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    SubjectRe: [PATCH 1/5] Add a global synchronization point for pvclock
    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.

     \ /
      Last update: 2010-04-19 16:29    [W:0.022 / U:113.648 seconds]
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