On Mon, 14 Aug 2017 17:16:22 +1000, Aristedes Maniatis wrote:
> On 14/8/17 3:08PM, Kevin Oberman wrote:
> > Again, the documentation lags reality. The default was changed for
> > 11.0. It is still conservative. In ALMOST all cases, Cmax will yield
> > the bast results. However, on large systems with many cores, Cmax
> > will trigger very poor results, so the default is C2, just to be
> > safe.
Given it's a server, anything beyond C2 is likely not worth trying.
OTOH, C2 is perhaps not worth avoiding; it's probably low latency and
should result in lower power consumption, so heat, and unlikely to hurt.
Or at least, I suspect that's the case .. cc'ing Alexander, as the wiki
article you referenced was his doing, so he's among those best placed.
> > As far as possible TSC impact, I think older processors had TSC
> > issues when not all cores ran with the same clock speed. That said,
> > I am not remotely expert on such issues, so don't take this too
> > seriously.
I wasn't aware that FreeBSD could yet do different freqs on different
cores? But I'm less expert than Kevin, and certainly behind the times.
> Thanks Kevin
>
> What does 'large' and 'many cores' mean here? Is 24 cores
large or
> small? For a server do we ever want the CPU to enter states other
> than C1?
If C2 works well on your box, I don't see why you wouldn't want to use
it .. but others might. I have no personal experience beyond 2 cores,
but I'm perennially curious about such issues, as Kevin knows :)
Are you using powerd? And what says, for example:
sysctl -a | egrep 'cx|available|_freq|freq_|choice' | grep -v net\.
which should include your timecounter and eventtimer setup too.
cheers, Ian