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For example RAS features or memory performance >> features that generate memory traffic may drive accesses that are >> counted differently by performance counters and MBM respectively, >> for instance generating "overhead" traffic which is not counted >> against any specific RMID. As a ratio, this different view of memory >> bandwidth becomes more apparent at low memory bandwidths. > > Interesting. > > I did some time back prototype with a change to MBM test such that instead > of using once=false I changed fill_buf to be able to run N passes through > the buffer which allowed me to know how many reads were performed by the > benchmark. This yielded numerical difference between all those 3 values > (# of reads, MBM, perf) which also varied from arch to another so it > didn't end up making an usable test. > > I guess I now have an explanation for at least a part of the differences. > >> It is not practical to enable/disable the various features that >> may generate memory bandwidth to give performance counters and >> resctrl an identical view. Instead, do not compare performance >> counters and resctrl view of memory bandwidth when the memory >> bandwidth is low. >> >> Bandwidth throttling behaves differently across platforms >> so it is not appropriate to drop measurement data simply based >> on the throttling level. Instead, use a threshold of 750MiB >> that has been observed to support adequate comparison between >> performance counters and resctrl. >> >> Signed-off-by: Reinette Chatre >> --- >> tools/testing/selftests/resctrl/mba_test.c | 7 +++++++ >> tools/testing/selftests/resctrl/resctrl.h | 6 ++++++ >> 2 files changed, 13 insertions(+) >> >> diff --git a/tools/testing/selftests/resctrl/mba_test.c b/tools/testing/selftests/resctrl/mba_test.c >> index cad473b81a64..204b9ac4b108 100644 >> --- a/tools/testing/selftests/resctrl/mba_test.c >> +++ b/tools/testing/selftests/resctrl/mba_test.c >> @@ -96,6 +96,13 @@ static bool show_mba_info(unsigned long *bw_imc, unsigned long *bw_resc) >> >> avg_bw_imc = sum_bw_imc / (NUM_OF_RUNS - 1); >> avg_bw_resc = sum_bw_resc / (NUM_OF_RUNS - 1); >> + if (avg_bw_imc < THROTTLE_THRESHOLD || avg_bw_resc < THROTTLE_THRESHOLD) { >> + ksft_print_msg("Bandwidth below threshold (%d MiB). Dropping results from MBA schemata %u.\n", >> + THROTTLE_THRESHOLD, >> + ALLOCATION_MAX - ALLOCATION_STEP * allocation); > > The second one too should be %d. > hmmm ... I intended to have it be consistent with the ksft_print_msg() that follows. Perhaps allocation can be made unsigned instead? Reinette