benh performance problem

Hollis Blanchard hollisb at us.ibm.com
Sat Feb 7 02:25:05 EST 2004


On Feb 5, 2004, at 11:14 PM, Benjamin Herrenschmidt wrote:
>
> On Thu, 2004-02-05 at 21:09, Mikolaj Krzewicki wrote:
>>
>> 2.6.1-benh1, HZ=1000: timing=36.5s
>> 2.6.1-benh1, HZ=100 : timing=32s
> The fact that pbbuttons makes a difference makes me think the
> interrupt handling is taking way too much time on your setup,
> and pbbuttons is loading the machine with PMU interrupts...

pbbuttons is loading the machine? What about all those extra
decrementer ticks? I assume benchmarks showed no performance
degradation on some class of ppc32; what was the slowest system tested?
For that matter, couldn't this hurt 4xx, 8xx(x), as well?

Perhaps HZ should be user-configurable? Although that doesn't help
distributions who'd like one kernel to work everywhere... How does
Linux handle increased HZ on old i386 anyways?

> Not sure if I can fix any of this at this point without doing a
> major rewrite of the exception handling code, I suspect those
> CPUs don't like running in real mode and our exception handling
> happens mostly in that mode in ppc32...

I didn't realize CPUs go slower in real mode..?

--
Hollis Blanchard
IBM Linux Technology Center


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