Cortex-A57 based on Cortex-A15 and not fully optimized for power-efficiency
The Cortex-A57 CPU core, which was announced in 2012, has significant similarities to Cortex-A15, ARM's long-standing high-performance 32-bit CPU core, which has been known for relatively high power consumption. As such, it is not unexpected that improvements on the Cortex-A57 architecture (in the form of the Cortex-A72) have proven to be possible. Cortex-A57-based SoCs such as Snapdragon 810 have been known to throttle, being forced to reduce the clock speed due to excessive heat production and power use, resulting in reduced sustained performance. Apple's A7 and A8 processors use CPU cores that most likely have strong similarities with Cortex-A57, but which exhibit little throttling due to a lower maxium clock speed, a lower number of cores and other factors related to the the chip design.
Increased level of sustained performance
ARM has made available a number of slides detailing the improvements in sustained performance and power efficiency in Cortex-A72 over Cortex-A57. On a 28 nm process and similar clock speed, ARM's charts indicate a roughly 20% improvement in power reduction.
Sustained performance is expected to be higher than Cortex-A57, implementations of which (such as Snapdragon 810 and Exynos 5433, and to a lesser degree Exynos 7420) have suffered from an inability to maintain high clock speeds and throttle back to a relatively low speed due to heat production and associated power consumption. ARM gives a figure of sustained 750 mW operation per core on a 16FF+ process with a clock speed around 2.5 GHz.
In terms of IPC (instructions per cycle), ARM's information shows improvements in all instruction-level performance segments, with a 1.16x improvement for "analytics", 1.38x for cryptography, 1.50x for memory, 1.26x for floating point and 1.16 for integer compute. The increase in memory performance appears to be significant.
Improved single-core performance evident in early Geekbench results
Early Geekbench results for the MT8173 SoC from MediaTek, which includes two Cortex-A72 cores, give an indication of practical peformance of the Cortex-A72 core, although the exact clock speed the Cortex-A72 cores are running at is hard to determine. The following table shows single-core performance from a recent MT8173 Geekbench entry, comparing it to Exynos 7420 as used in the Samsung Galaxy S6. Both use 64-bit AArch64 mode.
The MT8173 easily matches the single-core performance of Exynos 7420, while showing significant improvements in the Mandelbrot floating point subtest and the memory-intensive Dijkstra subtest, and also the Lua subtest. Memory subtest (Stream Copy) performance is also better than Exynos 7420, despite the likely much wider memory interface of the latter, providing clear evidence of the improved memory performance (largely due to smarter prefetching) in Cortex-A72. Overall, since the MT8173 results reflects a SoC using 28 mn or perhaps 20 nm process technology, while Exynos 7420 uses Samsung's leading-edge 14 nm FinFET process, the ability of the MT8173 to beat Exynos 7420 in single-core performance while using a less advanced process is impressive and illustrates the performance improvements in the Cortex-A72 core.
SoC JPEG Dijkstra Lua Mandelb. Stream SGEMM SFFT Compr. Copy 28nm? MT8173 (Cortex-A72) 1429 1287 1675 1750 2217 979 1345 14nm Exynos 7420 1475 1082 1409 1147 1993 954 1379
Reduced silicon area results in lower cost
Cortex-A72 has a silicon area that is 10% smaller than Cortex-A57 on an equivalent process, while delivering improvements in performance and power efficiency. Already SoCs have been announced or described that utilize Cortex-A72 cores, such as MediaTek's MT8173 for tablets, Qualcomm's Snapdragon 618 and 620 for smartphones, and MediaTek's MT6797 (Helio-X20) for smartphones.
There seems to be a clear trend of using just two Cortex-A72 cores (instead of the four cores used in many Cortex-A57 implementations), reducing cost and maximum power consumption. These are cores are augmented by low-power, small-area Cortex-A53 cores running at a lower frequency. MT8173, Snapdragon 618 and Helio-X20 all use such as configuration.
Use of Cortex-A72 may be more effective than high-clocked Cortex-A53 cores
There are indications that Cortex-A53 cores running at a high frequency (such as implemented in MediaTek's MT6752 and MT6795 (Helio-X10), HiSilicon's Kirin 930 and to a lesser degree in Snapdragon 615 and the announced Snapdragon 415 and 420) run into a power efficiency bottleneck at higher clock speed, due the relatively steep increase in power consumption as the clock speed of the Cortex-A53 core increases above 1.3-1.5 GHz. Solutions that combine a small number of Cortex-A72 with lower-clocked, power efficient Cortex-A53 cores may prove to be a sweet spot in terms of practical performance and power efficiency for mid-range SoCs.
Source: AnandTech (Cortex-A72 Architecture Details article), Geekbench Browser