On Thu, 2004-02-05 at 21:09, Mikolaj Krzewicki wrote:
the benchmark i used shows a speedup back to the values i'm used to.
The benchmark itself is an octave script the execution time of which i
tested with different kernels under similar circumstances, so here are
the details:
2.6.1-benh1, HZ=1000: timing=36.5s
2.6.1-benh1, HZ=100 : timing=32s
this is with X running and a lot more processes(not running).
the weird thing is it executes slightly faster (on average) with
pbbuttonsd off.
The machine is g3 500 ibook, the octave script is:
tic;
a = abs(randn(1500, 1500)/10);
b = a';
c= a*b;
a = reshape(b, 750, 3000);
b = a';
timing=toc;
so lots of system calls and cache flushing is in order.
Mikolaj.
Well, I don't knwo what the above means, I don't talk that language
anyway :)
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...
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...
Ben.
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On Feb 5, 2004, at 11:14 PM, Benjamin Herrenschmidt wrote:
On Thu, 2004-02-05 at 21:09, Mikolaj Krzewicki wrote:
quoted
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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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 :)
quoted
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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