linux-ext4 - Re: [PATCH] ext4: use percpu counter for extent cache count

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Message-ID: <20130301180035.GB17920@thunk.org>
Date:	Fri, 1 Mar 2013 13:00:35 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Dave Jones <davej@...hat.com>,
	"gnehzuil.liu" <gnehzuil.liu@...il.com>,
	Zheng Liu <wenqing.lz@...bao.com>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] ext4: use percpu counter for extent cache count

On Fri, Mar 01, 2013 at 10:42:25AM -0600, Eric Sandeen wrote:
> Use a percpu counter rather than atomic types for shrinker accounting.
> There's no need for ultimate accuracy in the shrinker, so this
> should come a little more cheaply.  The percpu struct is somewhat
> large, but there was a big gap before the cache-aligned
> s_es_lru_lock anyway, and it fits nicely in there.

I thought about using percpu counters, but I was worried about the
size on really big machines.  OTOH, it will be the really large NUMA
machines where atomic_t will really hurt, so maybe we should use
percpu countesr and not really worry about it.  It's on a per file
system basis, so even if it is a few hundred bytes it shouldn't break
the bank.

						- Ted
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