linux-kernel - Re: [patch v3 3/3] percpu_counter: use percpu data to track add start

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Message-ID: <1305622767.2850.19.camel@edumazet-laptop>
Date:	Tue, 17 May 2011 10:59:27 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	tj@...nel.org, cl@...ux.com
Subject: Re: [patch v3 3/3] percpu_counter: use percpu data to track add
 start

Le mardi 17 mai 2011 à 16:41 +0800, Shaohua Li a écrit :
> pièce jointe document texte brut (percpu-counter-percpustart.patch)
> add_start causes a lot of cache bouncing because it's updated by all
> cpus. We can actually make it a percpu variable. This will completely
> reduce the cache bouncing.
> With the patch and last patch, I get about 7x faster running the
> workload that last patch described. Only with last patch, the workload
> is only about 4x faster.
> This doesn't slow down _sum because we removed lock for _sum. I did
> a stress test. 23 CPU run _add, one cpu runs _sum. In _add fast path
> (don't hold) lock, _sum runs a little slow (about 20% slower). In
> _add slow path (hold lock), _sum runs much faster (about 9x faster);
> 
> V2: uses one percpu data as Eric suggested.

I NACK this patch very much.



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