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List:       freebsd-threads
Subject:    Re: CLOCK_MONOTONIC / CLOCK_UPTIME is not really monotonic between threads
From:       Seo Townsend <seotownsend () icloud ! com>
Date:       2016-06-04 10:09:44
Message-ID: B93363A9-DBA4-4D9B-81AE-B017C8D8ACE5 () icloud ! com
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Heyy,
Sure your machine isn't sitting next to a block hole?  :P

Many of the names (CLOCK_XXXXXX) seem to be just aliased.
https://github.com/freebsd/freebsd/blob/master/sys/kern/kern_time.c#L343 \
<https://github.com/freebsd/freebsd/blob/master/sys/kern/kern_time.c#L343>

I didn't thoroughly look at this, but it seems like there's a margin of noise
introduced by drift corrections that might (half ass-edly) explain the sub \
millisecond errors you were receiving?
https://github.com/freebsd/freebsd/blob/master/sys/kern/kern_tc.c#L1374 \
<https://github.com/freebsd/freebsd/blob/master/sys/kern/kern_tc.c#L1347>

For the larger >1ms errors, ...that's probably not good?  lol.  Might be worth it
to try the timersub library function: \
(http://www.unix.com/man-page/freebsd/3/timercmp/) to calculate the delta.

> On Jun 3, 2016, at 7:28 AM, Maxim Sobolev <sobomax@FreeBSD.org> wrote:
> 
> a. multiple cores.
> b. makes no difference
> c. yes, I believe so
> 
> kern.timecounter.tsc_shift: 1
> kern.timecounter.smp_tsc_adjust: 0
> kern.timecounter.smp_tsc: 1
> kern.timecounter.invariant_tsc: 1
> machdep.tsc_freq: 2658118740
> machdep.disable_tsc_calibration: 0
> machdep.disable_tsc: 0
> 
> d. no, single socket Intel Q6700. I've also seen this problem on core
> i7-4770 running virtualbox 5.x. I have a hints that this also happens on
> our bigger production boxes, but I have no specifics yet.
> 
> On Thu, Jun 2, 2016 at 10:05 PM, Adrian Chadd <adrian.chadd@gmail.com>
> wrote:
> 
> > [snip]
> > 
> > a) is it on one core, or multiple cores?
> > b) CLOCK_MONOTONIC_FAST?
> > c) is it on a system that /has/ invariant-TSC ?
> > d) is this a multi-socket system?
> > 
> > 
> > 
> > -adrian
> > 
> > 
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