Chapter 16. big.LITTLE Technology

Modern software stacks place conflicting requirements on mobile systems. On the one hand is a demand for very high performance for tasks such as games, while on the other is a continuing requirement to be frugal with energy reserves for low intensity applications like audio playback.Traditionally, it has not been possible to have a single processor design that can be capable of both high peak performance as well as high energy efficiency. This meant that a lot of energy was wasted because the high performance core would be used for low intensity tasks leading to reduced battery life. Performance would itself be affected by the thermal limits at which the cores could run for sustained periods.

big.LITTLE technology from ARM solves this problem by coupling together energy-efficient LITTLE cores with high-performance big cores. big.LITTLE is an example of a heterogeneous processing system. Such systems typically include several different processor types with different microarchitectures, like general-purpose processors and specialized ASICs.

big.LITTLE takes the heterogeneity one step further in that it includes general-purpose processors that are different in their micro-architecture but compatible in their instruction set architecture. A term that is often used with such systems is Heterogeneous Multiprocessing (HMP) (See Heterogeneous multi-processing). What makes HMP different from Asymmetric Multiprocessing (AMP) (Asymmetric multi-processing) is that all the processors in an HMP system are fully coherent and run the same operating system image.

Software can run on big or the LITTLE processors (or both) depending on performance requirements. When peak performance is required software can be moved to run only on big processors. For normal tasks, software can be run perfectly well on LITTLE processors. Through this combination, big.LITTLE provides a solution capable of delivering the peak performance required by the latest mobile devices, within the thermal bounds of the system, and with maximum energy efficiency.

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