From owner-freebsd-stable@freebsd.org Tue Aug 15 13:30:55 2017 Return-Path: Delivered-To: freebsd-stable@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id 742BADD450D for ; Tue, 15 Aug 2017 13:30:55 +0000 (UTC) (envelope-from smithi@nimnet.asn.au) Received: from sola.nimnet.asn.au (paqi.nimnet.asn.au [115.70.110.159]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 6CAEC6FD01; Tue, 15 Aug 2017 13:30:52 +0000 (UTC) (envelope-from smithi@nimnet.asn.au) Received: from localhost (localhost [127.0.0.1]) by sola.nimnet.asn.au (8.14.2/8.14.2) with ESMTP id v7FDUi1x067492; Tue, 15 Aug 2017 23:30:44 +1000 (EST) (envelope-from smithi@nimnet.asn.au) Date: Tue, 15 Aug 2017 23:30:44 +1000 (EST) From: Ian Smith To: Kevin Oberman cc: Aristedes Maniatis , freebsd-stable , Alexander Motin Subject: Re: TSC timekeeping and cpu states In-Reply-To: Message-ID: <20170815232509.Y12950@sola.nimnet.asn.au> References: <2ef99f5e-46f5-a185-2ac3-67d6afe68c89@ish.com.au> <20170815010943.B12950@sola.nimnet.asn.au> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-BeenThere: freebsd-stable@freebsd.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: Production branch of FreeBSD source code List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 15 Aug 2017 13:30:55 -0000 On Mon, 14 Aug 2017 09:48:07 -0700, Kevin Oberman wrote: > On Mon, Aug 14, 2017 at 8:38 AM, Ian Smith 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