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

Andrew Morton
2003-02-10T11:38:17Z

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.

> 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.