[Xenomai] Porting the I-pipe core to ARM big.LITTLE architecture

[Xenomai] Porting the I-pipe core to ARM big.LITTLE architecture

[Hongfei Cheng]

Searching the mailing list archives, I don't see mentioning of ARM's
big.LITTLE architecture. The list of the supported SoCs doesn't appear
to include any SoC based on ARM's big.LITTLE architecture.

Are there any special considerations for porting the I-pipe core to
such big.LITTLE architecture?

Thanks!

Hongfei

Re: [Xenomai] Porting the I-pipe core to ARM big.LITTLE architecture

[Gilles Chanteperdrix]

On Tue, Apr 28, 2015 at 01:08:12AM -0700, Hongfei Cheng wrote:
> Searching the mailing list archives, I don't see mentioning of ARM's
> big.LITTLE architecture. The list of the supported SoCs doesn't appear
> to include any SoC based on ARM's big.LITTLE architecture.
> 
> Are there any special considerations for porting the I-pipe core to
> such big.LITTLE architecture?

I would say this sounds like cpufreq:
- migrating a task from an A15 to an A7 will have a huge impact on
latencies
- the speed difference will have a huge impact on the timings of the
running tasks.

So, if I were to port the I-pipe core to a big/little processor, I
would simply get the system to run on the A15, always, or maybe
allow the system to only run on the A7, if the user knows that his
tasks will be schedulable with the reduced power. Until someone
comes up with the will to implement something else (for something
like 10 years now, it did not happen for cpufreq, so I doubt it will
happen for big/little either).

-- 
					    Gilles.

Re: [Xenomai] Porting the I-pipe core to ARM big.LITTLE architecture

[Gilles Chanteperdrix]

On Tue, Apr 28, 2015 at 10:24:34AM +0200, Gilles Chanteperdrix wrote:
> On Tue, Apr 28, 2015 at 01:08:12AM -0700, Hongfei Cheng wrote:
> > Searching the mailing list archives, I don't see mentioning of ARM's
> > big.LITTLE architecture. The list of the supported SoCs doesn't appear
> > to include any SoC based on ARM's big.LITTLE architecture.
> > 
> > Are there any special considerations for porting the I-pipe core to
> > such big.LITTLE architecture?
> 
> I would say this sounds like cpufreq:
> - migrating a task from an A15 to an A7 will have a huge impact on
> latencies
> - the speed difference will have a huge impact on the timings of the
> running tasks.

Note that xenomai never migrates a task from a cpu to another
automatically for the same reason.

-- 
					    Gilles.

Re: [Xenomai] Porting the I-pipe core to ARM big.LITTLE architecture

[Hongfei Cheng]

Thank you, Gilles, for the helpful info and the suggestion to work
around the inherent CPU power management scheme.

Hongfei

On Tue, Apr 28, 2015 at 1:32 AM, Gilles Chanteperdrix
<gilles.chanteperdrix@xenomai.org> wrote:
> On Tue, Apr 28, 2015 at 10:24:34AM +0200, Gilles Chanteperdrix wrote:
>> On Tue, Apr 28, 2015 at 01:08:12AM -0700, Hongfei Cheng wrote:
>> > Searching the mailing list archives, I don't see mentioning of ARM's
>> > big.LITTLE architecture. The list of the supported SoCs doesn't appear
>> > to include any SoC based on ARM's big.LITTLE architecture.
>> >
>> > Are there any special considerations for porting the I-pipe core to
>> > such big.LITTLE architecture?
>>
>> I would say this sounds like cpufreq:
>> - migrating a task from an A15 to an A7 will have a huge impact on
>> latencies
>> - the speed difference will have a huge impact on the timings of the
>> running tasks.
>
> Note that xenomai never migrates a task from a cpu to another
> automatically for the same reason.
>
> --
>                                             Gilles.