On Tue, Nov 27, 2018 at 12:12:28AM +0000, Elliott, Robert (Persistent Memory) wrote: > I ran a short test with: > * HPE ProLiant DL360 Gen9 system > * Intel Xeon E5-2699 CPU with 18 physical cores (0-17) and > 18 hyperthreaded cores (36-53) > * DDR4 NVDIMM-Ns (which run at regular DRAM DIMM speeds) > * fio workload generator > * cores on one CPU socket talking to a pmem device on the same CPU > * large (1 MiB) random writes (to minimize the threads getting CPU cache > hits from each other) > > Results: > * 31.7 GB/s four threads, four physical cores (0,1,2,3) > * 22.2 GB/s four threads, two physical cores (0,1,36,37) > * 21.4 GB/s two threads, two physical cores (0,1) > * 12.1 GB/s two threads, one physical core (0,36) > * 11.2 GB/s one thread, one physical core (0) > > So, I think it's important that the initialization threads run on > separate physical cores. Thanks for running this. And fair enough, in this test using both siblings gives only a 4-8% speedup over one, so it makes sense to use only cores in the calculation. As for how to actually do this, some arches have smp_num_siblings, but there should be a generic interface to provide that. It's also possible to calculate this from the existing topology_sibling_cpumask, but the first option is better IMHO. Open to suggestions. > For the number of cores to use, one approach is: > memory bandwidth (number of interleaved channels * speed) > divided by > CPU core max sustained write bandwidth > > For example, this 2133 MT/s system is roughly: > 68 GB/s (4 * 17 GB/s nominal) > divided by > 11.2 GB/s (one core's performance) > which is > 6 cores > > ACPI HMAT will report that 68 GB/s number. I'm not sure of > a good way to discover the 11.2 GB/s number. Yes, this would be nice to do if we could know the per-core number, with the caveat that a single number like this would be most useful for the CPU-memory pair it was calculated for, so the kernel could at least calculate it for jobs operating on local memory. Some BogoMIPS-like calibration may work, but I'll wait for ACPI HMAT support in the kernel.
