linux-ext4 - Re: [PATCH] percpu_counter: Fix __percpu_counter

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Message-ID: <20081208230047.GC2501@mit.edu>
Date:	Mon, 8 Dec 2008 18:00:47 -0500
From:	Theodore Tso <tytso@....edu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Eric Dumazet <dada1@...mosbay.com>,
	linux kernel <linux-kernel@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Mingming Cao <cmm@...ibm.com>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] percpu_counter: Fix __percpu_counter_sum()

On Mon, Dec 08, 2008 at 11:20:35PM +0100, Peter Zijlstra wrote:
> 
> atomic_t is pretty good on all archs, but you get to keep the cacheline
> ping-pong.
> 

Stupid question --- if you're worried about cacheline ping-pongs, why
aren't each cpu's delta counter cacheline aligned?  With a 64-byte
cache-line, and a 32-bit counters entry, with less than 16 CPU's we're
going to be getting cache ping-pong effects with percpu_counter's,
right?  Or am I missing something?

						- Ted

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