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SubjectRe: stochastic fair queueing in the elevator [Re: [BENCHMARK] 2.4.20-ck3 / aa / rmap with contest]
Nick Piggin <piggin@cyberone.com.au> wrote:
>
> Andrew Morton wrote:
>
> >Andrea Arcangeli <andrea@suse.de> wrote:
> >
> >>It's the readahead in my tree that allows the reads to use the max scsi
> >>command size. It has nothing to do with the max scsi command size
> >>itself.
> >>
> >
> >Oh bah.
> >
> >- *max_ra++ = vm_max_readahead;
> >+ *max_ra = ((128*4) >> (PAGE_SHIFT - 10)) - 1;
> >
> >
> >Well of course that will get bigger bonnie numbers, for exactly the reasons
> >I've explained. It will seek between files after every 512k rather than
> >after every 128k.
> >
> Though Andrea did say it is a "single threaded" streaming read.

Oh sorry, I missed that.

> That is what I can't understand. Movement of the disk head should
> be exactly the same in either situation and 128K is not exactly
> a pitiful request size - so it suggests a quirk somewhere. It
> is not as if the disk has to be particularly smart or know a
> lot about the data in order to optimise the head movement for
> a load like this.

Yes, that's a bit odd. Some reduction in CPU cost and bus
traffic, etc would be expected. Could be that sending out a
request which is larger than a track is saving a rev of the disk
for some reason.

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