[Xen-users] Question about cpufreq and scheduling in Xen.

[Austin S Hemmelgarn <ahferroin7@xxxxxxxxx>]

Background:
I've got a home server system running Xen, with about 8 user domains. 
All of the domains except one are idle more than 90% of the time.  The 
one domain that is never idle is a low priority domain I use to run 
BOINC based distributed computing applications, and is configured to use 
as much of the spare processor time as possible.  All of the things 
being run in this low priority domain are constrained by memory 
bandwidth and not processor speed, and I've determined through testing 
that increasing the CPU frequency provides almost no net performance 
benefit (the difference between running it at 1.4GHz and 3.2GHz is less 
than 2.5% on my hardware), and the power savings from running at a low 
frequency is more significant to me than the performance loss. However, 
running with the powersave governor and forcing all the CPU's down to 
1.4GHz produces a noticeable latency increase for the other domains, and 
that is more important to me than the power savings.  As far as 
scheduling is concerned, the regular credit scheduler works perfectly 
for what I want (namely, everything has scheduling priority over this 
domain), but the cpufreq driver always reads the system as 100% 
utilization, which brings me to my question.
Question:
Is there some way to have the Xen's cpufreq drivers ignore the low 
priority domain when computing system load to set the CPU frequencies?
In an ideal situation, I'd like to be running the ondemand cpufreq 
governor, and have it just ignore the low priority domain when 
calculating the target frequency, so that dom0 and the other domU's can 
still get good latency when they need it, but otherwise it runs at the 
minimum frequency for the CPU.
On Linux there is an option for the 'ondemand' governor to tell it to 
ignore tasks that have a positive nice value when computing system load, 
but this can't be used with Xen (because Xen treats all domains as equal 
when calculating system load), and I have been unable to find any 
equivalent to this in Xen.

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