Messages in this thread | | | Date | Fri, 26 Apr 2019 11:45:45 +0200 | From | Ingo Molnar <> | 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
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