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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