Re: adaptive readahead overheads

Fengguang Wu
2006-06-08T12:39:01Z

On Thu, Jun 08, 2006 at 02:25:56PM +0200, Voluspa wrote:
> On Thu, 8 Jun 2006 19:37:31 +0800 Fengguang Wu wrote:
> > I'd like to show some numbers on the pure software overheads come with
> > the adaptive readahead in daily operations.
> [...]
> >
> > # time find /usr -type f -exec md5sum {} \; >/dev/null
> >
> > ARA
> >
> > 406.00s user 325.16s system 97% cpu 12:28.17 total
>
> Just out of interest, all your figures show an almost maxed out CPU.
> Why is it that my own runs use so little CPU? I'm running the above

It does not have to wait for disk _seeks_, I guess.
All reads inside qemu actually hit the page cache in the host system ;)

> command as we 'speak' and on average only 40% is utilized, with the
> occasional spike at max 75%. And this is on the lowest CPU level
> 800MHz, which means that the 80% up_threshold of the ondemand cpufreq
> governor never is breached (there are 1800MHz, 2000MHz and 2200MHz
> levels above it).
>
> Are you only 'giving' qemu something like 400MHz to play with or is
> qemu so inefficient in itself

My qemu command line is:

qemu -m 156 /lab/wfg/linux_image -kernel ./arch/i386/boot/bzImage
-append "root=/dev /hda console=ttyS0,9600" -nographic

I do not have the qemu accelerator, though.

Thanks,
Wu