Re: [PATCH v2 1/2] perf: Add sampling of the raw monotonic clock

On Fri, 2014-09-26 at 15:38 +0100, Namhyung Kim wrote:

quoted

Then I have loads of normal normal samples, timestamped with sched clock
only, and every now and then one with both timestamps which then I can
use for time correlation. The whole point is that the frequency of such
"synchronisation" event can be much (much!) lower than of the normal
samples, but it still allows pretty good approximation (I was getting
accuracy of ~1 microsecond and better with sched_switch trace event
marked with additional raw monotonic timestamp).

Okay.  But in that case wouldn't it be enough to use just a single
timestamp for each event - sched_clock for cpu-cycles and monotonic raw
for sched_switch?

To do the correlation you need both timestamps to be "taken"
simultaneously:

        perf event     user event
       -----O--------------+-------------O------> t_mono
            :              |             :
            :              V             :
       -----O----------------------------O------> t_perf

Of course it's not possible get both values literally at the same time,
but placing them in a atomic context a couple of instructions from each
other still gives pretty good results. The larger this distance is, the
lower the accuracy will be. I must admit I haven't done such
experiments, but let me remind that I in my test I was getting results
in the range of 1000ns, with a single cycle of a 2GHz taking 0.5ns, so
moving the t_mono/t_perf value sampling further aside will reduce it
significantly...

Pawel

Pawel