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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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