Gadgetory

on our last episode of Android power user we talked about JIT Androids just-in-time compiler that takes advantage of the dalvik cache that's really powering all the apps that we run on our smartphones and tablets but what's dalvik what's a virtual machine what does it have to do with cache that's what we're going to talk about on today's episode Android power user dalvik is a word that gets thrown around quite a bit when we're talking about custom roms sometimes even when we're talking about stock roms and particularly when we're upgrading those stock roms from one version of the OS to the next kind of sounds a little bit different I'll tell you why in just a minute we have to understand a little bit more about computer architecture to understand what dalvik is what it does and why we have it why it's a really good thing so let's step away from our smartphones and tablets for just a minute and let's talk about computers whether that's a desktop a laptop a server really doesn't matter what we're going to do is we're going to describe how computers work I know this isn't the computer power user this is Android power user but stick with me computers are made up of physical components that might be your cd-rom drive or blu-ray player it might be your video card or your sound card any one of these things today we've got a lot of stuff integrated onto the motherboard so it doesn't look like you've got a lot of discrete components but you do each one of those components needs a driver for software to be able to talk to it that driver essentially bridges the gap between the software world asking for stuff to be done when you type on the keyboard or click on the mouse and the physical world when you actually want it to do something with that blu-ray player or make some sound for example well the physical world is all fine and dandy but it's somewhat fragile things can get broken you can have various different parts of your operating system or programs fighting over who has access to a particular device and it's really not all that cool when you think about it I mean it's going off we like reality here at pocket but we can virtualize that that's where the virtual and virtual machine comes into play you see it's not a physical machine it's a virtual machine it's a computer that doesn't exist anywhere except in software now why on earth would you do that there are lots and lots of reasons perhaps we'll go into a lot more detail in an upcoming episode of Android power user but suffice it to say the top just top couple reasons you do it number one is security stuff that's running inside a virtual machine pretty much stays inside that virtual machine so it protects the entire rest of the ecosystem also stability you remember back in the olden days when you were surfing the web and your web browser crashed and you had to reboot your computer one bad website cause you to have to reboot your entire computer not cool nowadays it's a lot less likely that that's going to happen it still might because we're not using virtual machines on our desktop computers yet but if you can virtualize that great that virtual machine goes down you'll restart it and you're done you still have your whole operating system and everything that you were working on you haven't lost anything but a little bit of time so two advantages now that's not exactly the way the dalvik VM works in Android but it's just giving you kind of an introduction to the way VMs work in general so dalvik it sounds kind of interesting the name where did it come from well dalvik is an open source project written by let me get this right Dan Bornstein according to one of his journals he named it dalvik because he has some ancestors who lived in a fishing village in Iceland you guessed it the name of that fishing village dalvik and I'm probably pronouncing that wrong so all of you who are watching from Iceland I apologize doing the best I can here so that's why it's named the way it is so how does an Android app work with the dalvik cache well I'm going to read a little bit of this so I don't get it completely wrong it's a little bit complex but I think you'll understand when I get through with it when you write an app for Android you write it in Java that Java is then filed into bytecode from there it's converted into a JVM compatible dot class file now Java applications they all run in a virtual machine it's the JVM so dalvik is it is a JVM just not the JVM that makes sense so what happens next we take that dot class file and we convert that into a dalvik compatible Dex file that's a dalvik executable for those of you keeping track of the file extensions here once that's done then they're installed on your Android pretty simple and then of course you open them and they're executed on your Android inside the dalvik VM sounds great but there's one step missing there okay let's go back just a minute why do we want to do that extra step well the dalvik executable format is quite a bit more compact than the dot class file for the JVM which means it takes up less space on memory constrained devices it runs in less memory space than regular dot class files and it's usually a little bit more processor friendly than some of those dot class files are so that's del Vic in the dalvik VM but what about that dalvik cache we're always hearing about specifically when you install a new ROM on your phone we're talking custom roms here primarily a lot of times the instructions say that you should wipe your cache well a lot of that is referring to the dalvik cache so what is that that takes us back to our JIT episode that we watched last week if you haven't watched that yet I'll have a link at the end of this episode so you can essentially what happens is these are essentially uncompelled applications well they have some compilation done to them they're not just raw code sometimes they are it's a little bit interesting there doesn't it but they're not compiled ready to run instead they have to be loaded up into this JIT compiler compiled on the fly and then run but that takes time so what do we do well we've got this nice dalvik cache where we kind of preheat things we optimize the apps so that they can run faster on first execution and we do that through something called if I can find it here the dalvik VM it inspects all of your apps and it creates a tree of dependencies and once it's got that done it stores it into your dalvik cache that enables your apps to run more efficiently they're optimized for the environment of course the the different apps that you have installed means that your cache is going to be different than everybody else's cash so there's not just a single cache out there everybody's cache is different so that's all kinds of great but it does explain why your phone takes a bit longer to boot up the first time after you've installed a new rom or updated your existing rom it's rebuilding that cache it's rebuilding that tree of interdependencies because well things have changed you've got a new rom on your phone now so it rebuilds that cache for you wiping the cache means you're going to have a nice clean slate for all of that new information to go into and can prevent some problems so make sure you wipe every time you install a new rom all righty then so does this have anything to do with Oh Dex and do text files yes yes it does but we're going to have to cover that on another episode of the Android power user if you like learning about this stuff please give the video a big thumbs up if you want to make sure you don't miss out on any of our future videos and all the extra learning that we're going to be doing there make sure you hit the subscribe button we'll have a link to that at the end of the video as well if you've got questions or comments would like to contribute to the discussion if you're watching this video on YouTube make sure you click the link right down there to go over to pocketnow.com and leave your comments on the article associated with this video where i've got a nice little summary of everything that we've talked about here today if you're already on pocketnow.com thank you go ahead and leave those comments right down below that's all the time we've got for today I'm Joe Levi this is PocketNow thanks for watching