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

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Date:	Wed, 3 Feb 2010 09:41:29 -0600 (CST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Richard Kennedy <richard@....demon.co.uk>
cc:	penberg <penberg@...helsinki.fi>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [RFC] slub: ARCH_SLAB_MINALIGN defaults to 8 on x86_32. is this
 too big?

On Wed, 3 Feb 2010, Richard Kennedy wrote:

> 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.

Note that 64 bit entities may  exist even under 32 bit (llong) that need
to be aligned properly.

struct buffer_head contains a sector_t which is 64 bit so you should align
to an 8 byte boundary.

> 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.

SLUB is not restricted to order 0 pages and can use order 1 or 2 pages as
long as this reduces the memory footprint (byte wastage in a slab page is
reduced) and as long as the kernel has contiguous memory available. It
will use order 0 when memory is fragmented.

> 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 ?

I'd say leave it alone.
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