Date: Tue, 15 Aug 2017 23:30:44 +1000 (EST) From: Ian Smith <smithi@nimnet.asn.au> To: Kevin Oberman <rkoberman@gmail.com> Cc: Aristedes Maniatis <ari@ish.com.au>, freebsd-stable <freebsd-stable@freebsd.org>, Alexander Motin <mav@freebsd.org> Subject: Re: TSC timekeeping and cpu states Message-ID: <20170815232509.Y12950@sola.nimnet.asn.au> In-Reply-To: <CAN6yY1u_Q1tAwEcb_%2BrNJbdNDijLtRK-nno3AJorzsp1EKz=6g@mail.gmail.com> References: <f023ffcf-b968-7cc4-2b5a-2ed7e437fc18@ish.com.au> <CAN6yY1tt8Aqhk3A2TWzWV6OcdeRE8nE0wQfOUVCr=HyOHnCZRg@mail.gmail.com> <2ef99f5e-46f5-a185-2ac3-67d6afe68c89@ish.com.au> <20170815010943.B12950@sola.nimnet.asn.au> <CAN6yY1u_Q1tAwEcb_%2BrNJbdNDijLtRK-nno3AJorzsp1EKz=6g@mail.gmail.com>
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On Mon, 14 Aug 2017 09:48:07 -0700, Kevin Oberman wrote: > On Mon, Aug 14, 2017 at 8:38 AM, Ian Smith <smithi@nimnet.asn.au> wrote: [..] > > > > 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. [..] > I guess I need to clarify. No, FreeBSD does not have the ability to tun > different cores at different frequencies. I seem to recall that TCC on some > processors could adjust the frequency of a core exceeding a defined > temperature, skipping N of every 8 clock cycles to slow the processor and > reduce the temperature. This is what TCC was designed for. It is entirely > possible that I am not correctly remembering the details of the issue, but > it could only be resolved by switching from TCC to another clocking system. > > If memory serves, and it may not, there was an issue a few years ago (jhb@ > worked the issue) where TSC was varying with frequency and that caused > clock drift. I believe all "modern" processors do not have this issue and > it seems unlikely that any system running 24 cores is old enough that this > might be an issue. > > Sorry for any confusion I may have caused. Not at all. It gave me an excuse to bug Alexander for some state-of-art details, to which he responded magnificently :) Thankyou both, and Ari for entertaining such sport at his expense .. cheers, Ian
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