On Thu, Sep 7, 2017 at 5:09 AM, Thibaut BEYLER <thibautbey...@gmail.com> wrote:
> Well, I found out that if i run a cpu stress test (i am using mprime
> 'torture test') while running chrony, suddenly my PPS source std dev goes
> stable below 300ns.
>
> So i guess this is realted to some kind of cpu scheduling. However i
> disabled the c-state options in the bios, my CPU frequency is not altered
> according to /proc/cpuinfo, and yet i still have bad results unless i run
> this stress test.
>
> Any idea ?

I found similar results when stressing the cpus (I saw the
synchronization get much more stable, although it wasn't with a pps
source).

In my case it was due to cpu c-states, even though I had also had the
bios set to "performance" mode (or something similar).

You can see if its c-states with `turbostat --debug' (in CentOS 6 its
part of the `cpupowerutils' package, in CentOS 7 its part of
`kernel-tools').  For other distro's, I'm not sure.

It'll show you where each processor is spending its time.

If it is, in fact, c-states, then you can either disable them via the
kernel (some combination of `processor.max_cstate',
`intel_idle.max_cstate' and maybe writing "performance" to
`/sys/devices/system/cpu/cpu*/cpufreq/scaling_governor') or you can
use the PM QOS interface from user space [1].

Hope some of that is useful.

[1] https://access.redhat.com/articles/65410

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