lkml.org 
[lkml]   [2011]   [May]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [patch V3] percpu_counter: scalability works
From
Date
Le lundi 16 mai 2011 à 14:37 +0800, Shaohua Li a écrit :
> On Mon, 2011-05-16 at 14:11 +0800, Eric Dumazet wrote:
> > Le lundi 16 mai 2011 à 08:58 +0800, Shaohua Li a écrit :
> >
> > > so if _sum starts and ends here, _sum can still get deviation.
> >
> > This makes no sense at all. If you have so many cpus 'here' right before
> > you increment fbc->sum_cnt, then no matter how precise and super
> > cautious you are in your _sum() implementation, as soon as you exit from
> > sum(), other cpus already changed the percpu counter global value.
> I don't agree here. The original implementation also just has quite
> small window we have deviation, the window only exists between the two
> lines:
> atomic64_add(count, &fbc->count);
> __this_cpu_write(*fbc->counters, 0);
> if you think we should ignore it, we'd better not use any protection
> here.
>

Not at all. Your version didnt forbid new cpu to come in _add() and
hitting the deviation problem.

There is a small difference, or else I wouldnt had bother.


> as I wrote in the email, the atomic and cacheline issue can be resolved
> with a per_cpu data, I just didn't post the patch. I post it this time,
> please see below. There is no cache line bounce anymore.
>

I am afraid we make no progress at all here, if you just try to push
your patch and ignore my comments.

percpu_counter is a compromise, dont make it too slow for normal
operations. It works well if most _add() operations only go through
percpu data.

Please just move vm_committed_as to a plain atomic_t, this will solve
your problem.

Thanks


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2011-05-16 08:57    [from the cache]
©2003-2011 Jasper Spaans