Re: [RFC PATCH v2 00/17] Core scheduling v2

[Ingo Molnar]

* Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> 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