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SubjectRe: stochastic fair queueing in the elevator [Re: [BENCHMARK] 2.4.20-ck3 / aa / rmap with contest]
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.
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.

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