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 _______________________________________________ freebsd-stable@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/freebsd-stable To unsubscribe, send any mail to "freebsd-stable-unsubscr...@freebsd.org"