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    SubjectRe: NTP dumps Linux, film at 11. [Fwd/FYI]
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    Linus Torvalds <torvalds@transmeta.com> writes:
    >
    > Btw, just in case people worry: don't worry too much. I use ntpd every
    > day, and it works beautifully. But I know that at least early 2.1.x SMP
    > kernels had horribly problems with ntpd - losing synchronization
    > completely several times a day, and not ever really getting truly good
    > time.

    I had symptoms like this, and that may be part of what you're
    remembering.

    It turned out that it had nothing to do with "xntpd" or kernel time
    interfaces; the "fdomain.c" SCSI driver had huge (300ms+) latencies.
    When the SCSI drivers were changed, en masse, to be SMP-friendly by
    putting a coarse "io_request_lock" around every SCSI interrupt
    handler, "fdomain.c" needed a patch to fine-grain those locks.

    My patch for that particular driver went in at 2.1.111 or thereabouts,
    but I don't think anyone looked through the rest of the SCSI drivers
    systematically. Certainly the drivers where the problem was most
    obvious have been patched, but perhaps there are a few SCSI drivers
    that lose a few timer ticks here and there, just enough so that only
    people using "xntpd" notice the slippage.

    I guess this is just a heads-up for people observing time
    synchronization problems: if your time is periodically stepped forward
    by relatively big (i.e., multisecond) chunks at seemingly random
    times, suspect a SCSI (or other) driver dropping interrupts.

    Kevin <buhr@stat.wisc.edu>

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