linux-kernel - Re: [PATCH 03/13] scsi: unify allocation of scsi command and sense buffer

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Message-ID: <20090525113251.GC24071@wotan.suse.de>
Date:	Mon, 25 May 2009 13:32:51 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Jens Axboe <jens.axboe@...cle.com>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, chris.mason@...cle.com,
	david@...morbit.com, hch@...radead.org, akpm@...ux-foundation.org,
	jack@...e.cz, yanmin_zhang@...ux.intel.com,
	Christoph Lameter <cl@...ux-foundation.org>,
	Matthew Wilcox <willy@...ux.intel.com>
Subject: Re: [PATCH 03/13] scsi: unify allocation of scsi command and sense buffer

On Mon, May 25, 2009 at 11:15:42AM +0300, Pekka Enberg wrote:
> On Mon, May 25, 2009 at 10:30 AM, Jens Axboe <jens.axboe@...cle.com> wrote:
> > Fold the sense buffer into the command, thereby eliminating a slab
> > allocation and free per command.
> >
> > Signed-off-by: Jens Axboe <jens.axboe@...cle.com>
> 
> Interesting. I wonder how this affects the SLAB vs. SLUB regression
> people are seeing on high end machines in OLTP benchmarks.

It could improve it. I think these (bios, requests, commands etc) allocations
are what SLUB has trouble with in that workload, so eliminating one of them
should help it. I guess it will help the other allocators as well, but maybe
a smaller relative improvement?

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