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

[Julien Grall <julien.grall@xxxxxxx>]

Hello Peng,

On 23/09/16 03:14, Peng Fan wrote:
> 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 
> that
> 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.
big.LITTLE is a generic term to have "power hungry and powerful core 
powerful" (big) with slower and battery-saving cores (LITTLE).
It is not mandatory to have different micro-architectures between big 
and LITTLE cores.
In any case, the interface should not be big.LITTLE specific. We don't 
want to tie us to one specific architecture.
Regards,

--
Julien Grall

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