[Xen-users] default CPU for 2nd domain...

[Tom Brown <tbrown@xxxxxxxxxxxxx>]

The default CPU for the first domU is wrong for SMP machines. Dead wrong
if there are only the two domains.  dom0 and the first domU end up on
hyperthreads of the first CPU. At least under 2.0.5 .... and I frankly
didn't realize that XEN doesn't seem to move domains between "CPUs". Once
a domain has been assigned a "CPU" it is stuck there until/unless someone
moves it with "xm pincpu"

You can have two busy dom's sharing one CPU and the other CPU effectively
idle. I hope and believe this will be fixed under 3.0 ?

I guess the lesson is be aware that despite the fact that not much is
mentioned about it in the docs, the "cpu=" setting is pretty darn
important if you have more than one CPU core, and run some CPU intensive
domains.

The funny thing is how I found this. I was measuring power consumption for
a server before sending it off to the co-lo (so we can have an idea of how
many boxes can go on a 15 amp circuit), and I couldn't figure out why the
first CPU drew an extra 50 watts going from idle to swamped, but the 2nd
CPU was only 10. Both doms were on the same core, XEN itself might have
been using the idle CPU...

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