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SubjectRe: [PATCH 207/208] x86/fpu: Add FPU performance measurement subsystem

* Andy Lutomirski <luto@amacapital.net> wrote:

> On May 5, 2015 11:30 PM, "Ingo Molnar" <mingo@kernel.org> wrote:
> >
> > Add a short FPU performance suite that runs once during bootup.
> >
> > It can be enabled via CONFIG_X86_DEBUG_FPU_PERFORMANCE=y.
>
> Neat!
>
> Can you change "cycles" to "TSC ticks"? They're not quite the same thing.

Yeah, with constant TSC we have the magic TSC frequency that is used
by RDTSC.

I'm torn: 'TSC ticks' will mean very little to most people reading
that output. We could convert it to nsecs with a little bit of
calibration - but that makes it depend on small differences in CPU
model frequencies, while the (cached) cycle costs are typically
constant per microarchitecture.

I suspect we could snatch a performance counter temporarily, to get
the real cycles count, and maybe even add a uops column. Most of this
needs to run in kernel space, so it's not a tooling project.

I also wanted to add cache-cold numbers which are very interesting as
well, just awfully hard to measure in a stable fashion. For cache-cold
numbers the natural unit would be memory bus cycles.

Thanks,

Ingo


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