Cool, so the measurements came through...
What I would like to see is the automatic detection of whether the
rdtsc instruction is usable,
a test for this already exists in the kernel and the question is
whether this info is currently exported
or whether we need to submit a patch to export it.
Then we should probably start looking at a simple choosing mechanism,
probably a function pointer?
/Nils
On Jul 6, 2010, at 8:12 PM, David Goulet wrote:
Hey,
After some talks with Nils from Ericsson, there was some questions
about using the TSC counter and not clock_gettime in include/ust/
clock.h
I ran some test after the meeting and was quite surprised by the
overhead of clock_gettime.
On an average run ...
WITH clock_gettime : ~ 266ns per events
WITH rdtsc instruction : ~ 235ns per events
And it is systematic... I'm getting stable result with rdtsc with
standard deviation of ~2ns.
As little as I know on TSC, one thing for sure, with SMP, it becomes
much more "fragile" to rely on it because we don't have assurance of
coherent counters between CPUs and also the CPU scaling policy
(ondemand is default on Ubuntu now). New CPUs support constant_tsc
and nonstop_tsc flags but still a small range of them.
Right now, UST is forcing the use of clock_gettime even if i386 or
x86_64 is used.
Should a change be consider ?
Thanks
David
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