Re: [Libevent-users] a dead looping bug when changing system time backward

[Nick Mathewson <nickm@xxxxxxxxxxxxx>]

On Tue, Apr 10, 2012 at 7:21 PM, Nir Soffer <nirsof@xxxxxxxxx> wrote:
>
> On Apr 10, 2012, at 6:15 PM, Nick Mathewson wrote:
>
>> And there's a third way for libevent to see a big jump forward in
>> time: if the program calls event_base_loop() sporadically, it is free
>> to wait as long as it wants between invocations.
>>
>> So, what's the right behavior for periodic events in these cases?  If
>> there is an event that's supposed to run every 5 seconds, and time has
>> jumped forward by 16 seconds, it seems reasonable to run the event 1
>> time, or maybe even 3 times... but if time has jumped forward by one
>> day, it seems unlikely that the programmer really wants us to run the
>> event 17280 times.
>>
>> Perhaps this argues for a cap on how far into the past we should be
>> willing to reschedule a periodic event, or how many "missed firings"
>> we'll compensate for before we drop some on the floor?
>
>
> Forward jumps should simply skip events in the past. It is just like events
> skipped because the event loop was blocked.
>
> If you should run every 5 seconds, but woke up 21 seconds late, you cannot
> fix the past - you just missed the events at +5, +10, +15 and +20. Run the
> next event at +25 according to the plan.
>
> Other event loops like Cocoa NSRunloop and GNUStep NSRunloop use this logic.

I've implemented approximately this logic in branch
"20_periodic_event_overload" in my github repository.  (That's where I
do most of my work before merging it onto an official branch; it lives
at https://github.com/nmathewson/libevent .)

I've also got a basic GetTickCount()-based monotonic timer callback
for windows merged into the master branch.  I'm going to be extending
it a little for accuracy, based on some clever hacks from Bittorrent's
libutp.

-- 
Nick
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