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

[Avi Kivity <avi@qumranet.com>]

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.
>


Affinity control is probably useful mostly for numa configurations, 
where you want to restrict virtual cpus to run on the cores closest to 
memory.  However it may well be that the scheduler is already good 
enough to do this on its own.


> 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.
>

In the brutal world of hypervisors, if your competitor has a feature, 
you must have it too.  I often get asked about cpu pinning in kvm.

[I'd like to see how Xen implements swapping, though]

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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