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SubjectRe: [patch] Performance Counters for Linux, v4

> On Tue, 2008-12-16 at 08:42 +1100, Paul Mackerras wrote:
> Furthermore, I think output of tools such as time and now timec are most
> relevant when compared between runs - that is, the change in values
> between runs, not the absolute values as such. At least, that's what I
> usually do:

That's doesn't do you any good when comparing results across different
machines, or even different kernels on the same machine.

perfmon shows that good results can be had, even if it's not the cleanest
way in the world. It would be a shame to lose that.

Small micro-benchmarks like this are important. You can't always trust
the performance counters to work, so being able to sanity check them with
exact test-cases is critical. Otherwise you might just be measuring
nonsense.

And while it might be able to subtract the exec() overhead for something
like retired instructions, it gets a lot more complicated when you have
something like cache bus snoops or branch mispredicts where it's hard to
tell what comes from the program and what is overhead from the monitoring
infrastructure.

Vince


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