| |||||||||||||
Linux support for ARM big.LITTLELinux support for ARM big.LITTLEPosted Feb 17, 2012 22:34 UTC (Fri) by dlang (✭ supporter ✭, #313)In reply to: Linux support for ARM big.LITTLE by BenHutchings 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,
doing the analysis every second (or even less frequently) should be pretty good in most cases
the kernel can do some fairly trivial choices, but they are limited to something along the lines of
here is a list of power modes, if you think you are being idle too much, move down the list, if you think you are not being idle enough move up the list
but anything more complicated than this will quickly get out of control
for example,
for the sake of argument, say that 'turbo mode' is defined as:
turn off half your cores and run the other half 50% faster, using the same amount of power. (loosing 25% of it's processing power, probably more due to memory pipeline stalls)
how would the kernel ever decide when it's appropriate to sacrifice so much of it's processing power for no power savings?
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.
but what if making that switch would result in all the 'turbo mode' cores being maxed out? it still may be faster to run overloaded for a short time to finish the cpuhog task faster.
I don't see any way that this sort of logic can possibly belong in the kernel. 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)
(Log in to post comments)
Linux support for ARM big.LITTLE Posted Feb 17, 2012 23:25 UTC (Fri) by khim (subscriber, #9252) [Link] 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.
Linux support for ARM big.LITTLE Posted Feb 18, 2012 0:47 UTC (Sat) by dlang (✭ supporter ✭, #313) [Link]
I'm trying to address the general problem, not just a tablet specific problem.
waking from some sleep modes may take 10ms, so if you have deadlines like that you better not put the processor to sleep in the first place.
I also think that a delay at the start of an app is forgivable, so if the system needs the faster cores to render things, it should find out quickly, start up those cores, and continue.
I agree that if you can specify an ordered list of configurations and hand that to the kernel you may be able to have the kernel use that.
on the other hand, the example that you give:
> 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.
sort of proves my point. how can the kernel know that the application completed it's work if it's busy 100% of the time? (especially if you have an algorithm that will adapt to not having quite enough processor and will auto-scale it's quality)
this sort of thing requires knowledge that the kernel does not have.
Also, the example of the 'turbo mode' where you can run some cores faster, but at the expense of not having the thermal headroom to run as many cores. In every case I am aware of, 'turbo mode' actually reduces the clock cycles available overall (and makes the cpu:memory speed ratio worse, costing more performance), but if you have a single threaded process that will finish faster in turbo mode, it may be the right thing to do to switch to that mode.
it doesn't matter if you are a 12 core Intel x86 monster, or a much smaller ARM chip.
Linux support for ARM big.LITTLE Posted Feb 18, 2012 11:09 UTC (Sat) by khim (subscriber, #9252) [Link] it doesn't matter if you are a 12 core Intel x86 monster, or a much smaller ARM chip. Well, sure. But differences between interactive tasks and batch processing modes are acute. With batch processing you are optimizing time for the [relatively] long process. With interactive tasks you optimize work in your tiny 16ms timeslice. It makes no sense to produce result in 5ms (and pay for it) but if you spent 20ms then you are screwed. Today the difference is not so acute because the most power-hungry part of the smartphone or tablet is LCD/OLED display. But if/when technologies like Mirasol will be adopted these decisions will suddenly start producing huge differences in the battery life.
Linux support for ARM big.LITTLE Posted Feb 18, 2012 11:58 UTC (Sat) by dlang (✭ supporter ✭, #313) [Link]
agreed, I'd love to have a passive LCD display netbook that was able to transparently sleep between keystrokes, but realistically we are a long way from that in terms of the hardware capabilities, and even further away from that in terms of being able to predict the future workloads correctly.
i don't think we are ever going to get away from having to make the choice between keeping things powered up to be able to be responsive, and powering things down aggressively to save power.
Linux support for ARM big.LITTLE Posted Feb 20, 2012 12:17 UTC (Mon) by khim (subscriber, #9252) [Link] Hardware is in labs already (and should reach the market in a few years), it's time to think about software side. If we are talking about small tweaks then such hardware it not yet on the radar, but if we plan to create the whole new subsystem (task which will itself need two or three years to mature) then it must be considered.
|
|||||||||||||