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Re: [Xen-devel] [RFC 0/5] xen/arm: support big.little SoC

On Thu, Sep 22, 2016 at 07:54:02PM +0100, Julien Grall wrote:
>Hi Stefano,
>On 22/09/2016 18:31, Stefano Stabellini wrote:
>>On Thu, 22 Sep 2016, Julien Grall wrote:
>>>Hello Peng,
>>>On 22/09/16 10:27, Peng Fan wrote:
>>>>On Thu, Sep 22, 2016 at 10:50:23AM +0200, Dario Faggioli wrote:
>>>>>On Thu, 2016-09-22 at 14:49 +0800, Peng Fan wrote:
>>>>>>On Wed, Sep 21, 2016 at 08:11:43PM +0100, Julien Grall wrote:
>>>>>A feature like `xl cpupool-biglittle-split' can still be interesting,
>>>>"cpupool-cluster-split" maybe a better name?
>>>You seem to assume that a cluster, from the MPIDR point of view, can only
>>>contain the same set of CPUs. I don't think this is part of the architecture,
>>>so this may not be true in the future.
>>Interesting. I also understood that a cluster can only have one kind if
>>cpus. Honestly it would be a little insane for it to be otherwise :-)
>I don't think this is insane (or maybe I am insane :)). Cluster usually
>doesn't share all L2 cache (assuming L1 is local to each core) and L3 cache
>may not be present, so if you move a task from one cluster to another you
>will add latency because the new L2 cache has to be refilled.
>The use case of big.LITTLE is big cores are used for short period of burst
>and little core are used for the rest (e.g listening audio, fetching
>mail...). If you want to reduce latency when switch between big and little
>CPUs, you may want to put them within the same cluster.
>Also, as mentioned in another thread, you may have a platform with the same
>micro-architecture (e.g Cortex A-53) but different silicon implementation
>(e.g to have a different frequency, power efficiency). Here the concept of
>big.LITTLE is more blurred.

That is possible that in one cluster, different pcpus runs with different cpu
frequency. This depends on hardware design. Some may require all the cores in
one cluster runs at the same frequency, some may have more complicated design 
supports different cores runs at different frequency.

This is just like you have a smp system, but different cores can run at
different cpu frequency. I think this is not what bit.LITTLE means.

For the pcpus in one cluster, xen needs to choose which pcpu for vcpu
for power or etc.


>That's why I am quite reluctant to name (even if it may be more handy to the
>user) "big" and "little" the different CPU set.
>Julien Grall


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