Date: Fri, 22 May 2015 13:59:11 -0700 From: Kevin Oberman <rkoberman@gmail.com> To: Adrian Chadd <adrian@freebsd.org> Cc: Stefan Esser <se@freebsd.org>, Nikos Vassiliadis <nvass@gmx.com>, freebsd-stable <freebsd-stable@freebsd.org> Subject: Re: CPU frequency doesn't drop below 1200MHz (like it used to) Message-ID: <CAN6yY1thVsuqgkxcqE3cdshA3-fbX_0O2KzcSxf9htenamO-ow@mail.gmail.com> In-Reply-To: <CAJ-VmonFPhwh_4oT_adCTXo%2B50AcgUug5PRTfaktjLoz-9gN7g@mail.gmail.com> References: <555C71C8.4080007@gmx.com> <555EDBBB.4090107@gmx.com> <555F7599.5000605@freebsd.org> <CAJ-VmonFPhwh_4oT_adCTXo%2B50AcgUug5PRTfaktjLoz-9gN7g@mail.gmail.com>
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On Fri, May 22, 2015 at 11:47 AM, Adrian Chadd <adrian@freebsd.org> wrote: > Hi, > > The whole point of throttling on modern hardware isn't to get really > low clock rates, it's to deal with being out of thermal envelope. > > But, the modern intel cores will do that for you without OS involvement. > > So, you don't have to actually use p4tcc and it may actually configure > your hardware wrong. Just throttle down to 1200MHz and go into deeper > sleep states (>C1). I checked this on a variety of older and modern > hardware; they all worked better just doing lowest ACPI P state and > lowest ACPI C state. > > > > -adrian > > It's actually worse than this. TCC (which was first available on the Pentium 4) was always documented by Intel as a Thermal Control Circuit. Nothing about power management. FreeBSD suborned it for power management a long time go, but it never actually saved power. EST , which adjusts frequency and voltage does save a bit. Cx states save a lot. Using TCC for this was a very bad idea. (I did research on this back when I worked for Berkeley Lab.) There are a couple of very limited corner cases where throttling MAY save an utterly insignificant amount of power, but when C-states came about, Intel never considered the impact of C-states when the OS was playing around with throttling. (Windows never did that.) If you want to save power, set both economy_cx_lowest (battery) and performance_cx_lowest (AC) to Cmax in rc.conf. (This is the default in head.) Do not set a minimum frequency for powerd or it may fail to start if you specify a "frequncy" that is no longer available. That capability was to prevent system lockups when TCC and Cx collided. With TCC off, there is no need to worry about it. Please don't just turn TCC back on. It really just makes things worse. Read mav's excellent article on the issues on the FreeBSD wiki at https://wiki.freebsd.org/TuningPowerConsumption. His research and mine came to virtually identical conclusions. -- Kevin Oberman, Network Engineer, Retired E-mail: rkoberman@gmail.com
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