cpufreq broken on core2duo

[Jeremy Chadwick]

On Sun, Jun 08, 2008 at 10:13:43PM -0700, Kevin Oberman wrote:
> > Date: Sat, 7 Jun 2008 09:48:12 -0700
> > From: Jeremy Chadwick <koitsu at FreeBSD.org>
> > Sender: owner-freebsd-stable at freebsd.org
> > 
> > On Sat, Jun 07, 2008 at 05:51:38PM +0300, Evren Yurtesen wrote:
> > > By the way, there is another thing I am wondering about. If I enable HTT 
> > > and Intel Enhanced SpeedStep in bios on a 3.00GHZ p4 CPU I see:
> > >
> > > cpu0: <ACPI CPU> on acpi0
> > > acpi_perf0: <ACPI CPU Frequency Control> on cpu0
> > > p4tcc0: <CPU Frequency Thermal Control> on cpu0
> > > cpu1: <ACPI CPU> on acpi0
> > > est1: <Enhanced SpeedStep Frequency Control> on cpu1
> > > est: CPU supports Enhanced Speedstep, but is not recognized.
> > > est: cpu_vendor GenuineIntel, msr f2700000f27
> > > device_attach: est1 attach returned 6
> > > p4tcc1: <CPU Frequency Thermal Control> on cpu1
> > >
> > > dev.cpu.0.freq_levels: 1500/27000 1312/23625 1200/13000 1050/11375 900/9750 
> > > 750/8125 600/6500 450/4875 300/3250 150/1625
> > > dev.acpi_perf.0.freq_settings: 1500/27000 1200/13000
> > > dev.cpufreq.0.%driver: cpufreq
> > > dev.cpufreq.0.%parent: cpu0
> > > dev.cpufreq.1.%driver: cpufreq
> > > dev.cpufreq.1.%parent: cpu1
> > > dev.p4tcc.0.freq_settings: 10000/-1 8750/-1 7500/-1 6250/-1 5000/-1 3750/-1 
> > > 2500/-1 1250/-1
> > > dev.p4tcc.1.freq_settings: 10000/-1 8750/-1 7500/-1 6250/-1 5000/-1 3750/-1 
> > > 2500/-1 1250/-1
> > >
> > > and it does not allow me to set the freq. of the cpu.
> > 
> > How are you setting the frequency?  Are you using powerd?  You do not
> > have to enable SpeedStep in your BIOS to achieve throttling CPU clock
> > speed.  In fact, I would highly recommend leaving EIST/SpeedStep in the
> > BIOS *disabled*, and let powerd adjust the clock frequency via ACPI.
> 
> I must strongly recommend against this. EST is MUCH more efficient on
> its control of power use than simple throttling. So much so that on my
> systems that support EST, I remove cpufreq from the kernel. (In all
> cases, throttling means either simple throttling or throttling by using
> TCC.) 

The reality of the situation is that users cannot use EIST reliably on
FreeBSD.  I cannot imagine removing cpufreq would solve the "runtime
went backwards" issue.  The EIST problem needs to be fixed.  If the
folks who work on it do not have present-day hardware (C2Ds, etc.), I
will be more than happy to purchase them whatever they need.

> I did quite a bit of testing on power management a year or so ago and
> found that throttling was of value only for controlling CPU temperature.
> For real power management, EST works far better as it adjust frequency
> (actual clock rate) and CPU voltage while throttling just stops and
> starts the clock without changing its actual frequency. (This came as a
> surprise to me about 5 years ago when I first discovered it.)

As far as I know, the ACPI frequency toggling does in fact adjust the
CPU clock rate -- because the values in dev.cpu.0.freq_levels are
defined by the ACPI configuration on the machine.

I'd confirm this, but looking at the kernel code doesn't help -- I
cannot find the definition of the CPUFREQ_LEVELS function or macro.

> At idle, throttling does exactly nothing. EST reduces voltage on the
> CPU and saves power even when idle. At full CPU load, throttling reduces
> performance and power consumption equally. EST beats it by a slim
> margin.

I thought the power-saving modes of the CPU (C1E/C2E/C3E/C4E) could be
used *without* enabling EIST.  I don't think FreeBSD has kernel or
userland support utilities to enable/disable CPU-level features.
Something like RMClock for Windows would be quite useful.  (If you have
the opportunity to run it, do so; you'll see what I mean, re: all the
features being toggleable individually).

> The big win is at moderate load. Throttling can result is very poor
> results for aps like video and music which will place a continuous load
> on the system, but only 20-60% (in the case of my test system). It
> tended to make the system seem sluggish. EST does a much better job in
> this case as it lowers CP voltage and clock rate to maximize performance
> while minimizing power.
>
> If your only concern is keeping the system cool, throttling will do the
> job, but if you want efficient power utilization, use EST if possible.

In the case est does not attach (e.g. no EIST support enabled in
FreeBSD), the throttling method resorts to simply suspending the system,
then yes I completely agree -- EIST is significantly better than that.

-- 
| Jeremy Chadwick                                jdc at parodius.com |
| Parodius Networking                       http://www.parodius.com/ |
| UNIX Systems Administrator                  Mountain View, CA, USA |
| Making life hard for others since 1977.              PGP: 4BD6C0CB |