16.1. Structure of a big.LITTLE system

Both types of core in a big.LITTLE system are fully cache coherent and share the same instruction set architecture (ISA). The same application binary runs unmodified on either. Differences in the internal microarchitecture of the processors enable them to provide the different power and performance characteristics that are fundamental to the big.LITTLE concept. These are typically managed by the operating system.

big.LITTLE software models require transparent and efficient transfer of data between big and LITTLE clusters. Hardware coherency enables this, transparently to the software. Coherency between clusters is provided by a cache-coherent interconnect such as the ARM CoreLink CCI-400 described in Chapter 14. Without hardware coherency, the transfer of data between big and LITTLE cores would always occur through main memory but this would be slow and not power efficient. In addition, it would require complex cache management software, to enable data coherency between big and LITTLE clusters.

In addition, such a system also requires a shared interrupt controller, such as the GIC-400, enabling interrupts to be migrated between any cores in the clusters. All cores can signal each other using distributed interrupt controllers such as the CoreLink GIC-400. Task switching is typically handled entirely within the OS scheduler, and invisible to the application software. An example system is shown in Figure 16.1.

Figure 16.1. Typical big.LITTLE system

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