On Fri, Apr 26, 2019 at 11:45:45AM +0200, Ingo Molnar wrote: > > * Mel Gorman <mgor...@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? >
Yes. Here is the comparison. Amean bt 678.75 ( 0.00%) 789.13 * -16.26%* Amean cg 261.22 ( 0.00%) 428.82 * -64.16%* Amean ep 55.36 ( 0.00%) 84.41 * -52.48%* Amean is 13.25 ( 0.00%) 17.82 * -34.47%* Amean lu 1065.08 ( 0.00%) 1090.44 ( -2.38%) Amean mg 89.96 ( 0.00%) 84.28 * 6.31%* Amean sp 1579.52 ( 0.00%) 1506.16 * 4.64%* Amean ua 611.87 ( 0.00%) 663.26 * -8.40%* This is the socket machine and with HT On, there are 80 logical CPUs versus HT Off with 40 logical CPUs. -- Mel Gorman SUSE Labs
