--- libev/ev.pod	2009/03/04 12:51:37	1.226
+++ libev/ev.pod	2009/03/04 14:33:10	1.227
@@ -1598,52 +1598,62 @@
 Periodic watchers are also timers of a kind, but they are very versatile
 (and unfortunately a bit complex).
 
-Unlike C<ev_timer>'s, they are not based on real time (or relative time)
-but on wall clock time (absolute time). You can tell a periodic watcher
-to trigger after some specific point in time. For example, if you tell a
-periodic watcher to trigger in 10 seconds (by specifying e.g. C<ev_now ()
-+ 10.>, that is, an absolute time not a delay) and then reset your system
-clock to January of the previous year, then it will take more than year
-to trigger the event (unlike an C<ev_timer>, which would still trigger
-roughly 10 seconds later as it uses a relative timeout).
-
-C<ev_periodic>s can also be used to implement vastly more complex timers,
-such as triggering an event on each "midnight, local time", or other
-complicated rules.
+Unlike C<ev_timer>, periodic watchers are not based on real time (or
+relative time, the physical time that passes) but on wall clock time
+(absolute time, the thing you can read on your calender or clock). The
+difference is that wall clock time can run faster or slower than real
+time, and time jumps are not uncommon (e.g. when you adjust your
+wrist-watch).
+
+You can tell a periodic watcher to trigger after some specific point
+in time: for example, if you tell a periodic watcher to trigger "in 10
+seconds" (by specifying e.g. C<ev_now () + 10.>, that is, an absolute time
+not a delay) and then reset your system clock to January of the previous
+year, then it will take a year or more to trigger the event (unlike an
+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
+those cannot react to time jumps.
 
 As with timers, the callback is guaranteed to be invoked only when the
-time (C<at>) has passed, but if multiple periodic timers become ready
-during the same loop iteration, then order of execution is undefined.
+point in time where it is supposed to trigger has passed, but if multiple
+periodic timers become ready during the same loop iteration, then order of
+execution is undefined.
 
 =head3 Watcher-Specific Functions and Data Members
 
 =over 4
 
-=item ev_periodic_init (ev_periodic *, callback, ev_tstamp at, ev_tstamp interval, reschedule_cb)
+=item ev_periodic_init (ev_periodic *, callback, ev_tstamp offset, ev_tstamp interval, reschedule_cb)
 
-=item ev_periodic_set (ev_periodic *, ev_tstamp at, ev_tstamp interval, reschedule_cb)
+=item ev_periodic_set (ev_periodic *, ev_tstamp offset, ev_tstamp interval, reschedule_cb)
 
-Lots of arguments, lets sort it out... There are basically three modes of
+Lots of arguments, let's sort it out... There are basically three modes of
 operation, and we will explain them from simplest to most complex:
 
 =over 4
 
-=item * absolute timer (at = time, interval = reschedule_cb = 0)
+=item * absolute timer (offset = absolute time, interval = 0, reschedule_cb = 0)
 
 In this configuration the watcher triggers an event after the wall clock
-time C<at> has passed. It will not repeat and will not adjust when a time
-jump occurs, that is, if it is to be run at January 1st 2011 then it will
-only run when the system clock reaches or surpasses this time.
+time C<offset> has passed. It will not repeat and will not adjust when a
+time jump occurs, that is, if it is to be run at January 1st 2011 then it
+will be stopped and invoked when the system clock reaches or surpasses
+this point in time.
 
-=item * repeating interval timer (at = offset, interval > 0, reschedule_cb = 0)
+=item * repeating interval timer (offset = offset within interval, interval > 0, reschedule_cb = 0)
 
 In this mode the watcher will always be scheduled to time out at the next
-C<at + N * interval> time (for some integer N, which can also be negative)
-and then repeat, regardless of any time jumps.
+C<offset + N * interval> time (for some integer N, which can also be
+negative) and then repeat, regardless of any time jumps. The C<offset>
+argument is merely an offset into the C<interval> periods.
 
 This can be used to create timers that do not drift with respect to the
-system clock, for example, here is a C<ev_periodic> that triggers each
-hour, on the hour:
+system clock, for example, here is an C<ev_periodic> that triggers each
+hour, on the hour (with respect to UTC):
 
    ev_periodic_set (&periodic, 0., 3600., 0);
 
@@ -1654,9 +1664,9 @@
 
 Another way to think about it (for the mathematically inclined) is that
 C<ev_periodic> will try to run the callback in this mode at the next possible
-time where C<time = at (mod interval)>, regardless of any time jumps.
+time where C<time = offset (mod interval)>, regardless of any time jumps.
 
-For numerical stability it is preferable that the C<at> value is near
+For numerical stability it is preferable that the C<offset> value is near
 C<ev_now ()> (the current time), but there is no range requirement for
 this value, and in fact is often specified as zero.
 
@@ -1665,15 +1675,16 @@
 will of course deteriorate. Libev itself tries to be exact to be about one
 millisecond (if the OS supports it and the machine is fast enough).
 
-=item * manual reschedule mode (at and interval ignored, reschedule_cb = callback)
+=item * manual reschedule mode (offset ignored, interval ignored, reschedule_cb = callback)
 
-In this mode the values for C<interval> and C<at> are both being
+In this mode the values for C<interval> and C<offset> are both being
 ignored. Instead, each time the periodic watcher gets scheduled, the
 reschedule callback will be called with the watcher as first, and the
 current time as second argument.
 
-NOTE: I<This callback MUST NOT stop or destroy any periodic watcher,
-ever, or make ANY other event loop modifications whatsoever>.
+NOTE: I<This callback MUST NOT stop or destroy any periodic watcher, ever,
+or make ANY other event loop modifications whatsoever, unless explicitly
+allowed by documentation here>.
 
 If you need to stop it, return C<now + 1e30> (or so, fudge fudge) and stop
 it afterwards (e.g. by starting an C<ev_prepare> watcher, which is the
@@ -1713,13 +1724,16 @@
 
 =item ev_tstamp ev_periodic_at (ev_periodic *)
 
-When active, returns the absolute time that the watcher is supposed to
-trigger next.
+When active, returns the absolute time that the watcher is supposed
+to trigger next. This is not the same as the C<offset> argument to
+C<ev_periodic_set>, but indeed works even in interval and manual
+rescheduling modes.
 
 =item ev_tstamp offset [read-write]
 
 When repeating, this contains the offset value, otherwise this is the
-absolute point in time (the C<at> value passed to C<ev_periodic_set>).
+absolute point in time (the C<offset> value passed to C<ev_periodic_set>,
+although libev might modify this value for better numerical stability).
 
 Can be modified any time, but changes only take effect when the periodic
 timer fires or C<ev_periodic_again> is being called.
@@ -2684,9 +2698,14 @@
 similar contexts (see the discussion of C<EV_ATOMIC_T> in the embedding
 section below on what exactly this means).
 
-This call incurs the overhead of a system call only once per loop iteration,
-so while the overhead might be noticeable, it doesn't apply to repeated
-calls to C<ev_async_send>.
+Note that, as with other watchers in libev, multiple events might get
+compressed into a single callback invocation (another way to look at this
+is that C<ev_async> watchers are level-triggered, set on C<ev_async_send>,
+reset when the event loop detects that).
+
+This call incurs the overhead of a system call only once per event loop
+iteration, so while the overhead might be noticeable, it doesn't apply to
+repeated calls to C<ev_async_send> for the same event loop.
 
 =item bool = ev_async_pending (ev_async *)
 
@@ -2699,8 +2718,10 @@
 it will reset the flag again. C<ev_async_pending> can be used to very
 quickly check whether invoking the loop might be a good idea.
 
-Not that this does I<not> check whether the watcher itself is pending, only
-whether it has been requested to make this watcher pending.
+Not that this does I<not> check whether the watcher itself is pending,
+only whether it has been requested to make this watcher pending: there
+is a time window between the event loop checking and resetting the async
+notification, and the callback being invoked.
 
 =back