linux-kernel - Re: Chaining sg lists for big I/O commands: Question

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Date:	Wed, 9 May 2007 15:38:30 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Justin Piszcz <jpiszcz@...idpixels.com>
Cc:	linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
	Alan Piszcz <ap@...arrain.com>
Subject: Re: Chaining sg lists for big I/O commands: Question

On Wed, May 09 2007, Justin Piszcz wrote:
> http://kerneltrap.org/node/8176

Oh

> I am a mdadm/disk/hard drive fanatic, I was curious:
> 
> >On i386, we can at most fit 256 scatterlist elements into a page,
> >and on x86-64 we are stuck with 128. So that puts us somewhere
> >between 512kb and 1024kb for a single IO.
> 
> How come 32bit is 256 and 64 is only 128?
>
> I am sure it is something very fundamental/simple but I was curious, I 
> would think x86_64 would fit/support more scatterlists in a page.

Because of the size of the scatterlist structure. As pointers are bigger
on 64-bit archs, the scatterlist structure ends up being bigger. The
page size on x86-64 is 4kb, hence the number of structures you can fit
in a page is smaller.

> Also, when this patch is implemented for x86_64 and if merged into 
> mainline, what does this mean for performance?

The sglist branch of block repo has x86-64 support now. I'll post a new
patchset tomorrow.

Performance wise, it's meant to help higher end hardware that need 2-4mb
(or bigger) commands to get good performance. That also includes things
like tapes that have big block sizes, getting a command of the right
size there is the difference between good and abysmal performance.

> I have an mdadm raid5 of 10 raptors and get 434MB/s write and 622MB/s 
> read, would I see an increase in performance with this patch?

Perhaps, depends on a lot of factors.

-- 
Jens Axboe

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