linux-kernel - Re: [patch V3] percpu_counter: scalability works

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Date:	Wed, 18 May 2011 09:00:43 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"cl@...ux.com" <cl@...ux.com>,
	"npiggin@...nel.dk" <npiggin@...nel.dk>
Subject: Re: [patch V3] percpu_counter: scalability works

On Tue, 2011-05-17 at 17:50 +0800, Tejun Heo wrote:
> Hello, Eric.
> 
> On Tue, May 17, 2011 at 11:45:41AM +0200, Eric Dumazet wrote:
> > _sum() is a bit more precise than percpu_counter_read(), but to make it
> > really precise, we means we have to stop concurrent activities, and we
> > never did in previous/current implementation.
> > 
> > We could add this (as Shaohua and myself tried in various patches)
> > later, if needed, but nowhere in kernel we currently need that.
> > 
> > Even /proc/meminfo doesnt call _sum(&vm_committed_as) but the lazy
> > percpu_counter_read_positive() function...
> > 
> > Reammy _sum() gives a good approximation of the counter, more precise
> > because of the percpu s32 folding, but no guarantee of deviation.
> 
> I'm not asking to make it more accurate but the initial patches from
> Shaohua made the _sum() result to deviate by @batch even when only one
> thread is doing _inc() due to the race window between adding to the
> main counter and resetting the local one.  All I'm asking is closing
> that hole and I'll be completely happy with it.  The lglock does that
> but it's ummm.... not a very nice way to do it.
> 
> Please forget about deviations from concurrent activities.  I don't
> care and nobody should.  All I'm asking is removing that any update
> having the possibility of that unnecessary spike and I don't think
> that would be too hard.
Hmm, we once again to talk about the deviation issue. I thought we
agreed the deviation issue should be resolved in last discussion, but
seems not...

I would suggest you guys seriously look at my v3 patches, which doesn't
use lglock but can solve the deviation issue and has no significant
overhead.

Thanks,
Shaohua

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