[ntp:questions] Sudden drop in frequency after software update

Hal Murray hal-usenet at ip-64-139-1-69.sjc.megapath.net
Mon Jan 19 06:28:22 UTC 2009


>> Linux seme to be having a real real problem with its time calibration
>> routines. It's drift rate jumps on reboot by up to 50PPM from one
>> reboot to the next.

>Really? I don't recall ever seeing that.

I thought it was well known.  It's been discussed here several/many
times.  I've seen jumps much bigger than 50 ppm.

The problem is the TSC calibration routine in recent kernels.

It doesn't get the same answer.  It's not just temperature.  At
one point, I hacked the kernel to call it several times and print
all the answers.  It matched the spread below.

It's close enough so that nobody but a time geek would notice.


Here are some handy samples from the line that gets printed out
at boot time:

Jan  1 14:23:16 shuksan kernel: Detected 2793.159 MHz processor.
Dec 13 10:14:27 shuksan kernel: Detected 2793.118 MHz processor.
Nov 13 02:15:22 shuksan kernel: Detected 2793.049 MHz processor.
Oct  2 03:29:00 shuksan kernel: Detected 2793.117 MHz processor.
Oct 13 12:26:08 shuksan kernel: Detected 2793.236 MHz processor.

If I did the math right, that's 66 ppm peak-to-peak.  I'm pretty
sure I've seen much worse.


-- 
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