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

Anthony Liguori
2008-03-04T19:28:24Z

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.

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.

Regards,

Anthony Liguori