I don't understand why nvidia insists on quad-core SoCs. Mobile devices need single threaded performance and graphics power more than they need more cores. A15 will be a huge help but I can't help but think that they'd be better off with a 2 + 1, a pure big.little design, or even just a standard dual core design while spending the extra die space on more graphics hardware, which is supposedly their strength anyway.
A large part of the die space of mobile arm cpu is everything else the cpu has...
- the cache which they are going to have no matter if it is a 2 or 4 core since the
- cache is shared
- the gpu
- the seperate image processor
- dedicated hd video decode (more power savings than using the gpu)
- dedicated hd video encode (more power savings than using the gpu)
- audio processor
- input/output
- etc
Here is an image of tegra 3 die size. Note the yellow parts those are the individual cortex a9 cores. Notice there are 5 of time and each individual core takes about 2% of the die space
So going from a 2 core to a 4+1 companion core increased the die space by about 6%.
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Furthermore you can scale the mhz speed and the voltage of each individually core separately. It is better to run two cores at 1000 mhz and 400 mhz then it would be to run one core at 1200 mhz from a heat and battery life standpoint.
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There is a reason why most manufactures are going for the big little approach. The future cortex a7 processors are extremely small from a die space persepctive from anandtech
ARM claims a single Cortex A7 core will measure only 0.5mm2 on a 28nm process. On an equivalent process node ARM expects customers will be able to implement an A7 in 1/3 - 1/2 the die area of a Cortex A8. As a reference, an A9 core uses about the same (if not a little less) die area as an A8 while an A15 is a bit bigger than both.
1/2 a mm2 that is frickin tiny. When you are dealing with dies that are 80 to 200mm2 putting 4 of these a7 cpus in there for low battery life makes sense when you are only dealing with 2 to 3mm2 of wasted die space.
From a die space scenario it makes sense that samsung is going for a 4 big+ 4 little cores. When your everything else is so much of your die space, yet it is your memory controller, cpu, and gpus that determine performance spending a fractional amount more on your cpu to get much better performance (from battery life and outright performance) makes sense.
Your app may only use 1 or 2 cores, but these devices are becomming more like computers running multiple things at one time with multiple processes as well as the os. Remember you do not need to run the additional cores at 100%