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

[Glauber Costa <gcosta@redhat.com>]

Anthony Liguori wrote:
> Glauber Costa wrote:
>> Hi guys,
>>
>> Here's a first series of patch aiming at vcpu pinning support in qemu.
>> Ideally, as vcpu as just normal threads, the usual userspace tools can 
>> be used
>> to set cpu affinities mask.
>>
>> However, It makes it very difficult to _start_ a vm with vcpus pinned, 
>> since
>> we don't know the thread ids from qemu in advance, nor do we know when 
>> are the
>> vcpus created.
>>
>> The patches introduce a -cpu-map option, that, if specified, starts 
>> the virtual cpus
>> with the specified affinities.
>>
>> Comments? Welcome. Random rants? Not welcome, but... how can I stop 
>> you? So go ahead!
>>
>>   
> 
> So why exactly is this useful?  I have a hard time constructing a 
> reasonable use-case in my mind for something like this.

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)




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