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Message-ID: <Pine.LNX.4.64.0607261239170.7520@schroedinger.engr.sgi.com>
Date: Wed, 26 Jul 2006 12:47:05 -0700 (PDT)
From: Christoph Lameter <clameter@....com>
To: Manfred Spraul <manfred@...orfullife.com>
cc: Pekka J Enberg <penberg@...helsinki.fi>,
Heiko Carstens <heiko.carstens@...ibm.com>,
Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [patch 2/2] slab: always consider arch mandated alignment
On Wed, 26 Jul 2006, Manfred Spraul wrote:
> Christoph Lameter wrote:
>
> > A slab user is setting alignment in order to optimize performance not for
> > correctness. Most users that I know of can live with misalignments. If that
> > would not be the case then this code would never have worked.
> >
>
> Which users do you know that set align and that can live with misalignments?
> As I wrote, there are no such users in my (i386) kernel.
The users of SLAB_HWCACHE_ALIGN can live with that.
Systems running with slab debugging on must be very buggy at this point or
we were very lucky:
The list is a bit strange:
>* the pmd structure (4096: hardware requirement)
It is already exempted from debug since the size is 4096.
>* the pgd structure (32 bytes: hardware requirement)
We were lucky on that one in the past? This should break.
>* the task structure (16 byte. fxsave)
Would only break if floating point is used I think.
>* sigqueue, pid: both request 4 byte alignment (based on __alignof__()).
>Doesn't affect debugging.
So also not relevant.
We now want to say that SLAB_HWCACHE_ALIGN is only a suggestion to be
disposed of if debug is on whereas an explicitly specified alignment must be enforced?
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