From: Jens Axboe <jens.axboe@oracle.com>
To: Jan Engelhardt <jengelh@linux01.gwdg.de>
Cc: Justin Piszcz <jpiszcz@lucidpixels.com>,
linux-kernel@vger.kernel.org, linux-raid@vger.kernel.org,
Alan Piszcz <ap@solarrain.com>
Subject: Re: Chaining sg lists for big I/O commands: Question
Date: Thu, 10 May 2007 23:34:28 +0200 [thread overview]
Message-ID: <20070510213428.GX4163@kernel.dk> (raw)
In-Reply-To: <Pine.LNX.4.61.0705102040090.6426@yvahk01.tjqt.qr>
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
prev parent reply other threads:[~2007-05-10 21:34 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-05-09 13:22 Chaining sg lists for big I/O commands: Question Justin Piszcz
2007-05-09 13:38 ` Jens Axboe
2007-05-10 18:41 ` Jan Engelhardt
2007-05-10 21:34 ` Jens Axboe [this message]
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