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

From: Nick Piggin (
Date: Mon Feb 10 2003 - 07:27:13 EST

Andrew Morton wrote:

>Nick Piggin <> wrote:
>>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.
Shouldn't be. Even at 128KB readahead we should always have
outstanding requests against the disk in a streaming read
scenario, right? Maybe if the track buffers are bigger than

Is there a magic number above which you see the improvement,
Andrea? Or does it steadily climb?

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