ARM Talk Cortex-A72's Improvements to Cortex-A57

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Yesterday at the ARM Holdings annual Tech Day, the business detailed their new Cortex-A72 reference design process core. The A72 is structurally similar to the current high end Cortex-A57 design, as used in the Snapdragon 810 and Exynos 7420, but with a number of important revisions and optimizations. The A72 has been designed with the 14nm to 16nm die size in mind. We already know that a few processor manufacturers are designing SoCs (System-on-Chips) based around the A72 at a larger die size, such as MediaTek using the 28nm, and I'll touch upon these designs later in the article.

ARM's chief architect for the Cortex A72, Mike Filippo, said this on the design: "Our focus on A72 was to achieve next-gen performance and pull a ton of power out of the design. We did that in spades." The revisions to the A57 core include a better performing branch prediction engine ("up to twenty percent better than the A57") and improved caching. In other words, the processor core is better able to predict what the software code will ask of it next and has a better memory of what it's just been told to do. As such, under ideal circumstances it may be significantly quicker. ARM have also accelerated memory bandwidth by fifty percent compared with the current core, which is what ARM believe will drive the real world performance improvements. ARM have refined and accelerated internal memory bandwidth between different components of the processor core. ARM's data claims that the A72 can be between 20% to 60% quicker, clock for clock, than the Cortex-A57 core. Because the ideal Cortex-A72 will be built on a smaller process than the A57, this is likely to mean that the newer core may be clocked higher than the outgoing core and this in turn means devices could see a double whammy increase in performance.


High performance is great, but what about power consumption? One of the images in the gallery below shows the same workload power consumption statistics for the Cortex-A72 versus the Cortex-A15 (the design used in the Shield Tablet, for example) and the A57 (used in the LG G Flex 2, HTC One M9 and Samsung Galaxy S6) at both current and next generation die sizes. As you can see from the slide, the Cortex-A72 shows a reduction in power consumption across the board and when built at the 28nm size, still uses less power than the older generation cores. Cutting through the numbers and considering energy efficiency, that is, performance per watt of power consumed, the Cortex-A72 is considered to be between 18% to 30% more efficient than the A57 core.

ARM Holdings spent time considering how well their new Cortex-A72 processor core fares when put up against the Intel Broadwell processor. The Broadwell processor is a revision to the Haswell line and is constructed on a 14nm die size. The lineup ranges from the Core i7 to the Core M, which is used in notebooks and tablets. And the A72 fares well under like for like circumstances. This is an important caveat: when the power or "thermal envelope" is limited (that is, how quickly the chassis the device the processor is in can dissipate heat from the processor cores), the Cortex-A72 is competitive with the Broadwell Core M processor. ARM's figures showed that for the same performance, the A72 uses around 75% less energy than the Broadwell competitor: however, we are likely almost a year away from being able to test this and mobile devices will likely evolve a lot in the coming year. It will be interesting to see how well Chromebooks running with a 16nm ARM Cortex-A72 based processor will fare compared with the Intel competition.

To summarize, the Cortex-A72 is showing improvements right across the board compared with the Cortex-A57. It's important to note that we don't have the details as to what the industry of ARM licensees will do with the A72 processor core. We know Qualcomm are working on their next generation custom processor core architecture, Kryo. And let's not discount Samsung as their implementation of the Cortex-A57 in the Exynos 7420 has proven itself able to consistently outperform similar competition in the real world. Plus Intel will not be standing still either; we are likely to be seeing newer and improved processors this time next year. A lot may change in twelve months.

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