Date: Fri, 03 Jun 2011 09:03:55 +0300 From: Andriy Gapon <avg@FreeBSD.org> To: Jung-uk Kim <jkim@FreeBSD.org> Cc: freebsd-hackers@FreeBSD.org Subject: Re: [RFC] Enabling invariant TSC timecounter on SMP Message-ID: <4DE8794B.60100@FreeBSD.org> In-Reply-To: <201106011655.51233.jkim@FreeBSD.org> References: <201105241356.45543.jkim@FreeBSD.org> <201105311616.31256.jkim@FreeBSD.org> <4DE5D0D1.1030903@FreeBSD.org> <201106011655.51233.jkim@FreeBSD.org>
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on 01/06/2011 23:55 Jung-uk Kim said the following: > Yes, it's still a work-in-progress. However, I thought it is good > enough for 9.0 inclusion. BTW, the latest patch is here: > > http://people.freebsd.org/~jkim/tsc_smp_test5.diff > > FYI, the only meaningful change from the previous version is that it's > limited to AMD single-socket Bulldozer platforms and Intel Core and > later platforms. We may add more quirks if needed, of course. Looks good, but I think that the check is a little bit unfair to AMD Family 10h+ CPUs. Although TSCs in those CPUs are per core I've never seen them drift out of sync if they started with the same value. [snip] > 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? -- Andriy Gapon
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