[Pkg-xen-devel] [PATCH 7/9] debian/xen.init: Load xen_acpi_processor on boot

[Elliott Mitchell]

On Mon, Nov 30, 2020 at 11:43:04PM +0100, Hans van Kranenburg wrote:
> On 9/14/20 6:23 AM, Elliott Mitchell wrote:
> > This allows more control of processor state, potentially resulting in
> > reduced power usage.  Alternatively simply more information on processor
> > use.
> 
> Ooh, nice.
> 
> Would you mind to explain how you discovered this one?

I have a Xen machine which isn't heavily loaded.  Tried running `sensors`
and what do I find?  Minimum power usage on the processor was
unexpectedly high for the processor and its usage.  So I try a handy
search engine to see what turns up.

The source of information I found mentioned the kernel module
xen_acpi_processor.  Also mentioned `xenpm get-cpuidle-states` and
`xenpm get-cpufreq-states` (which depend on xen_acpi_processor) as
sources of information.

xen_acpi_processor dropped power usage significantly by itself.
`xenpm get-cpuidle-states` though listed fewer levels of CPU-idle on
several cores.  The number of cores which were missing deeper power
saving states was the delta between Domain 0's vCPUs and the number of
cores.  Allowing Domain 0 to get vCPUs equal to the cores on boot, but
then hot-remove several (`xl vcpu-set 0 12`) got the rest of the cores to
low-power idle when possible.

Other hardware on the machine still consumes plenty of power, but the
main processor now uses about one-third of what it had been.


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