Re: Some dbench 32 results for 2.4.8-pre8, 2.4.7-ac10, and 2.4.7

[Linus Torvalds]

On Fri, 10 Aug 2001, Steven Cole wrote:
> Linus Torvalds wrote:
> >Does the numbers change if you do something like
> >
> >	killall -STOP kupdated
> >	echo 80 64 64 256 500 6000 90 > /proc/sys/vm/bdflush
> >
> >In particular, the dirty balancing worked really badly before, and was
> >just fixed.  I suspect that the bdflush numbers were tuned with the
> >badly-working case, and they might be a bit too aggressive for dbench
> >these days..
>
> Sorry Linus for the late reply, but I went to bed just before your message.
> I re-ran dbench 32 with the settings above.  Here are the results:

[ Good, looks more like it ]

Now, the problem with dbench is that no way in hell should you optimize
for dbench in general, because it is a sucky kind of benchmark.

For example, waiting until the last possible minute for writeouts is
definitely the best setting for dbench, but it's a pretty horrible setting
for usability.

I suspect that for optimal dbench performance we'll always have to let teh
system admin do the above kind of horrible tweaking stuff, but at the same
time I personally absolutely detest the need for tweaks in general, and I
would like the default behaviour to be reasonable.

Killing kupdated, for example, is not really "reasonable". But I also
suspect that now that dirty balancing works sanely, the "start writeout at
30% full" is a bit early too.

So instead of the 30/60% split (the first number is "when do we start
writing things out", and the second number is "when do we start actively
waiting for it"), a 50/75% setup might be more reasonable for regular
loads, while making dbench at least a bit happier.

Are you (or others) willing to play around with the numbers a bit and look
at both dbench performance and at interactive feel?

In general

	echo x 64 64 256 500 3000 y > /proc/sys/vm/bdflush

will set the "start writeout" to 'x'%, and the "start synchronous wait" to
'y'% (and restart kupdated with "killall -CONT kupdated"). It would be
interesting to hear where the sweet spot is.

		Linus