Messages in this thread | | | Subject | Re: NTP dumps Linux, film at 11. [Fwd/FYI] | From | (Kevin Buhr) | Date | 02 Dec 1998 14:15:41 -0600 |
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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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