SCHED_ULE vs SCHED_4BSD for XENHVM domU

[Carsten Heesch]

Oops, sorry for posting this twice. I thought it didn't go through  
with the first attempt.



On 29 Oct 2009, at 23:27, Carsten Heesch wrote:

> 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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Best regards,

Carsten Heesch

Mobile:	+44 (0)7889-129998
Mail:	carsten at heesch.me.uk
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