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

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Date:	Thu, 10 May 2007 23:34:28 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Jan Engelhardt <jengelh@...ux01.gwdg.de>
Cc:	Justin Piszcz <jpiszcz@...idpixels.com>,
	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 Thu, May 10 2007, Jan Engelhardt wrote:
> On May 9 2007 15:38, Jens Axboe wrote:
> >> 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.
> 
> I take it this problem "goes away" on arches with 8KB page_size?

Not really, the 8kb page size just doubles the sg size. On a 64-bit
arch, that would still only get you 1mb IO size.

-- 
Jens Axboe

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