Re: 2.6 vs 2.4 regression when running gnomemeeting

Ingo Molnar
2003-12-22T08:47:36Z

* Christian Meder <chris@onestepahead.de> wrote:

> > nice -19 ./loop &
> >
> > do a couple of such loops still degrade gnomemeeting?
>
> I found the culprit. It's sched_yield again. When I straced
> gnomemeeting even without load I saw a lot of sched_yields. [...]

this is definitely broken code. Such code already causes big CPU
overhead in certain circumstances (under 2.4 too) - but in 2.6 it also
shows up as an interactivity problem. So 2.4 hid the problem, 2.6
exposes it.

> So the questionable code in pwlib is probably:

> > BOOL PSemaphore::Wait(const PTimeInterval & waitTime)

yeah. pwlib should be fixed. The quick fix is, instead of sched_yield(),
to do:

{
struct timespec timer = { 0, 1 };

nanosleep (&timer, NULL);
}

this does what pwlib really wants to do: sleep for the shortest amount
of time posssible, because its semaphore implementation is polling
based.

(but pwlib should perhaps use sem_timedwait(sem, abs_timeout) instead -
which does exactly what PSemaphore::Wait() tries to implement.)

Ingo