Re: [PATCH 0/8] RFC: vcpu pinning at qemu start

["Daniel P. Berrange" <berrange@redhat.com>]

On Tue, Mar 04, 2008 at 01:28:24PM -0600, Anthony Liguori wrote:
> Glauber Costa wrote:
> > My main interest is in management tools being able to specify pinning
> > set ups at VM creation time.
> >
> > As I said, it can be done through tools like taskset, but then you'd 
> > have to know:
> >  * when are the threads created
> >  * which thread ids corresponds to each cpu
> >
> > And of course, for an amount of time, the threads will be running in a 
> > "wrong" cpu, which may affect workloads running there. (which is a 
> > case cpu pinning usually tries to address)
> 
> A management tool can start QEMU with -S to prevent any CPUs from 
> running, query the VCPU=>thread id relationship (modifying info cpus 
> would be a good thing to do for this), taskset, and then run 'cont' in 
> the monitor if they desperately need this functionality.  However, I 
> don't think the vast majority of people need this particular functionality.

I fully expected to have to run QEMU with -S and then use cont if I were
todo CPU pinning from libvirt.

The only info I'd need to get is the  PID <-> vCPU mapping data. Then
I can use regular Linux taskset capabilities from libvirt to assign the
initial pCPU <-> vCPU mapping and finally run 'cont'.

> My feeling is that adding an interface to do this in QEMU encourages 
> people to not use the existing Linux tools for this or worse yet, to 
> think they can do a better job than Linux.  The whole reason this exists 
> in Xen is that Xen's schedulers were incapable of doing CPU migration 
> historically (which is no longer true since the credit scheduler).  It 
> was necessary to specify pinning upon creation or you were stuck with 
> round-robin placement.  So libvirt has APIs for this because they were 
> part of the Xen API because it was needed to get reasonable performance 
> at some point in time on Xen.  I don't think this behavior is useful for 
> KVM though.  Just because Xen does it doesn't imply that we should do it.

I agree that adding QEMU commands for stuff which Linux already has APIs
and tools is a bad idea. QEMU/KVM is much nicer to manage than Xen, 
precisely because I can already use Linux APIs & process management tools.

Dan.
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