Messages in this thread | | | Date | Mon, 24 May 2010 16:19:44 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH] x86: Export tsc related information in sysfs |
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On 05/24/2010 04:16 PM, Dan Magenheimer wrote: >> From: john stultz [mailto:johnstul@us.ibm.com] >> Also, you don't really need extra accuracy, you just need it to be the >> same from boot to boot. NTP keeps the correction factor persistent from >> boot to boot via the drift file. The boot argument is just trying to >> save the time (possibly hours depending on ntp config) after a reboot >> for NTP to correct for the new error introduced by calibration. > > I was assuming that extra accuracy would decrease the ntp > convergence time by about the same factor (5-6 bits of extra > accuracy would decrease ntp convergence time by 32-64x). > Is that an incorrect assumption? >
Yes.
>> From: H. Peter Anvin [mailto:hpa@zytor.com] >> A longer sample would make sense if the goal is to freeze it into a >> kernel command line variable, but the real question is how many people >> would actually do that (and how many people would then suffer problems >> because they upgraded their CPU/mobo and got massive failures on post- >> boot.) > > Not sure why upgraded mobo's would fail due to a longer sample?
Not due to a longer sample, but a frozen sample.
> As more and more systems become dependent on clocksource==tsc > and more and more people assume nanosecond-class measurements > are relatively accurate, I'd expect the accuracy of tsc_khz > to become more important. While desktop users might bristle > at an extra second of boot delay, I'll bet many server > farm administrators would gladly pay that upfront cost > if they know an option exists.
Not really. The delta measurements aren't the issue here, but rather walltime convergence.
-hpa
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