Hi !
trying to measure cache efects I tried the folowing.
this is the modification to rtlinux/examples/measurements/rt_process.c
it simply grabs the time twice with nothing(?) inbetween . So is this now
the cache induced jitter ??
for (i = 0; i < ntests; i++) {
++cnt;
pthread_wait_np();
begin = clock_gethrtime(CLOCK_UST);
end = clock_gethrtime(CLOCK_UST);
diff = end - begin;
if (diff < min_diff) {
min_diff = diff;
}
if (diff > max_diff) {
max_diff = diff;
}
}
output from monitor:
--------------------
min: 5028, max: 7543
min: 5028, max: 6705
min: 5028, max: 6705
min: 5028, max: 6705
(snip...)
min: 5028, max: 6705
min: 5028, max: 6705
min: 5028, max: 6705
min: 5028, max: 7543
min: 5028, max: 6705
min: 5028, max: 8381
min: 5028, max: 6704
min: 5028, max: 6705
min: 5028, max: 6705
min: 5028, max: 6705
min: 5028, max: 20952 <- 15 us jitter due to ?
min: 5028, max: 6705
in this case min is the actual runtime of
clock_gethrtime(CLOCK_UST);
and max-min is the cache/RAM induced jitter.
so this means this is the absolute minimum jitter that this system (i486/75)
will always show , I guess there is no way to get below this?
comments ?
nmg
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