[CentOS-virt] Xen vs. iSCSI

Pasi Kärkkäinen pasik at iki.fi
Tue Jun 16 20:07:46 UTC 2009

On Tue, Jun 16, 2009 at 02:59:54PM -0400, Luke S Crawford wrote:
> Bill McGonigle <bill at bfccomputing.com> writes:
> > On 06/15/2009 11:33 PM, Luke S Crawford wrote:
> > > xm sched-credit -d 0 60000
> > 
> > Ah, ha!  This appears to work.  I didn't need to reserve a CPU for the 
> > dom0 (knock on wood).  Much obliged, Luke.
> > 
> > I'm academically curious, though - I seem to have created a CPU deadlock 
> > of some sort, yet in 'xm top' none of the CPU's were pegged.  I've got 
> > no reason to not give dom0 utmost priority - that makes perfect sense to 
> > me - but I'm surprised the Xen scheduler would allow me to get into this 
> > situation by default.
> My understanding of this is entirely janitor-level, but I believe what you
> are seeing is that the dom0 has exhausted it's 'credits'  and so if a
> DomU wants the CPU the dom0 gets kicked off the cpu, waits a timeslice
> (I think timeslices are on the order of tens of millaseconds...  I've
> read 60ms, which is quite a long time in terms of sending a packet to a
> nearby storage box.)  then gets back on the CPU.  

afaik Xen credit scheduler assigns/changes credits every 30ms.

although CPU scheduling happens more often?

-- Pasi

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