Re: [Xen-users] Xen Not Support multi-cpu??

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> > I get the idea. But I have another question about this.
> > Should we compile the dom0 and domU kernels with SMP support?
> > I have bought a server with 2 5130 cpu's (quad core), which
> > make the total cpu count 8.
> > How do I have to understand the arrangement of cpu's? One cpu
> > for the dom0 (which doesn't need SMP compiled then?), and
> > some domU's with two or 3 vcpu's (heavily loaded ones -> they
> > need SMP then?) and some domU's with only 1 vcpu.
> > So is it correct to say that I need SMP only for the domU kernels?
>
> If you want to make your own life easy, just compile one kernel for Dom0
> and DomU with SMP enabled (which is default).
>
> But if you want to eek out the last couple of percent performance out of
> the kernel (assuming your apps spend a fair bit in the kernel), you may
> want to get rid of SMP for kernels that you don't run more than one VCPU
> on. Note that there should be little difference between running SMP kernel
> or non-SMP kernel on a single (V)CPU, as the locks and such will never be
> contended, nor will there be cache-flushes for the locks. But of course,
> there are extra instructions to achieve locks in SMP, which will take some
> extra time from the execution. [And some locks will still be needed, as you
> still have the possibility of multiple threads running on the same CPU
> interacting with each other].

Does dom0 have to be SMP capable?  It used to be at one stage, but I might be 
massively out of date here.

If the "SMP alternatives" mode is compiled in the kernel will automatically 
reoptimise itself (even at runtime, last time I saw the patch!) for the 
number of (V)CPUs it has.  So the overhead of having SMP support compiled 
into a UP guest can actually be quite a bit lower than one would usually 
expect.

This is still marked experimental, I think.

> Note that the number of VCPU's in a 8 core system is not limited to 8. You
> can have 8 VCPU's for every domain should you wish to (and have a hundred
> domains -> 800 VCPU's). It's just that they can't all run at once, of
> course!

You could even have more than that - number of VCPUs for a domU can exceed the 
number of host CPUs (although that's obviously probably not going useful from 
a performance PoV!  Might be good for testing ... something, though.).

Cheers,
Mark

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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