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

From: Andrew Morton (
Date: Mon Feb 10 2003 - 06:48:08 EST

Andrea Arcangeli <> 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.

> You can wait 10 minutes and still such command can't grow. This is why
> claiming anticipatory scheduling can decrease the need for readahead
> doesn't make much sense to me, there are important things you just can't
> achieve by only waiting.

The anticipatory scheduler can easily permit 512k of reading before seeking
away to another file. In fact it can allow much more, without requiring that
readhead be cranked up.

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