linux-kernel - [RFC] slub: ARCH_SLAB_MINALIGN defaults to 8 on x86

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Message-ID: <1265206946.2118.57.camel@localhost>
Date:	Wed, 03 Feb 2010 14:22:26 +0000
From:	Richard Kennedy <richard@....demon.co.uk>
To:	Christoph Lameter <cl@...ux-foundation.org>,
	penberg <penberg@...helsinki.fi>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Cc:	lkml <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>
Subject: [RFC] slub: ARCH_SLAB_MINALIGN defaults to 8 on x86_32. is this
 too big?

Hi all,

slub.c sets the default value of ARCH_SLAB_MINALIGN to sizeof(unsigned
long long) if the architecture didn't already override it.

And as x86_32 doesn't set a value this means that slab objects get
aligned to 8 bytes, potentially wasting 4 bytes per object. Slub forces
objects to be aligned to sizeof(void *) anyway, but I don't see that
there is any need for it to be 8 on 32bits.

I'm working on a patch to pack more buffer_heads into each kmem_cache
slab page.
On 32 bits the structure size is 52 bytes and with the alignment applied
I end up with a slab of 73 x 56 byte objects. However, if the minimum
alignment was sizeof(void *) then I'd get 78 x 52 byte objects. So there
is quite a memory saving to be had in changing this.

Can I define a ARCH_SLAB_MINALIGN in x86_64 to sizeof(void *) ? 
or would it be ok to change the default in slub.c to sizeof(void *) ?

Or am I missing something ?

regards
Richard

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