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

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Date:	Mon, 16 May 2011 16:34:17 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Tejun Heo <tj@...nel.org>,
	"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 Mon, 2011-05-16 at 15:44 +0800, Eric Dumazet wrote:
> Le lundi 16 mai 2011 à 15:15 +0800, Shaohua Li a écrit :
> 
> > I can, but you can't prevent me to optimize percpu_counter.
> > 
> 
> Well, I have the right to say you're wrong.
sure, but please give a reason.

> Your last patch is not good, sorry. 
> Please take the time to read it
> again and fix obvious problems.
what kind of obvious problems?

> And also give us numbers if one process
> does the mmap()/munmap() loop, before and after your patch.
I did a stress test with one thread

while {
__percpu_counter_add(+count)
__percpu_counter_add(-count)
}
the loop do 10000000 times.
in _add fast path (no locking hold):
before my patch:
real    0m0.133s
user    0m0.000s
sys     0m0.124s
after:
real    0m0.129s
user    0m0.000s
sys     0m0.120s
the difference is variation.

in _add slow path (locking hold):
before my patch:
real    0m0.374s
user    0m0.000s
sys     0m0.372s
after:
real    0m0.245s
user    0m0.000s
sys     0m0.020s

My patch actually makes _add faster, because we removed spin_lock.

> A percpu_counter is already a beast as is, you're suggesting to double
> its size, for a pathological case.
> 
> Its absolutely not clear to me why vm_committed_as is using the default
> percpu_counter_batch. 
> 
> By the way could you make sure percpu_counter_batch has the right value
> on your 24 cpus machine ?
> 
> Your 128Mbyte mmap threshold sounds like percpu_counter_batch=32 instead
> of 48
let's not argue the batch size anymore. If we can make percpu_counter
faster, why we don't (even your patch mentioned this).

Thanks,
Shaohua

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