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

[Nick Mathewson <nickm@xxxxxxxxxxxxx>]

On Tue, Apr 10, 2012 at 1:03 AM, William Ahern
<william@xxxxxxxxxxxxxxxxxx> wrote:
> On Tue, Apr 10, 2012 at 11:44:39AM +0800, ååé wrote:
>> Hi all,
>>
>> I found an issue that it will trigger the timer callback dramatically
>> like a dead looping when moving the system time backward for a year
>> and then moving it forward to the accurate time.
>
> AFAICT, the libevent 2.x code uses clock_gettime(CLOCK_MONOTONIC) if
> available.
>
> Does your system provide CLOCK_MONOTONIC? Most do (including the ones I use
> the most, Linux and OpenBSD).

I think he's on Windows.

With Windows, your options for monotonic timekeeping seem to be
QueryPerformanceCounter, and GetTickCount.  Neither one is really
great from what I can tell: QueryPerformanceCounter is slow and has
known bugs on some older windows versions and some VMs, whereas
GetTickCount only has a 1 ms resolution and 15 ms accuracy by default.

> One notable exception is OS X. But OS X _does_
> provide a monotonic counter. If you're using OS X, try this compat wrapper:

I actually added support for the OSX monotonic counter to 2.1.x just
yesterday in commit b8fd6f918b4540.  If somebody who knows the
mach/osx stuff a bit better than I do wants to check it out, that
would be great.

I found PEP ( 418 http://www.python.org/dev/peps/pep-0418 ) to be a
pretty useful summary of cross-platform monotonic clocks, such as they
are.

One remaining issue with monotonic clocks is that they don't all
behave the same when the system sleeps and resumes.  Some of them act
as if the missing time never happened (which is probably more like
what Libevent needs), but others treat the clock as having jumped
forward, which could cause the same undesirable behavior that the
original poster is seeing with periodic timer events.

yrs,
-- 
Nick
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