Re: [PATCH 0/3] KVM-userspace: add NUMA support for guests

[Avi Kivity <avi@redhat.com>]

Andi Kleen wrote:
>> I don't think the first one works without the second.  Calling getcpu() 
>> on startup is meaningless since the initial placement doesn't take the 
>>     
>
> Who said anything about startup? The idea behind getcpu() is to call
> it every time you allocate someting.
>
>   

Qemu only allocates on startup (though of course the kernel actually 
allocates the memory lazily).

Please explain.  When would you call getcpu() and what would you do at 
that time?

>> This could happen completely in the kernel (not an easy task), or by 
>>     
>
> There were experimental patches for tieing memory migration to cpu migration 
> some time ago from Lee S.
>
>   
>> having a second-level scheduler in userspace polling for cpu usage an 
>> rebalancing processes across numa nodes.  Given that with virtualization 
>> you have a few long lived processes, this does not seem too difficult.
>>     
>
> I think I would prefer to fix that in the kernel. user space will never
> have the full picture.
>   

On the other hand, getting everyone happy so this can get into the 
kernel will be very difficult.  Many workloads will lose from this; 
we're trying to balance both memory affinity and cpu balancing, and each 
workload has a different tradeoff.

-- 
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