AFAIK this optimization in Linux is only available on some architectures and there is a knob with several possible settings.
Without this, gettimeofday() can be quite expensive. On Wed, Jul 27, 2011 at 01:10:05PM -0700, William Ahern wrote: > On Wed, Jul 27, 2011 at 02:49:26PM -0400, Nick Mathewson wrote: > <snip> > > So because I'm at a conference this week, I can't examine the code too > > closely. But my guess is that this is happening because Libevent > > caches its calls to gettimeofday()/clock_gettime() during its event > > loop, so that your event is being scheduled relative to the time when > > epoll returned, not quite relative to the time when event_add() is > > called. > > > > In Libevent 2.0, you can override this behavior with the > > EVENT_BASE_FLAG_NO_CACHE_TIME flag. This will result in more needless > > system calls, but more precise timing. > > > > If you're running on Linux and you're interested in precise timings, > > you should also be aware that the time resolution of epoll() is only > > one ms. I'd like for Libevent 2.1 to add timerfd support to the epoll > > backend, but I don't have a patch for that right now. > > Has anyone tested the performance gain of caching? Linux's gettimeofday() > doesn't trap into the kernel; it effectively just reads a shared memory > page. IIRC it's something like 100x-1000x faster than gettimeofday() in > OpenBSD, which (again, IIRC) reads from the hardware clock every call (Linux > usually has an HPET update the clock asynchronously). > > After I did those measurements I quit pursuing a caching strategy in my own > code. (I don't use OpenBSD for it's performance.) Maybe the default setting > could be different on different platforms, or the default inverted so > caching must be requested. > > *********************************************************************** > To unsubscribe, send an e-mail to majord...@freehaven.net with > unsubscribe libevent-users in the body. *********************************************************************** To unsubscribe, send an e-mail to majord...@freehaven.net with unsubscribe libevent-users in the body.
