linux-kernel - Re: [PATCH v2 3/5] slab: restrict the number of objects in a slab

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Date:	Thu, 17 Oct 2013 19:01:38 +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,
	Wanpeng Li <liwanp@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2 3/5] slab: restrict the number of objects in a slab

n Thu, 17 Oct 2013, Joonsoo Kim wrote:

> To prepare to implement byte sized index for managing the freelist
> of a slab, we should restrict the number of objects in a slab to be less
> or equal to 256, since byte only represent 256 different values.
> Setting the size of object to value equal or more than newly introduced
> SLAB_MIN_SIZE ensures that the number of objects in a slab is less or
> equal to 256 for a slab with 1 page.

Ok so that results in a mininum size object size of 2^(12 - 8) = 2^4 ==
16 bytes on x86. This is not true for order 1 pages (which SLAB also
supports) where we need 32 bytes.

Problems may arise on PPC or IA64 where the page size may be larger than
64K. With 64K we have a mininum size of 2^(16 - 8) = 256 bytes. For those
arches we may need 16 bit sized indexes. Maybe make that compile time
determined base on page size? > 64KByte results in 16 bit sized indexes?

Otherwise I like this approach. Simplifies a lot and its very cache
friendly.

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