linux-kernel - Re: [REPOST PATCH 3/4] slab: introduce byte sized index for the freelist of a slab

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Date:	Fri, 6 Sep 2013 15:58:18 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
cc:	Pekka Enberg <penberg@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Joonsoo Kim <js1304@...il.com>,
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [REPOST PATCH 3/4] slab: introduce byte sized index for the
 freelist of a slab

On Fri, 6 Sep 2013, Joonsoo Kim wrote:

> Currently, the freelist of a slab consist of unsigned int sized indexes.
> Most of slabs have less number of objects than 256, since restriction
> for page order is at most 1 in default configuration. For example,
> consider a slab consisting of 32 byte sized objects on two continous
> pages. In this case, 256 objects is possible and these number fit to byte
> sized indexes. 256 objects is maximum possible value in default
> configuration, since 32 byte is minimum object size in the SLAB.
> (8192 / 32 = 256). Therefore, if we use byte sized index, we can save
> 3 bytes for each object.

Ok then why is the patch making slab do either byte sized or int sized
indexes? Seems that you could do a clean cutover?

As you said: The mininum object size is 32 bytes for slab. 32 * 256 =
8k. So we are fine unless the page size is > 8k. This is true for IA64 and
powerpc only I believe. The page size can be determined at compile time
and depending on that page size you could then choose a different size for
the indexes. Or the alternative is to increase the minimum slab object size.
A 16k page size would require a 64 byte minimum allocation. But thats no
good I guess. byte sized or short int sized index support would be enough.

> This introduce one likely branch to functions used for setting/getting
> objects to/from the freelist, but we may get more benefits from
> this change.

Lets not do that.
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