14.1.5. Asymmetric multi-processing

An Asymmetric Multi-processing (AMP) system enables you to statically assign individual roles to a core within a cluster so that, in effect, you have separate cores, each performing separate jobs within each cluster. This is known as a function-distribution software architecture and typically means that you have a separate OS running on the individual cores. The system can appear to you as a single-core system with dedicated accelerators for certain critical system services. AMP does not refer to systems in which tasks or interrupts are associated with a particular core.

In an AMP system, each task can have a different view of memory and there is no scope for a core that is highly loaded to pass work to one that is lightly loaded. There is no requirement for hardware cache coherency in such systems, although there are typically mechanisms for communication between the cores through shared resources, possibly requiring dedicated hardware. The system described in Cache coherency can help reduce the overheads associated with sharing data between the systems.

Reasons for implementing an AMP system using a multi-core processor might include security, a requirement for guaranteeing meeting of real-time deadlines, or because individual cores are dedicated to perform specific tasks.

There are classes of systems that have both SMP and AMP features. This means that there are two or more cores running an SMP OS, but the system has additional elements that do not operate as part of the SMP system. The SMP sub-system can be regarded as one element within the AMP system. Cache coherency is implemented between the SMP cores, but not necessarily between SMP cores and AMP elements within the system. In this way, independent subsystems can be implemented within the same cluster.

It is entirely possible (and normal) to build AMP systems in which individual cores are running different operating systems (these are called Multi-OS systems).

Note

Where synchronization is required between these separate cores, it can be provided through message passing communication protocols, for example, the Multicore Communications Association API (MCAPI). These can be implemented by using shared memory to pass data packets and by the use of software-triggered interrupts to implement a so-called door-bell mechanism.

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