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

Glauber Costa
2008-03-04T18:10:06Z

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)