On Thu, 29 Aug 2024, Reinette Chatre wrote:
The MBA test incrementally throttles memory bandwidth, each time followed by a comparison between the memory bandwidth observed by the performance counters and resctrl respectively.
While a comparison between performance counters and resctrl is generally appropriate, they do not have an identical view of memory bandwidth. 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 reinette.chatre@intel.com
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.