SCHED_ULE vs SCHED_4BSD for XENHVM domU

Carsten Heesch sysconfig at ossafe.org
Thu Oct 29 23:27:13 UTC 2009


Hi guys,

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  
same level.

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  
(virtual) CPUs.

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?


Thanks!

Carsten




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