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Date:   Fri, 26 Apr 2019 11:45:45 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Aubrey Li <aubrey.intel@gmail.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Vineeth Remanan Pillai <vpillai@digitalocean.com>,
        Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Paul Turner <pjt@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Subhra Mazumdar <subhra.mazumdar@oracle.com>,
        Fr?d?ric Weisbecker <fweisbec@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        Greg Kerr <kerrnel@google.com>, Phil Auld <pauld@redhat.com>,
        Aaron Lu <aaron.lwe@gmail.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Jiri Kosina <jkosina@suse.cz>
Subject: Re: [RFC PATCH v2 00/17] Core scheduling v2
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* Mel Gorman <mgorman@techsingularity.net> wrote:

> > > I can show a comparison with equal levels of parallelisation but with 
> > > HT off, it is a completely broken configuration and I do not think a 
> > > comparison like that makes any sense.
> > 
> > I would still be interested in that comparison, because I'd like
> > to learn whether there's any true *inherent* performance advantage to 
> > HyperThreading for that particular workload, for exactly tuned 
> > parallelism.
> > 
> 
> It really isn't a fair comparison. MPI seems to behave very differently
> when a machine is saturated. It's documented as changing its behaviour
> as it tries to avoid the worst consequences of saturation.
> 
> Curiously, the results on the 2-socket machine were not as bad as I
> feared when the HT configuration is running with twice the number of
> threads as there are CPUs
> 
> Amean     bt      771.15 (   0.00%)     1086.74 * -40.93%*
> Amean     cg      445.92 (   0.00%)      543.41 * -21.86%*
> Amean     ep       70.01 (   0.00%)       96.29 * -37.53%*
> Amean     is       16.75 (   0.00%)       21.19 * -26.51%*
> Amean     lu      882.84 (   0.00%)      595.14 *  32.59%*
> Amean     mg       84.10 (   0.00%)       80.02 *   4.84%*
> Amean     sp     1353.88 (   0.00%)     1384.10 *  -2.23%*

Yeah, so what I wanted to suggest is a parallel numeric throughput test 
with few inter-process data dependencies, and see whether HT actually 
improves total throughput versus the no-HT case.

No over-saturation - but exactly as many threads as logical CPUs.

I.e. with 20 physical cores and 40 logical CPUs the numbers to compare 
would be a 'nosmt' benchmark running 20 threads, versus a SMT test 
running 40 threads.

I.e. how much does SMT improve total throughput when the workload's 
parallelism is tuned to utilize 100% of the available CPUs?

Does this make sense?

Thanks,

	Ingo