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

From: Andrea Arcangeli (
Date: Mon Feb 10 2003 - 05:15:39 EST

On Mon, Feb 10, 2003 at 01:07:25PM +0300, Hans Reiser wrote:
> Andrew Morton wrote:
> >
> >Large directories tend to be spread all around the disk anyway. And I've
> >never explicitly tested for any problems which the loss of readahead might
> >have caused ext2. Nor have I tested inode table readahead. Guess I
> >should.
> >
> >
> >-
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> >
> >
> >
> readahead seems to be less effective for non-sequential objects. Or at

yes, this is why I said readahead matters mostly to generate the big dma
commands, so if the object is sequential it will be served by the
lowlevel with a single dma using SG. this is also why when I moved the
high dma limit of scsi to 512k (from 128k IIRC) I got such a relevant
throughput improvement. Also watch the read speed in my tree compared to
2.4 and 2.5 in bigbox.html from Randy (bonnie shows it well).

> least, you don't get the order of magnitude benefit from doing only one
> seek, you only get the better elevator scheduling from having more
> things in the elevator, which isn't such a big gain.


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