After FreeBSD 8.0-RC2 is working well (for me) on Xen 3.3.1, I somehow
made an interesting observation:
Even if the FreeBSD domU does absolutely nothing, each of the assigned
vCPUs will be reported by xentop as using 15%-25% CPU time. It doesn't
matter, if domU is running with 1 or 4 vCPUs. All of them float at the
On the same Xen box five other domU's (CentOS 5.2-5.4) have an idle
CPU percentage of 0 to 0.5%.
So I played a bit and figured out that changing the scheduler in the
XENHVM kernel configuration from "option SCHED_ULE" to "option
SCHED_4BSD" resolves this problem. The idle state of the FreeBSD domU
is now identical to the CentOS domU's.
However, this of course means using an old (maybe even deprecated?)
scheduler, which moreover isn't the best choice for multiple
The configuration I tested on is:
Intel i7 920 Quad-Core with Hyperthreading (8 logical CPUs)
Citrix XenServer 5.5 (based on CentOS 5.2, Xen 3.3.1)
Any thoughts, ideas, solutions?
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