Re: stochastic fair queueing in the elevator [Re: [BENCHMARK]

[Andrew Morton]

Andrea Arcangeli <[e-mail]> 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.