RE: Direct rdtsc call side-effect

David Laight
2023-06-05T10:28:09Z

...
> Who would have thought that rdtsc() in applications can be a problem.
> Interfaces to query time exist for a reason and it's documented by
> Microsoft:
>
> https://learn.microsoft.com/en-us/windows/win32/dxtecharts/game-timing-and-multicore-processors
>
> But sure, reading documentation is overrated...

That eve says:
"Multiprocessor and dual-core systems do not guarantee synchronization
of their cycle counters between cores."

.
> Synchronizing TSC by writing the TSC MSR is fragile as hell. This has
> been tried so often and never reliably passed all synchronization tests
> on a wide range of systems.
>
> It kinda works on single socket, but not on larger systems.
>
> We spent an insane amount of time to make timekeeping correct and I'm
> not interested at all to deal with the fallout of such a mechanim.

I've wondered whether the TSC ought to be deliberately mis-synchronised?
So the high order bits are effectively the cpu number.

It has to be said that using it as a time source was fundamentally
a bad idea.
Sometimes (eg micro benchmarks) you really want a TSC.
You can extract one from the performance counters, but it is hard,
root only, and the library functions have high and variable overhead.

David

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