Workings/effectiveness of the xen-acpi-processor driver

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

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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.

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
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
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.

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.

Thanks,
Stefan


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