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    Subjectgettimeofday() jumping into the future

    We've been seeing some strange behaviour on some of our applications
    recently. I've tracked this down to gettimeofday() returning spurious
    values occasionally.

    Specifically, gettimeofday() will suddenly, for a single call, return
    a value about 4398 seconds (~1 hour 13 minutes) in the future. The
    following call goes back to a normal value.

    This seems to be occurring when the clock source goes slightly
    backwards for a single call. In
    kernel/time/timekeeping.c:__get_nsec_offset(), we have this:
    cycle_delta = (cycle_now - clock->cycle_last) & clock->mask;

    So a small decrease in time here will (this is all unsigned
    arithmetic) give us a very large cycle_delta. cyc2ns() then multiplies
    this by some value, then right shifts by 22. The resulting value (in
    nanoseconds) is approximately 4398 seconds; this gets added on to the
    xtime value, giving us our jump into the future. The next call to
    gettimeofday() returns to normal as we don't have this huge nanosecond

    This system is a 2-socket core 2 quad machine (8 cpus), running 32 bit
    mode. It's a dell poweredge 1950. The kernel selects the TSC as the
    clock source, having determined that the tsc runs synchronously on
    this system. Switching the systems to use a different time source
    seems to make the problem go away (which is fine for us, but we'd like
    to get this fixed properly upstream).

    We've also seen this behaviour with a synthetic test program (which
    just runs 4 threads all calling gettimeofday() in a loop as fast as
    possible and testing that it doesn't jump) on an older machine, a dell
    poweredge SC1425 with two p4 hyperthreaded xeons.

    Can anyone advise on what's going wrong here? I can't find much in the
    way of documentation on whether the TSC is guaranteed to be
    monotonically increasing on intel systems. Should the code choose not
    to use the TSC? Or should the TSC reading code ensure that the
    returned values are monotonic?

    Is there any more information that would be useful? I'll be on a plane
    for most of tomorrow, so might be a little slow responding.


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      Last update: 2007-08-23 13:11    [W:0.021 / U:36.116 seconds]
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