Re: Workings/effectiveness of the xen-acpi-processor driver

[Stefan Bader <stefan.bader@canonical.com>]

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On 26.04.2012 17:50, Konrad Rzeszutek Wilk wrote:
> On Wed, Apr 25, 2012 at 03:00:58PM +0200, Stefan Bader wrote:
>> Since there have been requests about that driver to get backported into 3.2, I
>> was interested to find out what or how much would be gained by that.
>>
>> The first system I tried was an AMD based one (8 core Opteron 6128@2GHz). Which
>> was not very successful as the drivers bail out of the init function because the
>> first call to acpi_processor_register_performance() returns -ENODEV. There is
>> some frequency scaling when running without Xen, so I need to do some more
>> debugging there.
> 
> Did you back-port the other components - the ones that turn off the native
> frequency scalling?
> 
>       provide disable_cpufreq() function to disable the API.
> 	xen/acpi-processor: Do not depend on CPU frequency scaling drivers.
>       xen/cpufreq: Disable the cpu frequency scaling drivers from loading
>>

Yes, here is the full set for reference:

* xen/cpufreq: Disable the cpu frequency scaling drivers from loading.
* xen/acpi: Remove the WARN's as they just create noise.
* xen/acpi: Fix Kconfig dependency on CPU_FREQ
* xen/acpi-processor: Do not depend on CPU frequency scaling drivers.
* xen/acpi-processor: C and P-state driver that uploads said data to hyper
* provide disable_cpufreq() function to disable the API.

>> The second system was an Intel one (4 core i7 920@2.67GHz) which was
>> successfully loading the driver. Via xenpm I can see the various frequencies and
>> also see them being changed. However the cpuidle data out of xenpm looks a bit odd:
>>
>> #> xenpm get-cpuidle-states 0
>> Max C-state: C7
>>
>> cpu id               : 0
>> total C-states       : 2
>> idle time(ms)        : 10819311
>> C0                   : transition [00000000000000000001]
>>                        residency  [00000000000000005398 ms]
>> C1                   : transition [00000000000000000001]
>>                        residency  [00000000000010819311 ms]
>> pc3                  : [00000000000000000000 ms]
>> pc6                  : [00000000000000000000 ms]
>> pc7                  : [00000000000000000000 ms]
>> cc3                  : [00000000000000000000 ms]
>> cc6                  : [00000000000000000000 ms]
>>
>> Also gathering samples over 30s does look like only C0 and C1 are used. This
> 
> Yes. 
>> might be because C1E support is enabled in BIOS but when looking at the
>> intel_idle data in sysfs when running without a hypervisor will show C3 and C6
>> for the cores. That could have been just a wrong output, so I plugged in a power
>> meter and compared a kernel running natively and running as dom0 (with and
>> without the acpi-processor driver).
>>
>> Native: 175W
>> dom0:   183W (with only marginal difference between with or without the
>>               processor driver)
>> [yes, the system has a somewhat high base consumption which I attribute to a
>> ridiculously dimensioned graphics subsystem to be running a text console]
>>
>> This I would take as C3 and C6 really not being used and the frequency scaling
> 
> To go in deeper modes there is also a need to backport a Xen unstable
> hypercall which will allow the kernel to detect the other states besides
> C0-C2.
> 
> "XEN_SET_PDC query was implemented in c/s 23783:
>     "ACPI: add _PDC input override mechanism".
>     

I see. There is a kernel patch about enabling MWAIT that refers to that...

> 
>> having no impact on the idle system is not that much surprising. But if that was
>> true it would also limit the usefulness of the turbo mode which I understand
>> would also be limited by the c-state of the other cores.
> 
> Hm, I should double-check that - but somehow I thought that Xen independetly
> checks for TurboMode and if the P-states are in, then they are activated.
> 
Turbo mode should be enabled. I had been only looking at a generic overview
about it on Intel site which sounded like it  would make more of a difference on
how much one core could get overclocked related to how many cores are active
(and I translated active or not into deeper c-states or not).
Looking at the verbose output of turbostat it seems not to make that much
difference whether 2-4 cores are running. A single core alone could get one more
increment in clock stepping. That does not immediately sound a lot. And of
course how much or long the higher clock is used depends on other factors as
well and is not under OS control.

In the end it is probably quite dynamic and hard to come up with hard facts to
prove its value. Though if I can lower the idle power usage by reaching a bit
further, that would greatly help to justify the effort and potential risk of
backporting...

>>
>> Do I misread the data I see? Or maybe its a known limitation? In case it is
>> worth doing more research I'll gladly try things and gather more data.
> 
> Just missing some patches. 
> 
> Oh, and this one:
>       xen/acpi: Fix Kconfig dependency on CPU_FREQ
> 
> Hmm.. I think a patch disappeared somewhere.
> 
>>
>> Thanks,
>> Stefan
>>
> 
> 
> 
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