linux-kernel - Re: [PATCH 207/208] x86/fpu: Add FPU performance measurement subsystem

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Date:	Wed, 6 May 2015 06:52:39 +0200
From:	Ingo Molnar <mingo2.kernel.org@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Borislav Petkov <bp@...en8.de>, Fenghua Yu <fenghua.yu@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 207/208] x86/fpu: Add FPU performance measurement
 subsystem

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

> On May 5, 2015 11:30 PM, "Ingo Molnar" <mingo@...nel.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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