On Sat, 2006-03-11 at 14:50 +1100, Con Kolivas wrote:
> On Saturday 11 March 2006 09:35, Andrew Morton wrote:
> > Con Kolivas <[email protected]> wrote:
> > > + /*
> > > + * get_page_state is super expensive so we only perform it every
> > > + * SWAP_CLUSTER_MAX prefetched_pages.
> >
> > nr_running() is similarly expensive btw.
>
> Yes which is why I do it just as infrequently as get_page_state.
> >
> > > * We also test if we're the only
> > > + * task running anywhere. We want to have as little impact on all
> > > + * resources (cpu, disk, bus etc). As this iterates over every cpu
> > > + * we measure this infrequently.
> > > + */
> > > + if (!(sp_stat.prefetched_pages % SWAP_CLUSTER_MAX)) {
> > > + unsigned long cpuload = nr_running();
> > > +
> > > + if (cpuload > 1)
> > > + goto out;
> >
> > Sorry, this is just wrong. If swap prefetch is useful then it's also
> > useful if some task happens to be sitting over in the corner calculating
> > pi.
> >
> > What's the actual problem here? Someone's 3d game went blippy? Why? How
> > much? Are we missing a cond_resched()?
>
> No, it's pretty easy to reproduce, kprefetchd sits there in uninterruptible
> sleep with one cpu on SMP pegged at 100% iowait due to it. This tends to have
> noticeable effects everywhere on HT or SMP. On UP the yielding helped it but
> even then it still causes blips. How much? Well to be honest it's noticeable
> a shipload. Running a game, any game, that uses 100% (and most fancy games
> do) causes stuttering on audio, pauses and so on. This is evident on linux
> native games, games under emulators or qemu and so on. That iowait really
> hurts, and tweaking just priority doesn't help it in any way.
That doesn't really make sense to me. If a task can trigger audio
dropout and stalls by sleeping, we have a serious problem. In your
SMP/HT case, I'd start crawling over the load balancing code. I can't
see how trivial CPU with non-saturated IO can cause dropout in the UP
case either. Am I missing something?
-Mike
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