[ltt-dev] UST clock rdtsc vs clock_gettime
nils.carlson at ludd.ltu.se
Tue Jul 6 15:39:50 EDT 2010
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?
On Jul 6, 2010, at 8:12 PM, David Goulet wrote:
> After some talks with Nils from Ericsson, there was some questions
> about using the TSC counter and not clock_gettime in include/ust/
> 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 ?
> ltt-dev mailing list
> ltt-dev at lists.casi.polymtl.ca
More information about the lttng-dev