netdev - Re: [rfc][patch 2/3] slab: introduce SMP alignment

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Date:	Mon, 3 Mar 2008 13:23:48 -0800 (PST)
From:	Christoph Lameter <clameter@....com>
To:	Pekka Enberg <penberg@...helsinki.fi>
cc:	Nick Piggin <npiggin@...e.de>, netdev@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	yanmin_zhang@...ux.intel.com, David Miller <davem@...emloft.net>,
	Eric Dumazet <dada1@...mosbay.com>
Subject: Re: [rfc][patch 2/3] slab: introduce SMP alignment

On Mon, 3 Mar 2008, Pekka Enberg wrote:

> Hmm... Can't we just fix SLAB_HWCACHE_ALIGN in SLUB to follow the
> semantics of SLAB?

AFAICT it follows SLAB semantics. The only small difference is for objects 
small than cache_line_size() / 2 where SLUB does not bother to align to a 
fraction of a cacheline since we are already placing multile object into a 
cacheline. We effectively have made the decision to give up the 
organization of objects in separatate cache lines.

Lets say you have a 64 byte cache line size. Then the alignment can be 
as follows. (8 byte alignment is the basic alignment requirement).

Objsize	[C	SLAB	SLUB
-----------------------------
> 64		X	X
33 .. 64	64	64
32		32	32
24		32	24	-> 3 object per cacheline sizes = 72 so overlap.
16		16	16
8		8	8

So there is only one difference for 24 byte sizes slabs.

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