benh performance problem

[Benjamin Herrenschmidt]

> 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?

Heh, well, I did see any obvious degradation on my powerbook but
I didn't benchmark much actually. Maybe I should, or make kernel HZ
a config option.

> 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?

I don't know :)

> > 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..?

It is the case with 970's and I think with IBM G3s, I'm not sure
about Motorola G4s and other CPUs. In real mode, all memory is
treated like it has the G bit set, preventing speculation (among
others, I suppose prefetch gets killed too).

I's very visible on lmbench with a G5: The null syscall overhead
of the ppc64 kernel is lower than the one of the ppc32 kernel
despite actually running more code and saving more & bigger registers.

Ben.


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