Re: New readahead - ups and downs

[Fengguang Wu <fengguang.wu@gmail.com>]

Hi Helge,

On Tue, Jun 27, 2006 at 03:07:16PM +0200, Helge Hafting wrote:
> I made my own little io-intensive test, that shows a case where
> performance drops.
> 
> I boot the machine, and starts "debsums", a debian utility that
> checksums every file managed by debian package management.
> As soon as the machine starts swapping, I also start
> start a process that applies an mm-patch to the kernel tree, and
> times this.
> 
> This patching took 1m28s with cold cache, without debsums running.
> With the 2.6.15 kernel (old readahead), and debsums running, this
> took 2m20s to complete, and 360kB in swap at the worst.
> 
> With the new readahead in 2.6.17-mm3 I get 6m22s for patching,
> and 22MB in swap at the most.  Runs with mm1 and mm2 were
> similiar, 5-6 minutes patching and 22MB swap.
> 
> My patching clearly takes more times this way.  I don't know
> if debsums improved though, it could be as simple as a fairness
> issue.  Memory pressure definitely went up.

There are a lot changes between 2.6.15 and 2.6.17-mmX. Would you use
the single 2.6.17-mm5 kernel for benchmarking? It's easy:

        - select old readahead:
                echo 1 > /proc/sys/vm/readahead_ratio

        - select new readahead:
                echo 50 > /proc/sys/vm/readahead_ratio


Thanks,
Wu