Date: Sun, 13 Aug 2017 22:08:31 -0700 From: Kevin Oberman <rkoberman@gmail.com> To: Aristedes Maniatis <ari@ish.com.au> Cc: freebsd-stable <freebsd-stable@freebsd.org> Subject: Re: TSC timekeeping and cpu states Message-ID: <CAN6yY1tt8Aqhk3A2TWzWV6OcdeRE8nE0wQfOUVCr=HyOHnCZRg@mail.gmail.com> In-Reply-To: <f023ffcf-b968-7cc4-2b5a-2ed7e437fc18@ish.com.au> References: <f023ffcf-b968-7cc4-2b5a-2ed7e437fc18@ish.com.au>
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On Sun, Aug 13, 2017 at 8:25 PM, Aristedes Maniatis <ari@ish.com.au> wrote: > I note that in FreeBSD 11, we now have this: > > # grep performance_cx_lowest /etc/defaults/rc.conf > performance_cx_lowest="C2" # Online CPU idle state > > However this wiki page suggests that C1 is the default > > https://wiki.freebsd.org/TuningPowerConsumption > > > Are these inconsistent? > > > I went looking for this because I've been having trouble with the TSC-low > timecounter hardware choice and my system clock running at about 80% of > normal speed. Moving to ACPI-fast solved this problem. > > Could the power saving CPU states be related to this problem, or should I > look elsewhere for the TSC issue? > > This is a server, not a laptop. > > > Thanks > Ari > > > > -- > --------------------------> > Aristedes Maniatis > CEO, ish > https://www.ish.com.au > GPG fingerprint CBFB 84B4 738D 4E87 5E5C 5EFA EF6A 7D2E 3E49 102A > 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. 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. -- Kevin Oberman, Part time kid herder and retired Network Engineer E-mail: rkoberman@gmail.com PGP Fingerprint: D03FB98AFA78E3B78C1694B318AB39EF1B055683
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