Date: Tue, 15 Aug 2017 01:38:30 +1000 (EST) From: Ian Smith <smithi@nimnet.asn.au> To: Aristedes Maniatis <ari@ish.com.au> Cc: Kevin Oberman <rkoberman@gmail.com>, freebsd-stable <freebsd-stable@freebsd.org>, Alexander Motin <mav@freebsd.org> Subject: Re: TSC timekeeping and cpu states Message-ID: <20170815010943.B12950@sola.nimnet.asn.au> In-Reply-To: <2ef99f5e-46f5-a185-2ac3-67d6afe68c89@ish.com.au> References: <f023ffcf-b968-7cc4-2b5a-2ed7e437fc18@ish.com.au> <CAN6yY1tt8Aqhk3A2TWzWV6OcdeRE8nE0wQfOUVCr=HyOHnCZRg@mail.gmail.com> <2ef99f5e-46f5-a185-2ac3-67d6afe68c89@ish.com.au>
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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
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