[Scip] Timing issues

[Giacomo Nannicini]

Dear Mr SCIP,
we are getting some strange readings when running the same process
multiple times. A (very smart) student who's working with me reports
that if he runs the same job (same input, parameters etc) multiple
times, using 4 parallel jobs on a machine that has 4 distinct CPUs and
no other load, some jobs may take considerably longer. Number of
iterations, nodes etc. is exactly the same of course.

For example, almost all runs out of 30 runs take ~500 seconds with
tiny variance as expected, and a couple of runs take an extra 200
seconds (!!!). The process is not swapping and the only shared
resources are HD and RAM: each CPU has its own cache. This happens on
a few problem instances. Note that typically, we do not have this
issue and all jobs take almost exactly the same time, as expected.

We are measuring user CPU time, clocktype = 1.

I could not replicate this, but the logs and settings look correct and
I have no idea where the discrepancy comes from. Did anybody
experience a similar issue? Is it a hardware related thing (TurboBoost
is disabled, but who knows...)? And finally, is there any way to get a
more stable reading of the CPU time (I see that SCIP uses times(),
while I am more familiar with getrusage())?

Any ideas are welcome because I am completely in the dark. I am
willing to pay a beer for a hint in the right direction.

Giacomo