Gadgetory

hello my name is Gary Sims from Android authority now system-on-a-chip designers have a problem main memory the RAM on your mobile phone is actually quite slow now to solve this problem is a thing called cache memory if you don't know what cache memory is please let me explain you're probably quite surprised that I've said that main memory is slow of course this is all relative a hard disk is slow a cd-rom is slow but let me give an illustration the average mobile CPU is clocked at somewhere between 1.5 gigahertz and 2.2 gigahertz now the average Ram module in your mobile phone is clocked at just 200 megahertz so you can see a big difference in 1.5 gigahertz and 200 megahertz whenever the CPU wants something it has to talk to a device that's running ten times slower than it is before it can get the information back now in CPU terms that's an absolute age as to sit around waiting for something and therefore that affects the performance okay I'll admit that as an oversimplified view of the problem however that is basically the issue that every system of a chip designer has to cope with now because of things like double data rate memory which can send to lots of data per clock 200 megahertz suddenly now equals 400 megahertz and in fact the latest low-power ddr3 is actually about eight times the base rate clock speed that can push the data out to the CPU in fact the low-power data rate for memory motors are working effectively the same CPU frequency is 1.8 gigahertz so if the CPU is running at 1.8 gigahertz and the memory effectively is running at 1.8 gigahertz there shouldn't be any problem or should there well in fact we don't remember that modern day CPUs have four or even eight cores on them so those eight cores are all accessing the same one set of memory banks and therefore there is a contention here on who can get the data and when they can get it so fact even though that's running at 1.8 gigahertz because there are eight CPU trying to get hold of the data actually it's still a lot lot a lot lot slower now this is actually a very well known problem in computer science and it's what's known as the von Neumann bottleneck now if you watch my video on assembly language and machine code you'll remember the von Neumann was one of the key players in developing and designing what is a modern-day computer now the von Neumann bottleneck is basically when the CPU is waiting for data in a resource that's slower than it that creates a performance bottleneck where in fact the system is running at the speed of the slowest device which in this case would be the RAM now the several ways around this problem and the most popular is cache memory now cache memory what is it well basically it's a small amount of memory that's actually on the CPU itself or right next to it and what that means is that when it wants data you can go and get this memory at the same speeds while it's operating so if it's operating at 1.8 gigahertz you can get it at one point a Giga it's running at 2.2 gigahertz it can get it at 2.2 gigahertz and not only that every CPU core has its own cache memory so there's no contention about who can get access to it because they all have their own little cache memory now I can see you thinking well if that's the case then Gary why isn't all memory at cache memory on the CPU at the same speed as the CPU well the basic answer is price cache memory is very expensive the fabrication process is very difficult very complicated and to get large amounts of memory onto a chip is very very expensive so therefore when we're talking about cache memory we're talking about just maybe 64 K or 80 K or 128 K very very small amount of memory per core on the CPU so how does cache memory work well basically it stores a copy of information that's in the main memory when the CPU wants a particular piece of memory it says hey cache do you have that he goes yes I've got it and therefore you can get it at a great speed and that's called a cache hit however sometimes it will say do have a copy of that memory goes no sorry I don't have that you have to get a main memory to go and get it and that's called a cache miss now the greater the cache hits the greater the performance the more cache misses then the lower the performance now if you imagine there are a whole bunch of different ways of Phil English cash to make sure it has the optimum memory in it the optimum information in it now one of the systems they use to get that optimum information is to split the cash in two they split it into data cache and instruction cache now they do that because the instruction cache is actually easier to fill because normally a computer program excuse one instruction then the next one then the next one so you pretty well know that the next instructor could be the one after this one now there are things called branching which means the program jumps to another place and that will be a whole different set of cache but it's actually pretty easy to work out what the next instruction can be needed so for example on the cortex a 72 core from arm there is 48 K of instruction cache and 32 k of data cache and that is for every one of the four calls on the chip now another technique that cache designers use is to use multiple levels of cache so this cache right next to the CPU running at the best speed ever just maybe thirty to forty eight K of memory is called a level one cache now after level one you can have level two cache now level two cache can be measured maybe in megabytes four megabytes let's say now that is shared across all the CPU cores however it's a bigger pool of memory and therefore again there is a greater chance of having a cache hit but because four megabytes is expensive to build it's actually slightly slower memory it's slightly cheaper memory making it more feasible in fact on some systems for example the ARM architecture chips that are put into servers an AMD make server chip forearm and Qualcomm make chips for arm now those chips actually use a level three cache and that may be even as much as 32 megabytes there's one other piece of this jigsaw we need to talk about how does the CPU know where in the cache memory is the data that it needs from main memory the way it does this is basically using what's called a hashing function it takes the address and it wanting main memory applies a hash to it and that gives it a location in that 32 K every time you put in the same address you get the same answer and what happens is is that each address will give you location 1 cache location 2 lat cache location 3 and then that will go through when it gets the end of its 32k or 48k whatever it's got it has to loop round again and so therefore for many many Ram locations you have one cache location and of course the problem comes about when you want to cache two things in the same cache location you can't put two things in there at once so there's another thing which is called a two-way cache 2-way associative cache and what that does is it gives you two slots for every memory address and now when the CPU goes to look there it has to look between is it in the first one no is it in the second one okay I'll use that now obviously that's much quicker than looking through 32 K's worth in fact you can actually get four-way and 8-way and even 16 Way associative cache caches but of course the problem is there is the balance between the complexity of the chip the amount of power that takes because we're running on mobile phones in the amount of power that takes and the performance gains so let me quickly sum up for you a cache is a small amount of memory that runs at the same speed of the CPU and it's there so the CPU has a local copy of the most important bits of information the next instructions to execute or the next bit of data that it wants and it's much faster than going out to the main memory now the bigger the cache the better well organize the cache the greater the performance a smaller cache no cache even will mean lower performance and caches can come in three levels level 1 l1 here's a small one on board the chip may be 30 to 48 K level 2 may be 4 megabytes and level 3 may be 32 megabytes so next time you look at which chip you're going to choose for your smartphone maybe you should look too much cache it's got because that's going to affect its performance well my name is Gary Sims from Android authority and I hope you enjoyed this video if you did please do give it a thumbs up also please don't forget to follow me on social media you can use a little comments here below to tell me what you know about cache memory is important to you have you even looked about the cache memory of the CPU in your smartphone also don't forget to use this link here over at the android authority forums where you can talk to me about cache memory we can have a discussion there if you want to and of course don't forget to stay tuned to android authority comm because we are your 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