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

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Date:   Thu, 2 Feb 2017 17:29:01 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     "Ghannam, Yazen" <Yazen.Ghannam@....com>
Cc:     Borislav Petkov <bp@...en8.de>, 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


* Ghannam, Yazen <Yazen.Ghannam@....com> wrote:

> Here are my results on a 32C Bulldozer system with an SSD. Also, I use ccache so 
> I added "ccache -C" in the pre-build script so the cache gets cleared.
> 
> Before:
> Performance counter stats for 'make -s -j65 bzImage' (3 runs):
> 
>     2375752.777479      task-clock (msec)         #   23.589 CPUs utilized            ( +-  0.35% )
>          1,198,979      context-switches          #    0.505 K/sec                    ( +-  0.34% )
>      8,964,671,259      cache-misses                                                  ( +-  0.44% )
>             79,399      cpu-migrations            #    0.033 K/sec                    ( +-  1.92% )
>         37,840,875      page-faults               #    0.016 M/sec                    ( +-  0.20% )
>  5,425,612,846,538      cycles                    #    2.284 GHz                      ( +-  0.36% )
>  3,367,750,745,825      instructions              #    0.62  insn per cycle                                              ( +-  0.11% )
>    750,591,286,261      branches                  #  315.938 M/sec                    ( +-  0.11% )
>     43,544,059,077      branch-misses             #    5.80% of all branches          ( +-  0.08% )
> 
>      100.716043494 seconds time elapsed                                          ( +-  1.97% )
> 
> After:
> Performance counter stats for 'make -s -j65 bzImage' (3 runs):
> 
>     1736720.488346      task-clock (msec)         #   23.529 CPUs utilized            ( +-  0.16% )
>          1,144,737      context-switches          #    0.659 K/sec                    ( +-  0.20% )
>      8,570,352,975      cache-misses                                                  ( +-  0.33% )
>             91,817      cpu-migrations            #    0.053 K/sec                    ( +-  1.67% )
>         37,688,118      page-faults               #    0.022 M/sec                    ( +-  0.03% )
>  5,547,082,899,245      cycles                    #    3.194 GHz                      ( +-  0.19% )
>  3,363,365,420,405      instructions              #    0.61  insn per cycle                                              ( +-  0.00% )
>    749,676,420,820      branches                  #  431.662 M/sec                    ( +-  0.00% )
>     43,243,046,270      branch-misses             #    5.77% of all branches          ( +-  0.01% )
> 
>       73.810517234 seconds time elapsed                                          ( +-  0.02% )

That's pretty impressive: ~35% difference in wall clock performance of this 
workload.

And that while both the cycles and the instructions count is within 2.5% of each 
other. The only stat the differs beyond the level of noise is cache-misses:

      8,964,671,259      cache-misses                                                  ( +-  0.44% )
      8,570,352,975      cache-misses                                                  ( +-  0.33% )

which is 4.5%, but I have trouble believing that just 4.5% more cachemisses can 
have such a massive effect on performance.

So unless +4.5% cachemisses can cause a 35% difference in performance this is a 
really weird result. Where did the extra performance come from - was the 'good' 
workload perhaps running at higher CPU frequencies for some reason?

Thanks,

	Ingo