Gadgetory

hello there my name's Gary Sims from Android authority now maybe you've heard the term CPU scaling or maybe CPU frequency scaling or even dynamic voltage and frequency scaling and you've wondered what they mean well today I'm going to try and explain so if you're ready let's go one of the key characteristics of mobile computing and maybe the most obvious characteristic is that all of our smartphones and tablets and laptops run from a battery desktops and servers run from the mains and therefore they don't have power worries they can just draw as much power as they need but smartphones in particular have a battery and the more power they draw the quicker that battery will drain now it turns out there's a direct correlation between the frequency the clock frequency of a CPU one gigahertz 1.5 gigahertz 2 gigahertz and the amount of power that it needs that's because there are all these transistors inside of the chip and every time they have to switch that uses air power and therefore if you switch them less frequency then there's less power used you could think about it maybe like the Reds of the engine in your car the more you rev your engine the more petrol the more gas will be used now when you're using your smart phone maybe you're reading a web page for example while you're actually reading the text as phone is doing very very little so it doesn't need to be running at 2 gigahertz just to keep displaying that webpage to you so actually there's a system that's inside the limits kernel and inside Android that allows the frequency to be scaled to be changed automatically when the CPU isn't very busy and so this is called CPU scaling and when the CPU seated for nothing to do it's in the idle mode then it works that it needs to slow down the CPU fat there's also a stake where you can switch off a core or switch off a couple of calls but that's a different subject than today now of course once you then start to use your web application again we started playing game or you have to do something else then of course the CPU frequency needs to go back up again so this scaling works in both directions and it actually also turns out that when you're running the CPU at a lower clock frequency you actually don't need as many volts to power that chip so you can actually or to the vault at the same time in tandem with the clock frequency and that's where we get the term dynamic voltage and frequency scaling devfs because both the vault and the clock frequency are changed at the same time now controlling this is a thing called the governor the CPU scaling governor and on Android it's set in the mode interactive and it's the interactive mode because it has a little trick inside of it and that as soon as you touch the screen then the CPU is wrapped up instantly to give the user that fast feedback to a UI interaction now if you want to see the CPU frequency scaling in action you can download an app like cpu-z CPU Z depending on how you pronounce it now inside that there's information about your make of your CPU and the model number the number of cores and there's also a section which talks about the clock frequencies and the way the clock frequencies change inside the app are actually dynamic you can see maybe some of them are stopped some of them are running at 300 megahertz and then if you do something like scroll the app up and down you'll see that it jumps up to a higher frequency in fact if you see running at a low frequency let's say 300 megahertz if you just touch the screen you'll see it quickly jump up because this is the interactive governor working in anticipation of some UI interaction with the user now it's also possible to change the governor on an android but to do that you need to have a rooted phone there are several apps in the App Store that allow you to change it and there are some stories that if you change it you get better performance and maybe different battery life and so on and some of it's true and some of it's also a load of Miss information you have to remember this one thing that smartphone manufacturers spend millions and millions of dollars designing and building and selling these smartphones and you would think that if they knew there was a better governor out there they would change it to give to use that new governor in fact the interactive governor is the best governor for Android otherwise as I said they would all be changing to the other one or even writing their own which has been known in the past but that's a whole another story not for now now if you're a developer I just wanted to point out that this change from a low-frequency state to a high-frequency state can take around 20 milliseconds so if you're doing some system profiling and you see that you have a dropped frame may be the beginning of a particular segment in your program that is probably likely due to the fact that the CPU has to ramp up from a low frequency state to pay high frequency state so there's nothing much you can really do about it but now you know why it's happening now of course there is more going on inside your CPU than just DV FS one of the big changes that we've seen last year that will continue this year in many many phones is the use of heterogeneous multi-processing or big dot little from arm and that's what we find these octa-core phones where you have maybe four cortex a 72 cores and four cortex a53 cores and what happens is when the phone gets busy it switches from using the lower performance a53 calls but they have a higher energy efficiency and it starts using the more power-hungry but faster cortex a 72 cause now if the dynamic voltage and frequency scaling is like revving the engine in your car then this is like changing gear you change from one to the other and the original implementations are big got little the software component or big got little use the devfs system to switch from one quarter other when it got to a certain limit it would say well this core can't cope this anymore let's switch over to a higher power corner that's still actually one of the basic principles but now the whole thing is actually a lot more complicated because the scheduler takes into account not only what's going on but it take into account maybe the temperature of this of the CPU so it knows whether it can reach a certain level or not because otherwise there's overheating and armor actually working on an energy aware scheduler for Linux for Android which will actually mean that maybe rather than putting on a non active core which maybe would be the best choice in terms of performance it might actually put a task on to an already running core because actually that's more energy efficient because it's an energy aware scheduler rather than starting up a whole new core and ramping it up and start using energy for that but that's coming out in this year 2016 and we'll start seeing that in phones maybe towards the end of the year that isn't there yet so as you can see there's lots going on inside of the processor to make sure it uses the optimum out of battery the option amount of power but yet delivers you the best level of performance my name is Gary Sims from Andrew authority I hope you really enjoyed this video if you did please don't forget to give it a thumbs up also ever get to use the comments blow to tell me what you think about devfs what you think about edge to Dias multi-processing what you think about big dot little what you think about energyaware scheduling what you think about any of the technologies are being used inside of your processor in your smartphone to make sure that gives you the best performance yet with the best battery life also don't forget to subscribe to and Reuters youtube channel also you can follow me on social media and ask for me I'll see you in my next video