British company ARM Holdings designs and licenses technology used in the manufacture of processors that power a vast array of mobile devices. ARM has various Cortex A-series CPUs that are designed for a wide range of devices; some focus on providing maximum performance in a mobile or enterprise power envelope, while others tend to run with energy efficiency and cost as the leading attributes.
With battery life a key concern for mobile devices, ARM understands that a single processor architecture isn’t ideally suited to powering entry-level, mid-range and high-end smartphones and tablets, as each have a different set of power and efficiency requirements. Exploring the high-end market further, there is little need for a powerful CPU when running low-intensity tasks such as emailing or MP3 playback – a smaller, more efficient CPU is able to complete the same task just as proficiently while sipping on considerably less power. Conversely, a powerful CPU is the best candidate for heavy-duty processing required for gaming or multi-page web browsing, where using a lower-power CPU could lead to a substandard experience.
How big. LITTLE works
Running a particular application on the most appropriate CPU/cluster requires that a device’s operating system and power-management algorithms, called dynamic voltage and frequency scaling (DVFS), monitor and allocate workload correctly. The operating system is in charge of deciding which cluster and CPUs are active, depending upon load, and ARM has fine-tuned the DVFS profiles to ensure that.
It takes around 30 microseconds for the big. LITTLE software to move the workload from one of the paired cores to another, based on load requirements, but the trade-off between lost time and improved energy efficiency is worth it. This paired approach of CPU Migration, or In-Kernel Switching, requires that the ARM Cortex processors have an equal number of cores. What this also means is that a maximum of four cores can be run at any time.