linux-ext4 - Re: [PATCH] ext2/ext3: allocate ->s_blockgroup

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Message-Id: <1226840289.8172.8.camel@lappy.programming.kicks-ass.net>
Date:	Sun, 16 Nov 2008 13:58:09 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Pekka J Enberg <penberg@...helsinki.fi>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-ext4@...r.kernel.org, cl@...ux-foundation.org,
	mpm@...enic.com, eduard.munteanu@...ux360.ro
Subject: Re: [PATCH] ext2/ext3: allocate ->s_blockgroup_lock separately to
	avoid wasting space

On Fri, 2008-11-14 at 11:17 +0200, Pekka J Enberg wrote:
> From: Pekka Enberg <penberg@...helsinki.fi>
> 
> As spotted by kmemtrace, struct ext2_sb_info is 17024 bytes and ext3_sb_info is
> 17152 bytes on 64-bit which makes them a very bad fit for SLAB allocators. In
> fact, both allocations are round up to the next available page size of 
> order 3 which is 32 KB.
> 
> The culprit if the wasted memory is the ->s_blockgroup_lock which can be as big
> as 16 KB when CONFIG_NR_CPUS is set to 32. As struct blockgroup_lock is a
> perfect fit for order 2 page in the worst case, allocate ->s_blockgroup_lock
> separately to avoid wasting space.

And here I was thinking that NR_CPUS=4096 is currently our worst
case ;-)

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