Hi Ilpo,
On 8/30/24 4:42 AM, Ilpo Järvinen wrote:
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@xxxxxxxxx>
---
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
