big.LITTLE: ARM's Strategy For Efficient Computing 73
MojoKid writes "big.LITTLE is ARM's solution to a particularly nasty problem: smaller and smaller process nodes no longer deliver the kind of overall power consumption improvements they did years ago. Before 90nm technology, semiconductor firms could count on new chips being smaller, faster, and drawing less power at a given frequency. Eventually, that stopped being true. Tighter process geometries still pack more transistors per square millimeter, but the improvements to power consumption and maximum frequency have been falling with each smaller node. Rising defect densities have created a situation where — for the first time ever — 20nm wafers won't be cheaper than the 28nm processors they're supposed to replace. This is a critical problem for the mobile market, where low power consumption is absolutely vital. big.LITTLE is ARM's answer to this problem. The strategy requires manufacturers to implement two sets of cores — the Cortex-A7 and Cortex-A15 are the current match-up. The idea is for the little cores to handle the bulk of the device's work, with the big cores used for occasional heavy lifting. ARM's argument is that this approach is superior to dynamic voltage and frequency scaling (DVFS) because it's impossible for a single CPU architecture to retain a linear performance/power curve across its entire frequency range. This is the same argument Nvidia made when it built the Companion Core in Tegra 3."
Re:It's not necessarily ARM's solution (Score:2, Interesting)
where on phones every extra hour of battery life is a cherished commodity. Such may not be true for tablets that can stand to have larger batteries and where performance at "some reasonable expectation" of battery life may be the more important.
This isn't directly for phones and tablets and it isn't "a lazy way out of the power problem".
We are not talking about a gradual increase in efficiency here, this is to solve the standby energy requirements for permanently powered consumer devices like TV-sets. (See the One Watt Initiative [wikipedia.org])
The first generation of devices that solved the problem had dual power supplies. One that was optimized for high efficiency for a low load. This was used to power a microcontroller that dealt with the remote control and started the primary power supply and the rest of the electronics.
Later there where pretty large improvements in switched power supplies that made it possible to go back to just having a single transformer.
The problem is that there aren't really any devices in the 32bit-range that can get down below the 1mA-range without being completely shut down. (This isn't just ARM, it's also true for PIC32, ColdFire, AVR32 and other competing controllers, and no, Atom is not even trying to get down in this range.)
Because of this the common solution is to have a small 8bit/16bit controller to handle the standby mode and possibly some of the low latency tasks that the larger controllers have problems with.
The big.LITTLE solution to this problem isn't new, controllers with asymmetric cores have been available for a while. The benefit of it isn't a power saving, it is a space saving. This will allow developers to remove the external controllers.
It also doesn't add complexity compared to a system with multiple controllers that are completely separate form each other.