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Chapter 14. Multi-core processors

The ARMv8-A architecture provides a significant level of support for systems containing multiple processing elements. An ARM multi-core processor such as the Cortex-A57MPCore and Cortex-A53MPCore processors can contain between one and four cores. Systems that use the Cortex-A57 or Cortex-A53 processors are almost always implemented in this way. A multi-core processor might contain several cores capable of independent instruction execution that can be treated as a single unit or cluster. ARM multi-core technology enables any of the four component cores within a cluster to shut down when not in use to save power, for example when the device is lightly loaded or in standby mode. When higher performance is required, every processor is in use to meet the demand while still sharing the workload to keep power consumption as low as possible.

Multi-processing can be defined as running two or more sequences of instructions simultaneously within a single device containing two or more cores. It is now a widely adopted technique in both systems intended for general-purpose application processors and in areas that are more traditionally defined as embedded systems.

The overall energy consumption of a multi-core system can be significantly lower than that of a system based on a single processor core. Multiple cores may enable execution to be completed faster and so some elements of the system might be completely powered down for longer periods. Alternatively, a system with multiple cores might be able to operate at a lower frequency than that required by a single processor to achieve the same throughput. A lower power silicon process or a lower supply voltage can result in lower power consumption and reduced energy usage. Most current systems do not permit the frequency of cores to be changed independently. However, each core can be dynamically clock gated, giving additional power and energy savings.

Having multiple cores at our disposal also enables more options for system configuration. For example, you might have a system that uses separate cores, one to handle a hard real-time requirement and another for an application requiring high, uninterrupted performance. These could be consolidated into a single multi-processor system.

A multi-core device is also likely to be more responsive than one with a single core. When interrupts are distributed between cores there is more than one core available to respond to an interrupt and fewer interrupts per core to be serviced. Multiple cores also enable an important background process to progress simultaneously with an important but unrelated foreground process.