linux-kernel - Re: [PATCH 10/11] VFS: Cache request_queue in struct block

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Message-ID: <20110819153622.GD25996@one.firstfloor.org>
Date:	Fri, 19 Aug 2011 17:36:22 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Andi Kleen <ak@...ux.intel.com>, Andi Kleen <andi@...stfloor.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	hch@...radead.org, Jens Axboe <jaxboe@...ionio.com>
Subject: Re: [PATCH 10/11] VFS: Cache request_queue in struct block_device

On Fri, Aug 19, 2011 at 10:14:09AM -0400, Vivek Goyal wrote:
> On Thu, Aug 18, 2011 at 02:03:56PM -0700, Andi Kleen wrote:
> > 
> > >Is the performance gain because of this one less dereference really
> > >substantial.
> > Yes it is measurable on a large macro benchmark.
> > 
> > The gain is from doing the prefetch early enough, and that needs the
> > additional pointer.
> 
> So it gives you extra .3% (as mentioned in your first mail). IMHO, for
> .3% we should not cache extra request queue pointer.

Note this is on a benchmark which is primarily userland.  Kernel 
is only a small part, so it's a much higher percentage for the kernel
time.

Also on that large benchmark it's hard to any improvement at all,
and this isn't even a particularly ugly or intrusive change. Not sure 
why you're against it.

Cache optimizations are important.

-Andi
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