linux-kernel - Re: [RFC PATCH] x86/CPU/AMD: Bring back Compute Unit ID

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Date:   Thu, 2 Feb 2017 17:09:16 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     "Ghannam, Yazen" <Yazen.Ghannam@....com>, x86-ml <x86@...nel.org>,
        Yves Dionne <yves.dionne@...il.com>,
        Brice Goglin <Brice.Goglin@...ia.fr>,
        Peter Zijlstra <peterz@...radead.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] x86/CPU/AMD: Bring back Compute Unit ID


* Borislav Petkov <bp@...en8.de> wrote:

> On Thu, Feb 02, 2017 at 01:10:54PM +0100, Borislav Petkov wrote:
> > Now on to run the same thing on a bigger bulldozer.
> 
> It looks differently on the bigger box:
> 
> before:
> 
>  Performance counter stats for 'make -s -j17 bzImage' (3 runs):
>      143.807894000 seconds time elapsed                                          ( +-  0.89% )
> 
>  Performance counter stats for 'make -s -j17 bzImage' (3 runs):
>      147.109189694 seconds time elapsed                                          ( +-  0.49% )

If there's any doubt about the validity of the measurement I'd suggest doing:

	perf stat -a --sync --repeat 3 ...

... so that there's no perf overhead and skew from the many processes of a kernel 
build workload, plus the --sync should reduce IO related noise.

Or:

	perf stat --null --sync --repeat 3 ...

... will only measure elapsed time, but will do that very precisely and with very 
little overhead.

Thanks,

	Ingo