On Mon, Apr 19, 2010 at 03:25:43PM -0300, Glauber Costa wrote: > On Mon, Apr 19, 2010 at 09:19:38AM -0700, Jeremy Fitzhardinge wrote: > > On 04/19/2010 07:26 AM, Glauber Costa wrote: > > >> Is the problem that the tscs are starting out of sync, or that they're > > >> drifting relative to each other over time? Do the problems become worse > > >> the longer the uptime? How large are the offsets we're talking about here? > > >> > > > The offsets usually seem pretty small, under a microsecond. So I don't think > > > it has anything to do with tscs starting out of sync. Specially because the > > > delta-based calculation has the exact purpose of handling that case. > > > > > > > So you think they're drifting out of sync from an initially synced > > state? If so, what would bound the drift? > I think delta calculation introduces errors. Yes. > Marcelo can probably confirm it, but he has a nehalem with an appearently > very good tsc source. Even this machine warps. > > It stops warping if we only write pvclock data structure once and forget it, > (which only updated tsc_timestamp once), according to him. Yes. So its not as if the guest visible TSCs go out of sync (they don't on this machine Glauber mentioned, or even on a multi-core Core 2 Duo), but the delta calculation is very hard (if not impossible) to get right. The timewarps i've seen were in the 0-200ns range, and very rare (once every 10 minutes or so). > Obviously, we can't do that everywhere. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
