Performance regression in 1.10.3

[jsquyres]

@hjelmn is working on identifying a performance regression that was introduced in 1.10.3. He doesn't have any further info yet beyond the fact that one of their apps runs a bunch slower with 1.10.3 vs 1.10.2.

He thinks this will also affect v2.0.x and v2.x.

This is potentially a blocker for v1.10.5 and v2.0.2.

@hjelmn Says he'll have more information shortly.

[hjelmn]

Bisect shows for 1.10 the regression starts with 9f2a6da. This commit forces the use of clock_gettime () over the rtdtsc instruction. IMHO this is a really bad idea. This forces us to call clock_gettime () on every single call to opal_progress ().

@bosilca We do not need a monotonic timer for opal_progress (), correct? If not we should force opal_progress to always use the asm timer and not clock_gettime ().

[hjelmn]

@rhc54, @jladd-mlnx. I would think this would affect message rates as well. The pre-1.10.3 behavior can be forced using --mca timer_require_monotonic 0. Might be worth rerunning some of the performance regressions with this option set.

[bosilca]

@hjelmn I think we can survive in opal_progress without a monotonic timer, in which case we might run the libevent progress at an erratic rate. However, this will force us to expose 2 timers, one that is monotonic and one that might not be, and we do not have right now the infrastructure.

[hjelmn]

@bosilca. Yeah. Not sure how to handle that.

For now I can do something for newer Intel processors. There is a cpuid bit to indicate whether the cpu has a core invariant tsc. I am adding a function to check that bit. If it is set it will use the rtdtsc instruction as a monotonic timer.

[bosilca]

👍

[ggouaillardet]

Iirc, we moved to clock_gettime() because MPI_Wtime() could go back in time if a (thread of a) task migrates within a node.
@hjelmn do you plan to test the cpuid bit at configure time or at run time ?
Iirc once again, we take several shortcuts in the timer, and it does not behave as a standard component.
Having two timers (one internal, one external and always monotonic) could be the most efficient and "portable" way