Re: Disable cpufreq on modern X86 processors

[Andre Przywara <andre.przywara@amd.com>]

On 07/24/2012 03:22 PM, Andreas Herrmann wrote:
> CC-ing Andre
>
> On Tue, Jul 24, 2012 at 03:15:14PM +0200, Thomas Renninger wrote:
>> Hi,
>>
>> I recently got pointed to performance losses measured
>> with and without cpufreq enabled when people worked on
>> scheduler tunables/improvements.
>>
>> Depending on whether processes are bound to cores, tunables
>> inside the cpufreq subsystem, etc. there can be rather big
>> differences.
>>
>> While there have been improvements (for example do not poll
>> that often if constantly running at highest frequency and
>> others), dynamic cpufreq adjusting as it currently is
>> implemented via ondemand/conservative governors always
>> will cost performance.
>>
>> Arjan mentioned quite some time ago, that for modern X86
>> processors it does not make much sense to control the
>> frequency of the CPU via OS, because idle states are
>> much more efficient and should get entered asap.
>>
>> Especially on bigger X86 systems with dozens or even hundreds
>> of cores, cpufreq polling sounds like a bad idea.
>> Especially if the CPUs do achieve the same or even
>> better performance/power results via entering C-states quickly.
>>
>> I would like to come up with a init_default_governor()
>> or similar function which choses the performance governor
>> for such CPUs.
>> Hm, maybe it could get a driver callback, then this one could
>> be picked up by acpi-cpufreq (and powernow-k8 if applicable)
>> and those drivers could choose the right governor for the
>> platform/cpu.

Actually we are currently also looking into this issue.
So I'd refrain at least for now from changing the default governor.
Let's see what Arjan comes up with, I'd also like to see a reworked 
governor instead of changing the default one.

To answer your questions nevertheless:

>> Ideally identifying the CPUs where performance governor should get
>> used is a one liner checking for a cpu flag.
>> But this might not get that easy? CPU family/model would need
>> maintenance if there is no cpu flag/feature to test for.

 From the AMD side it looks like family >= 0x11 should do the trick. The 
actual difference is the availability of >C1 per core sleep states. 
Family 10h and 11h have C1 and C1e, but no real deeper states.
Let me see if there is some register or bit we can reliably query to 
check for this instead of relying on fragile f/m/s values.

On Phenoms I could measure much lower power usage with ondemand (or 
conservative) governor on idle. This was not true for Bulldozers 
anymore, so there is some truth in your idea.

Regards,
Andre.

>>
>> Just some ideas..., if it's doable with some lines of code without
>> the need of maintaining/adding new cpu families, I'd like to have
>> a better default behavior.
>>
>> One main problem I am facing is: Measuring power consumption
>> in different workloads.
>>
>> I can measure the power consumption in idle (deeper sleep
>> states entered) when CPU frequency is set to lowest and highest
>> and compare. If both are the same, the CPU is a good candidate
>> to not do OS controlled CPU frequency scaling.
>>
>> What do you think?
>>
>> Thanks,
>>
>>     Thomas
>>



-- 
Andre Przywara
AMD-OSRC (Dresden)
Tel: x29712