Re: Clock Skew Due to CPU Frequency Management? (was Re: Dual-core system will not create NTP peers)
On Tue, Feb 19, 2008 at 07:37:32AM -0600, Moshe Yudkowsky wrote: I had a very kind private communication from a list member. Let me reformulate my question. On the AMD64 dual-core platform, the system clock has far too much drift even though the hardware clock seems well behaved. I don't seem to be experiencing the double-speed clock problem that I see references to on the net as of a few years ago, but I am experiencing some other odd behavior. The most likely culprit, as far as I can tell from research, is some interaction between the hardware's ability to adjust CPU clock speed to save power/control temperature (which then influences the system clock's theory of what a CPU tick means in terms of actual time passing) with the dual-core mode. But that seems rather wacky. I find it hard to believe that a kernel release wouldn't work on dual core CPUs. But if the problem really is that's the case, I would like to know what boot-time settings I can use to test this hypothesis. For that matter I'd like to know why the kernel itself isn't picking up on this theoretical problem -- ISTR reading about an enhanced CPU clock mode that could solve this problem, but I can't find references to it or how to build a kernel or modules from source that would include this capability. I've spent a substantial amount of time on this problem, and I can't come to any conclusion. I have to wonder if it's a problem with NTP itself, but if so it's a problem that only manifests itself on the AMD64 box, although I continue to fiddle with NTP settings. Someone here at work was having trouble getting NTP working on their system a few weeks ago. Disabling spread spectrum in the BIOS solved the problem. Of course as far as I know disabling spread spectrum violate emmisions certifications in europe, but I could be wrong about that since I don't actually live there (anymore at least). Spread spectrum clocking really screws with the timings NTP depends on. -- Len Sorensen -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of unsubscribe. Trouble? Contact [EMAIL PROTECTED]
Re: Clock Skew Due to CPU Frequency Management? (was Re: Dual-core system will not create NTP peers)
Len, Thanks for writing with the spread-spectrum idea. I don't know if you saw the final email in this thread, but it turns out that (as far as I can tell) adjtimexconfig made a bad estimate when it was installed, and put a TICK value of 9750 in /etc/defaults/adjtimex, which of course works out to a clock that's always 2.5% slow. -- Moshe Yudkowsky * [EMAIL PROTECTED] * www.pobox.com/~moshe Poor Mexico -- so far from G-d and so near the United States. -- President Porfirio Diaz -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of unsubscribe. Trouble? Contact [EMAIL PROTECTED]
Clock Skew Due to CPU Frequency Management? (was Re: Dual-core system will not create NTP peers)
I had a very kind private communication from a list member. Let me reformulate my question. On the AMD64 dual-core platform, the system clock has far too much drift even though the hardware clock seems well behaved. I don't seem to be experiencing the double-speed clock problem that I see references to on the net as of a few years ago, but I am experiencing some other odd behavior. The most likely culprit, as far as I can tell from research, is some interaction between the hardware's ability to adjust CPU clock speed to save power/control temperature (which then influences the system clock's theory of what a CPU tick means in terms of actual time passing) with the dual-core mode. But that seems rather wacky. I find it hard to believe that a kernel release wouldn't work on dual core CPUs. But if the problem really is that's the case, I would like to know what boot-time settings I can use to test this hypothesis. For that matter I'd like to know why the kernel itself isn't picking up on this theoretical problem -- ISTR reading about an enhanced CPU clock mode that could solve this problem, but I can't find references to it or how to build a kernel or modules from source that would include this capability. I've spent a substantial amount of time on this problem, and I can't come to any conclusion. I have to wonder if it's a problem with NTP itself, but if so it's a problem that only manifests itself on the AMD64 box, although I continue to fiddle with NTP settings. -- Moshe Yudkowsky * [EMAIL PROTECTED] * www.pobox.com/~moshe If, after hearing my songs, just one human being is inspired to say something nasty to a friend, it will all have been worthwhile. -- Tom Lehrer -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of unsubscribe. Trouble? Contact [EMAIL PROTECTED]
