Linux support for ARM big.LITTLE
Posted Feb 17, 2012 23:25 UTC (Fri) by khim
In reply to: Linux support for ARM big.LITTLE
Parent article: Linux support for ARM big.LITTLE
I don't see what's so performance critical about this, you shouldn't be making significant power config changes to your system hundreds of times a second,
Why do you think so?
And it's also stuff that's not very timing sensitive (if delaying a second to make the decision results in the process finishing first, it was probably not that important a decision to make, for example)
What you are talking about? It looks like this whole discussion comes from different universe. Perhaps it's the well-discussed phenomenon where an important requirement that was not at all obvious to one party is so obvious to other one that they didn't think to state it.
We are discussing all that in the context of big.LITTLE processing, right? Which is used by things like tablets and mobile phones, right?
Well, the big question here is: do I need to unfreeze and use hot and powerful Cortex-A15 core to perform some kind of UI task or will slim and cool Cortex-A7 be enough to finish it? And the cut-off is dictated by physiology: the task should be finished in less then 70-100ms if it's reaction to user input or in 16ms if it's part of the animation. This means that decision to wake up Cortex-A15 or not must be taken in 1-2ms, tops. Better to do it in about 300-500µs. Any solution which alters power config once per second is so, so, SO beyond the event horison it's not even funny.
I could say that I would want to do so if a single thread is using 100% of a cpu in a non-turbo mode.
Wrong criteria. If Cortex-A7 core can calculate the next frame in 10ms then there are no need to wake up Cortex-A15 core even if for these 10ms Cortex-A7 is 100% busy.
The problems here are numerous and indeed quite time-critical. The only model which makes sense is in-kernel demon which actually does the work quickly and efficiently - but it uses information collected by userspace daemon.
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