Hi Ilpo,
On 5/20/24 5:30 AM, Ilpo Järvinen wrote:
For MBM/MBA tests, measure_vals() calls get_mem_bw_imc() that performs
the measurement over a duration of sleep(1) call. The memory bandwidth
numbers from IMC are derived over this duration. The resctrl FS derived
memory bandwidth, however, is calculated inside measure_vals() and only
takes delta between the previous value and the current one which
besides the actual test, also samples inter-test noise.
Rework the logic in measure_vals() and get_mem_bw_imc() such that the
resctrl FS memory bandwidth section covers much shorter duration
closely matching that of the IMC perf counters to improve measurement
accuracy. Open two the resctrl mem bw files twice to avoid opening
after the test during measurement period (reading the same file twice
returns the same value so two files are needed).
I think this is only because of how the current reading is done, resctrl
surely supports keeping a file open and reading from it multiple times.
There seems to be two things that prevent current code from doing this
correctly:
(a) the fscanf() code does not take into account that resctrl also
prints a "\n" ... (this seems to be the part that may cause the same
value to be returned).
So:
if (fscanf(fp, "%lu", mbm_total) <= 0) {
should be:
if (fscanf(fp, "%lu\n", mbm_total) <= 0) {
(b) the current reading does not reset the file position so a second
read will attempt to read past the beginning. A "rewind(fp)"
should help here.
A small program like below worked for me by showing different values
on every read:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
const char *mbm_total_path = "/sys/fs/resctrl/mon_data/mon_L3_00/mbm_total_bytes";
int main(void)
{
unsigned long mbm_total;
FILE *fp;
int count;
fp = fopen(mbm_total_path, "r");
if (!fp) {
perror("Opening data file\n");
exit(1);
}
for (count = 0; count < 100; count++) {
if (fscanf(fp, "%lu\n", &mbm_total) <= 0) {
perror("Unable to read from data file\n");
exit(1);
}
printf("Read %d: %lu\n",count ,mbm_total );
sleep(1);
rewind(fp);
}
fclose(fp);
return 0;
}
Reinette
