on 03/06/2011 14:50 John Baldwin said the following: > On Friday, June 03, 2011 2:03:55 am Andriy Gapon wrote: >>> Consecutive RDTSCs used on a same CPU is always incremental but we >>> cannot 100% guarantee that on two cores, even if TSC is derived from >>> the same clock. I am hoping at least latency difference (I believe >>> it's about few tens of cycles max) is "eaten up" by lowering >>> resolution. It's not perfect but it's better than serialization >>> (Linux) or heuristics (OpenSolaris), just because there are few rare >>> conditions to consider. Thoughts? >> >> I am still not sure which case this code should solve. >> >> Thread T1: x1 = rdtsc() on CPU1; >> Thread T1: x2 = rdtsc() on CPU2; >> x2 < x1 ? >> Or? > > Yes, that can happen.
Well, I think that the test based on smp_rendezvous should ensure that difference in TSC values is "small enough"; that is, I expect that cost (in TSC ticks) of migrating a thread from CPU to CPU should be larger than that difference if the test was passed. Is this an unreasonable expectation? -- Andriy Gapon _______________________________________________ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "freebsd-hackers-unsubscr...@freebsd.org"
