additional generation counter and comparing that against the
+events to filter out spurious ones, recreating the set when required.
 While stopping, setting and starting an I/O watcher in the same iteration
-will result in some caching, there is still a system call per such incident
+will result in some caching, there is still a system call per such
-(because the fd could point to a different file description now), so its
+incident (because the same I<file descriptor> could point to a different
-best to avoid that. Also, C<dup ()>'ed file descriptors might not work
+I<file description> now), so its best to avoid that. Also, C<dup ()>'ed
-very well if you register events for both fds.
+file descriptors might not work very well if you register events for both
+file descriptors.
 Best performance from this backend is achieved by not unregistering all
 watchers for a file descriptor until it has been closed, if possible,
 i.e. keep at least one watcher active per fd at all times. Stopping and
 starting a watcher (without re-setting it) also usually doesn't cause
-extra overhead.
+extra overhead. A fork can both result in spurious notifications as well
+as in libev having to destroy and recreate the epoll object, which can
+take considerable time and thus should be avoided.
+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.
 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 was
+Kqueue deserves special mention, as at the time of this writing, it
-broken on all BSDs except NetBSD (usually it doesn't work reliably with
+was broken on all BSDs except NetBSD (usually it doesn't work reliably
-anything but sockets and pipes, except on Darwin, where of course it's
+with anything but sockets and pipes, except on Darwin, where of course
-completely useless). For this reason it's not being "auto-detected" unless
+it's completely useless). Unlike epoll, however, whose brokenness
-you explicitly specify it in the flags (i.e. using C<EVBACKEND_KQUEUE>) or
+is by design, these kqueue bugs can (and eventually will) be fixed
-libev was compiled on a known-to-be-good (-enough) system like NetBSD.
+without API changes to existing programs. For this reason it's not being
+"auto-detected" unless you explicitly specify it in the flags (i.e. using
+C<EVBACKEND_KQUEUE>) or libev was compiled on a 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 and it
+two event changes per incident. Support for C<fork ()> is very bad (but
-drops fds silently in similarly hard-to-detect cases.
+sane, unlike 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
 everywhere, so you might need to test for this. And since it is broken
 might perform better.
 On the positive side, with the exception of the spurious readiness
 notifications, this backend actually performed fully to specification
 in all tests and is fully embeddable, which is a rare feat among the
-OS-specific backends.
+OS-specific backends (I vastly prefer correctness over speed hacks).
 This backend maps C<EV_READ> and C<EV_WRITE> in the same way as
 C<EVBACKEND_POLL>.
 =item C<EVBACKEND_ALL>
 the loop.
 A flags value of C<EVLOOP_ONESHOT> will look for new events (waiting if
 necessary) and will handle those and any already outstanding ones. It
 will block your process until at least one new event arrives (which could
-be an event internal to libev itself, so there is no guarentee that a
+be an event internal to libev itself, so there is no guarantee that a
 user-registered callback will be called), and will return after one
 iteration of the loop.
 This is useful if you are waiting for some external event in conjunction
 with something not expressible using other libev watchers (i.e. "roll your
      else
        {
          // callback was invoked, but there was some activity, re-arm
          // the watcher to fire in last_activity + 60, which is
          // guaranteed to be in the future, so "again" is positive:
-         w->again = timeout - now;
+         w->repeat = timeout - now;
          ev_timer_again (EV_A_ w);
        }
    }
 To summarise the callback: first calculate the real timeout (defined
 =head2 C<ev_stat> - did the file attributes just change?
 This watches a file system path for attribute changes. That is, it calls
-C<stat> regularly (or when the OS says it changed) and sees if it changed
+C<stat> on that path in regular intervals (or when the OS says it changed)
-compared to the last time, invoking the callback if it did.
+and sees if it changed compared to the last time, invoking the callback if
+it did.
 The path does not need to exist: changing from "path exists" to "path does
-not exist" is a status change like any other. The condition "path does
+not exist" is a status change like any other. The condition "path does not
-not exist" is signified by the C<st_nlink> field being zero (which is
+exist" (or more correctly "path cannot be stat'ed") is signified by the
-otherwise always forced to be at least one) and all the other fields of
+C<st_nlink> field being zero (which is otherwise always forced to be at
-the stat buffer having unspecified contents.
+least one) and all the other fields of the stat buffer having unspecified
+contents.
-The path I<should> be absolute and I<must not> end in a slash. If it is
+The path I<must not> end in a slash or contain special components such as
+C<.> or C<..>. The path I<should> be absolute: If it is relative and
-relative and your working directory changes, the behaviour is undefined.
+your working directory changes, then the behaviour is undefined.
-Since there is no standard kernel interface to do this, the portable
+Since there is no portable change notification interface available, the
-implementation simply calls C<stat (2)> regularly on the path to see if
+portable implementation simply calls C<stat(2)> regularly on the path
-it changed somehow. You can specify a recommended polling interval for
+to see if it changed somehow. You can specify a recommended polling
-this case. If you specify a polling interval of C<0> (highly recommended!)
+interval for this case. If you specify a polling interval of C<0> (highly
-then a I<suitable, unspecified default> value will be used (which
+recommended!) then a I<suitable, unspecified default> value will be used
-you can expect to be around five seconds, although this might change
+(which you can expect to be around five seconds, although this might
-dynamically). Libev will also impose a minimum interval which is currently
+change dynamically). Libev will also impose a minimum interval which is
-around C<0.1>, but thats usually overkill.
+currently around C<0.1>, but that's usually overkill.
 This watcher type is not meant for massive numbers of stat watchers,
 as even with OS-supported change notifications, this can be
 resource-intensive.
 At the time of this writing, the only OS-specific interface implemented
-is the Linux inotify interface (implementing kqueue support is left as
+is the Linux inotify interface (implementing kqueue support is left as an
-an exercise for the reader. Note, however, that the author sees no way
+exercise for the reader. Note, however, that the author sees no way of
-of implementing C<ev_stat> semantics with kqueue).
+implementing C<ev_stat> semantics with kqueue, except as a hint).
 =head3 ABI Issues (Largefile Support)
 Libev by default (unless the user overrides this) uses the default
 compilation environment, which means that on systems with large file
 support disabled by default, you get the 32 bit version of the stat
 structure. When using the library from programs that change the ABI to
 use 64 bit file offsets the programs will fail. In that case you have to
 compile libev with the same flags to get binary compatibility. This is
 obviously the case with any flags that change the ABI, but the problem is
-most noticeably disabled with ev_stat and large file support.
+most noticeably displayed with ev_stat and large file support.
 The solution for this is to lobby your distribution maker to make large
 file interfaces available by default (as e.g. FreeBSD does) and not
 optional. Libev cannot simply switch on large file support because it has
 to exchange stat structures with application programs compiled using the
 default compilation environment.
 =head3 Inotify and Kqueue
-When C<inotify (7)> support has been compiled into libev (generally
+When C<inotify (7)> support has been compiled into libev and present at
-only available with Linux 2.6.25 or above due to bugs in earlier
+runtime, it will be used to speed up change detection where possible. The
-implementations) and present at runtime, it will be used to speed up
+inotify descriptor will be created lazily when the first C<ev_stat>
-change detection where possible. The inotify descriptor will be created
+watcher is being started.
-lazily when the first C<ev_stat> watcher is being started.
 Inotify presence does not change the semantics of C<ev_stat> watchers
 except that changes might be detected earlier, and in some cases, to avoid
 making regular C<stat> calls. Even in the presence of inotify support
 there are many cases where libev has to resort to regular C<stat> polling,
-but as long as the path exists, libev usually gets away without polling.
+but as long as kernel 2.6.25 or newer is used (2.6.24 and older have too
+many bugs), the path exists (i.e. stat succeeds), and the path resides on
+a local filesystem (libev currently assumes only ext2/3, jfs, reiserfs and
+xfs are fully working) libev usually gets away without polling.
 There is no support for kqueue, as apparently it cannot be used to
 implement this functionality, due to the requirement of having a file
 descriptor open on the object at all times, and detecting renames, unlinks
 etc. is difficult.
+=head3 C<stat ()> is a synchronous operation
+Libev doesn't normally do any kind of I/O itself, and so is not blocking
+the process. The exception are C<ev_stat> watchers - those call C<stat
+()>, which is a synchronous operation.
+For local paths, this usually doesn't matter: unless the system is very
+busy or the intervals between stat's are large, a stat call will be fast,
+as the path data is suually in memory already (except when starting the
+watcher).
+For networked file systems, calling C<stat ()> can block an indefinite
+time due to network issues, and even under good conditions, a stat call
+often takes multiple milliseconds.
+Therefore, it is best to avoid using C<ev_stat> watchers on networked
+paths, although this is fully supported by libev.
 =head3 The special problem of stat time resolution
-The C<stat ()> system call only supports full-second resolution portably, and
+The C<stat ()> system call only supports full-second resolution portably,
-even on systems where the resolution is higher, most file systems still
+and even on systems where the resolution is higher, most file systems
-only support whole seconds.
+still only support whole seconds.
 That means that, if the time is the only thing that changes, you can
 easily miss updates: on the first update, C<ev_stat> detects a change and
 calls your callback, which does something. When there is another update
 within the same second, C<ev_stat> will be unable to detect unless the
 =over 4
 =item ev_async_init (ev_async *, callback)
 Initialises and configures the async watcher - it has no parameters of any
-kind. There is a C<ev_asynd_set> macro, but using it is utterly pointless,
+kind. There is a C<ev_async_set> macro, but using it is utterly pointless,
 trust me.
 =item ev_async_send (loop, ev_async *)
 Sends/signals/activates the given C<ev_async> watcher, that is, feeds
    #define EV_STANDALONE 1
    #include "ev.h"
 Both header files and implementation files can be compiled with a C++
-compiler (at least, thats a stated goal, and breakage will be treated
+compiler (at least, that's a stated goal, and breakage will be treated
 as a bug).
 You need the following files in your source tree, or in a directory
 in your include path (e.g. in libev/ when using -Ilibev):
 loop, as long as you don't confuse yourself). The only exception is that
 you must not do this from C<ev_periodic> reschedule callbacks.
 Care has been taken to ensure that libev does not keep local state inside
 C<ev_loop>, and other calls do not usually allow for coroutine switches as
-they do not clal any callbacks.
+they do not call any callbacks.
 =head2 COMPILER WARNINGS
 Depending on your compiler and compiler settings, you might get no or a
 lot of warnings when compiling libev code. Some people are apparently
    ==2274==    definitely lost: 0 bytes in 0 blocks.
    ==2274==      possibly lost: 0 bytes in 0 blocks.
    ==2274==    still reachable: 256 bytes in 1 blocks.
 Then there is no memory leak, just as memory accounted to global variables
-is not a memleak - the memory is still being refernced, and didn't leak.
+is not a memleak - the memory is still being referenced, and didn't leak.
 Similarly, under some circumstances, valgrind might report kernel bugs
 as if it were a bug in libev (e.g. in realloc or in the poll backend,
 although an acceptable workaround has been found here), or it might be
 confused.
 =back
 =head1 AUTHOR
-Marc Lehmann <libev@schmorp.de>.
+Marc Lehmann <libev@schmorp.de>, with repeated corrections by Mikael Magnusson.

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Comparing libev/ev.pod (file contents): Revision 1.204 by root, Mon Oct 27 11:08:29 2008 UTC vs. Revision 1.216 by root, Thu Nov 13 15:55:38 2008 UTC

Diff Legend

Comparing libev/ev.pod (file contents):
Revision 1.204 by root, Mon Oct 27 11:08:29 2008 UTC vs.
Revision 1.216 by root, Thu Nov 13 15:55:38 2008 UTC