Messages in this thread | | | Date | Tue, 29 Jan 2013 12:13:46 +0530 | From | Santosh Shilimkar <> | Subject | Re: One of these things (CONFIG_HZ) is not like the others.. |
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Jon,
On Tuesday 29 January 2013 05:31 AM, John Stultz wrote: > On 01/27/2013 10:08 PM, Santosh Shilimkar wrote: >> On Tuesday 22 January 2013 08:35 PM, Santosh Shilimkar wrote: >>> On Tuesday 22 January 2013 08:21 PM, Russell King - ARM Linux wrote: >>>> On Tue, Jan 22, 2013 at 03:44:03PM +0530, Santosh Shilimkar wrote:
[..]
>>> Thanks for expanding it. It is really helpful. >>> >>>> And I think further discussion is pointless until such research has >>>> been >>>> done (or someone who _really_ knows the time keeping/timer/sched code >>>> inside out comments.) >>>> >>> Fully agree about experimentation to re-asses the drift. >>> From what I recollect from past, few OMAP customers did >>> report the time drift issue and that is how the switch >>> from 100 --> 128 happened. >>> >>> Anyway I have added the suggested task to my long todo list. >>> >> So I tried to see if any time drift with HZ = 100 on OMAP. I ran the >> setup for 62 hours and 27 mins with time synced up once with NTP server. >> I measure about ~174 millisecond drift which is almost noise considering >> the observed duration was ~224820000 milliseconds. > > So 174ms drift doesn't sound great, as < 2ms (often much less - though > that depends on how close the server is) can be expected with NTP. > Although its not clear how you were measuring: Did you see a max 174ms > offset while trying to sync with NTP? Was that offset shortly after > starting NTP or after NTP converged down? > To avoid the server latency, we didn't do continuous sync. The time was synced in the beginning and after 62.5 hours (#ntpd -qg) and the drift of about 174 ms was observed. As you said this could be because of server sync time along with probably some addition from system calls from #ntpd. As mentioned, the other run with HZ = 128 which started 15 hours 20 mins is already showing about 24 mS drift now. I will let it run for couple of more days just to have similar duration run.
Regards, santosh
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