On Mar 8, 2011, at 5:56 PM, Steve Kostecke wrote: > On 2011-03-08, Chuck Swiger <cswi...@mac.com> wrote: [ ... ] >>> NTP disciplines the system (i.e. kernel) clock, not the hardware >>> clock on the mother board. >> >> That's right, although in reasonably common for platforms to >> periodically write the system clock time back to the hardware >> clock-- variously called the RTC/TOD/TOY clock which is in the >> BIOS/EFI/firmware and keeps time when the system is off. > > The RTC is _updated_, not synced, by the kernel.
Right. Perhaps I'm mistaken, but I don't recall suggesting otherwise...? [ ... ] >>> I have a Debian 6.0 system running as a VMWare guest. ntpd on this >>> system has no problem disciplining the clock. >> >> OK. Does it do any better than using VMWare's "tools.syncTime = true"? > > I don't have access to the host. OK. If I was going to compare two things, well, I would actually try them both while measuring relevant factors to my comparison. If I had never tried one of the two, and I didn't have any other data available, then I wouldn't try to draw a conclusion about which alternative works better. Note [1]. >> Your jitter values are well over an order of magnitude worse than that >> of ntpd running on a non-virtualized machine, and your offsets are >> nearly an order of magnitude worse: > > You're comparing apples and oranges. Absolutely. In fact, this is exactly the point I was making. >> For all of that, your VM is doing pretty well running ntpd compared to >> others I'd seen. I'd imagine the host running the VM isn't especially >> busy; if it was, I wouldn't be surprised if ntpd can't manage to >> discipline the clock without "tinker panic 0". > > The default panic threshold is 1024 seconds. Right. Depending on which VM technology is being used, it's entirely possible to suspend a guest OS for longer than 1024 seconds. Again, there's a point lurking here about the quality of timekeeping which is possible within a VM. A real machine, or the host ESX/Dom 0/whatever, ought to be able to keep good time without this option, and I would be looking for hardware failure, interrupt routing problems, etc if it could not. >>>> You are better off running ntpdate (or sntp) periodically via cron >>>> in the DomUs. >>> >>> Perhaps in certain cases, but not across the board. >> >> I'd be happy to review counterexamples to my generalization... > > There's my example. See note [1] above. To my mind, you don't have enough data to draw a conclusion from your example. Regards, -- -Chuck _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
