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

Comparing libev/ev.pod (file contents):
Revision 1.105 by root, Sun Dec 23 03:50:10 2007 UTC vs.
Revision 1.108 by root, Mon Dec 24 10:39:21 2007 UTC

 semantics of C<ev_stat> watchers, which means that libev sometimes needs
 to fall back to regular polling again even with inotify, but changes are
 usually detected immediately, and if the file exists there will be no
 polling.
+=head3 Inotify
+When C<inotify (7)> support has been compiled into libev (generally only
+available on Linux) and present at runtime, it will be used to speed up
+change detection where possible. The inotify descriptor will be created lazily
+when the first C<ev_stat> watcher is being started.
+Inotify presense 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 presense of inotify support
+there are many cases where libev has to resort to regular C<stat> 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).
+=head3 The special problem of stat time resolution
+The C<stat ()> syscall only supports full-second resolution portably, and
+even on systems where the resolution is higher, many filesystems still
+only support whole seconds.
+That means that, if the time is the only thing that changes, you might
+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 it.
+The solution to this is to delay acting on a change for a second (or till
+the next second boundary), using a roughly one-second delay C<ev_timer>
+(C<ev_timer_set (w, 0., 1.01); ev_timer_again (loop, w)>). The C<.01>
+is added to work around small timing inconsistencies of some operating
+systems.
 =head3 Watcher-Specific Functions and Data Members
 =over 4
 =item ev_stat_init (ev_stat *, callback, const char *path, ev_tstamp interval)
 =item const char *path [read-only]
 The filesystem path that is being watched.
 =back
+=head3 Examples
 Example: Watch C</etc/passwd> for attribute changes.
   static void
   passwd_cb (struct ev_loop *loop, ev_stat *w, int revents)
   }
   ...
   ev_stat passwd;
-  ev_stat_init (&passwd, passwd_cb, "/etc/passwd");
+  ev_stat_init (&passwd, passwd_cb, "/etc/passwd", 0.);
   ev_stat_start (loop, &passwd);
+Example: Like above, but additionally use a one-second delay so we do not
+miss updates (however, frequent updates will delay processing, too, so
+one might do the work both on C<ev_stat> callback invocation I<and> on
+C<ev_timer> callback invocation).
+  static ev_stat passwd;
+  static ev_timer timer;
+  static void
+  timer_cb (EV_P_ ev_timer *w, int revents)
+  {
+    ev_timer_stop (EV_A_ w);
+    /* now it's one second after the most recent passwd change */
+  }
+  static void
+  stat_cb (EV_P_ ev_stat *w, int revents)
+  {
+    /* reset the one-second timer */
+    ev_timer_again (EV_A_ &timer);
+  }
+  ...
+  ev_stat_init (&passwd, stat_cb, "/etc/passwd", 0.);
+  ev_stat_start (loop, &passwd);
+  ev_timer_init (&timer, timer_cb, 0., 1.01);
 =head2 C<ev_idle> - when you've got nothing better to do...
 Idle watchers trigger events when no other events of the same or higher
 =item Starting and stopping timer/periodic watchers: O(log skipped_other_timers)
 This means that, when you have a watcher that triggers in one hour and
 there are 100 watchers that would trigger before that then inserting will
-have to skip those 100 watchers.
+have to skip roughly seven (C<ld 100>) of these watchers.
-=item Changing timer/periodic watchers (by autorepeat, again): O(log skipped_other_timers)
+=item Changing timer/periodic watchers (by autorepeat or calling again): O(log skipped_other_timers)
-That means that for changing a timer costs less than removing/adding them
+That means that changing a timer costs less than removing/adding them
 as only the relative motion in the event queue has to be paid for.
 =item Starting io/check/prepare/idle/signal/child watchers: O(1)
 These just add the watcher into an array or at the head of a list.
 =item Stopping check/prepare/idle watchers: O(1)
 =item Stopping an io/signal/child watcher: O(number_of_watchers_for_this_(fd/signal/pid % EV_PID_HASHSIZE))
 These watchers are stored in lists then need to be walked to find the
 correct watcher to remove. The lists are usually short (you don't usually
 have many watchers waiting for the same fd or signal).
-=item Finding the next timer per loop iteration: O(1)
+=item Finding the next timer in each loop iteration: O(1)
+By virtue of using a binary heap, the next timer is always found at the
+beginning of the storage array.
 =item Each change on a file descriptor per loop iteration: O(number_of_watchers_for_this_fd)
 A change means an I/O watcher gets started or stopped, which requires
-libev to recalculate its status (and possibly tell the kernel).
+libev to recalculate its status (and possibly tell the kernel, depending
+on backend and wether C<ev_io_set> was used).
-=item Activating one watcher: O(1)
+=item Activating one watcher (putting it into the pending state): O(1)
 =item Priority handling: O(number_of_priorities)
 Priorities are implemented by allocating some space for each
 priority. When doing priority-based operations, libev usually has to
-linearly search all the priorities.
+linearly search all the priorities, but starting/stopping and activating
+watchers becomes O(1) w.r.t. prioritiy handling.
 =back
 =head1 AUTHOR

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Comparing libev/ev.pod (file contents): Revision 1.105 by root, Sun Dec 23 03:50:10 2007 UTC vs. Revision 1.108 by root, Mon Dec 24 10:39:21 2007 UTC

Diff Legend

Comparing libev/ev.pod (file contents):
Revision 1.105 by root, Sun Dec 23 03:50:10 2007 UTC vs.
Revision 1.108 by root, Mon Dec 24 10:39:21 2007 UTC