Hacker News new | ask | show | jobs
by toast0 303 days ago
I think most current systems have invariant tsc, I skimmed your article and was surprised to see an offset (but not totally shocked), but the rate looked the same.

You could cpu pin the thread that's reading the tsc, except you can't pin threads in OpenBSD :p
1 comments
wahern 303 days ago
But just to be clear (for others), you don't need to do that because using RDTSC/RDTSCP is exactly how gettimeofday and clock_gettime work these days, even on OpenBSD. Where using the TSC is practical and reliable, the optimization is already there.

OpenBSD actually only implemented this optimization relatively recently. Though most TSCs will be invariant, they still need to be trained across cores, and there are other minutiae (sleeping states?) that made it a PITA to implement in a reliable way, and OpenBSD doesn't have as much manpower as Linux. Some of those non-obvious issues would be relevant to someone trying to do this manually, unless they could rely on their specific hardware behavior.

link
RossBencina 303 days ago
Out of interest, does training across cores result in any residual offset? If so, is the offset nondeterministic?
link
wahern 301 days ago
I was curious myself, poked around, and found some references. But I'm still woefully incapable of answering that with any confidence and don't want to risk saying anything misleading, so here's the code and some other breadcrumbs:

1. Apparently OpenBSD gave up on trying to fix desync'd TSCs. See https://github.com/openbsd/src/commit/78156938567f79506a923c...

2. Relevant OpenBSD kernel code: https://github.com/openbsd/src/blob/master/sys/arch/amd64/am...

3. Relevant Linux kernel code: https://github.com/torvalds/linux/blob/master/arch/x86/kerne..., https://github.com/torvalds/linux/blob/master/arch/x86/kerne...

4. Linux kernel doc (out-of-date?): https://www.kernel.org/doc/Documentation/virtual/kvm/timekee...

5. Detailed SUSE blog post with many links: https://www.suse.com/c/cpu-isolation-nohz_full-troubleshooti...

6. Linux patch (uncommitted?) to attempt to directly sync TSCs: https://lkml.rescloud.iu.edu/2208.1/00313.html

link