Just FYI,
gettimeofday() is expensive on platforms (like FreeBSD) which actually attempt
to return very precise timestamps.
Adrian
On Tue, Apr 14, 2009, Haiping Zhao wrote:
> Niels,
>
> I found out why. 1.4.9 has a tv_cache in gettime():
>
> static int
> gettime(struct event_base *base, struct timeval *tp){
> if (base->tv_cache.tv_sec) {
> *tp = base->tv_cache;
> return (0);
> }
> ....(omitted)...
> }
>
> We happened to use timer as our message pump (my original statement about
> using timer as timeout management wasn't quite right after I read the code)
> to process some UDP packets. But then, this cached timestamp can be 1 to 3ms
> out of date in our code. So the whole pumping was slowed down 2x. I confirmed
> this by dumping all timeout_process()'s timestamps when an event is detected
> as "timed-out", and all of them were shifted or lagged behind than the ones
> collected with 1.3c build.
>
> Anyways, I'll have to re-think our model. At the same time, may I ask why the
> change? Was that for calling gettimeofday() less number of times to be more
> efficient? But the code only updates base->tv_cache once per loop, each of
> which may take several milliseconds to finish. Doesn't that leave huge space
> for an inaccurate timestamp?
>
> -Haiping
>
>
> On 4/14/09 4:59 PM, "Niels Provos" <[email protected]> wrote:
>
> On Tue, Apr 14, 2009 at 3:37 PM, Haiping Zhao <[email protected]> wrote:
> > We have a piece of code that was using 1.3c, and when I switched it to use
> > 1.4.9-stable, I found it's 2x slower, spending almost all extra time in I/O
> > waiting. This piece of code does simple UDP handling, and we do have timers
> > going on for timeout management.
>
> I am not aware of anything that would have made libevent slower
> between 1.3 and 1.4. It should be slightly faster actually.
>
> What do you mean by I/O waiting? Your code is blocking on IO?
>
> Niels.
>
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