On Fri, Apr 16, 2010 at 01:36:34PM -0700, Jeremy Fitzhardinge wrote: > On 04/15/2010 11:37 AM, Glauber Costa wrote: > > In recent stress tests, it was found that pvclock-based systems > > could seriously warp in smp systems. Using ingo's time-warp-test.c, > > I could trigger a scenario as bad as 1.5mi warps a minute in some systems. > > > > Is that "1.5 million"? Yes it is. But as I said, this seem to be a very deep worst case scenario. Most of boxes are not even close to being that bad. > > > (to be fair, it wasn't that bad in most of them). Investigating further, I > > found out that such warps were caused by the very offset-based calculation > > pvclock is based on. > > > > 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. -- 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
