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Re: [Xen-devel] Report on Virtualization in Clusters

To: Håvard Bjerke <havarbj@xxxxxxxxxxx>
Subject: Re: [Xen-devel] Report on Virtualization in Clusters
From: Ian Pratt <Ian.Pratt@xxxxxxxxxxxx>
Date: Sun, 28 Nov 2004 11:17:03 +0000
Cc: Jacob Gorm Hansen <jacobg@xxxxxxx>, runejoha@xxxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxxx, Ian.Pratt@xxxxxxxxxxxx
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> > It seems from your tests without interrupt coalescing that Xen has a 
> > higher interrupt-handling overhead than native Linux.  I suppose that 
> > with the new IO model, Xen needs to flush the TLBs for every interrupt 
> > that arrives. Do you think this is the reason for the overhead, or are 
> > the performance problems only due to your hardware being badly supported 
> > by Xen?
> My guess is that it is the former, because we "solved" the problem with the 
> IOAPIC bug with 'ignorebiostables'. Thus using legacy PIC, everything seemed 
> to behave normally except when not using IC.
> I think it boils down to whether or not using legacy PIC as opposed to (a 
> supported) IOAPIC has an impact on interrupt handling. If it does, I think 
> it's the latter reason, otherwise I think it's the former.

The fact that you had to disable the IOAPIC with ignorebiostables
is rather concerning. It certainly sounds like Xen is really
struggling on your platform. What motherboard chipset are the
machines in your cluster?

We're hoping that this kind of problem will go away when Xen 2.1
gets released, as we're planing on leaving IOAPIC initialisation
to domain 0, hence using the existing Linux code. Domain 0 will
then have an interface to tell Xen just what it needs to know
about the IOAPIC configuration (i.e. how to ACK interrupts).


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