Tuesday, March 10, 2015

Early benchmarks appear for Cortex-A72-based SoC

ARM recently announced the new Cortex-A72 processor core, which is an improved version of the existing high-performance Cortex-A57 processor core.

Alongside the Cortex-A72 CPU core, ARM also announced the CCI-500 interconnect technology as well as the high-end Mali-T880 GPU. Devices incorporating the combination of these technologies are expected to become available in 2016.

However, SoCs using the Cortex-A72 CPU are likely to become available earlier. Qualcomm and MediaTek have both announced SoCs using the Cortex-A72 core with commercial availability in the second half of 2015, suggesting that the CPU core itself is at an advanced stage of introduction. Already, early benchmarks for MediaTek's MT8173 tablet SoC that incorporates the Cortex-A72 have become available.

Cortex-A72 appears to be enhanced version Cortex-A57 optimized for next-generation processes

In its announcement press release from 3 February 2015, ARM claims that more than ten partners have already licensed Cortex-A72, including HiSilicon, MediaTek and Rockchip. Cortex-A72 is based on ARM's ARMv8-A instruction set architecture, and can be combined with the existing Cortex-A53 in a big.LITTLE configuration. Cortex-A72 seems to be positioned as a replacement for Cortex-A57. The similarities with Cortex-A57 are very apparent, for example in the identically sized L1 instruction and data caches, and a feature set that is otherwise very similar.

On a 16 nm FinFET process, the core can sustain operation at speeds up to 2.5 GHz within the constraints of a mobile power envelope (e.g. smartphones), with scalability to higher speeds for larger form-factor devices. However, the first announced devices, such as MediaTek's MT8173, appear to use older processes such as the tried-and-trusted 28 nm HPM process at TSMC, so they are likely to have a lower maximum clock speed.

ARM claims increased performance and power efficiency, although these claims seem to be based on implementation on next-generation processes such as 16 nm FinFET that deliver a significant intrinsic improvement in these metrics. ARM mentions micro-architectural improvements that result in enhancements in floating point, integer and memory performance. When implemented on a 16 nm FinFET process, ARM expects Cortex-A57 to provide 85% higher performance when compared to the Cortex-A57 core on a 20 nm process within a similar smartphone power budget.

Overall, the differences with Cortex-A57 appear to be relatively minor, so that Cortex-A72 is best viewed as an enhanced version of Cortex-A57 that is optimized for next-generation processes such as 16 nm FinFET. Nevertheless, the first SoCs to use the Cortex-A72 core will be manufactured using a less advanced process.

Benchmarks appear for MediaTek's MT8173

MediaTek's MT8173 is a mid-range tablet processor mainly targeting Wi-Fi-only tablets, since it does not have an integrated modem. It has two Cortex-A72 cores and two Cortex-A53 cores in a big.LITTLE configuration. Probably manufactured using the established 28HPM process at TSMC, the maximum clock speed of the Cortex-A57 cores is likely to be lower that the target for 16 nm FinFET, although MediaTek claims a clock speed up to 2.4 GHz, while a much lower frequency is apparent in early benchmarks results.

The chip also features a PowerVR GX6250 GPU, which delivers higher performance than the G6200 GPU used inside MediaTek's existing MT8135 and MT6795.

Recently, early benchmarks for a MT8173 development board have appeared both in the Geekbench Browser and in the results database of GFXBench. The first Geekbench results already appeared in December 2014. The latest set of Geekbench results date from the end of February 2015, although they do show a certain amount variation that may reflect thermal throttling.

Single-core performance good, but not spectacular

As expected, the Geekbench results show good single-core performance, albeit not spectacular. As shown in the following table, singe-core performance is in line with Cortex-A57-based SoCs such as Exynos 5433 and Exynos 7420. It should be noted that the MT8173 test SoC is most likely manufactured at 28 nm with a corresponding relatively low maximum CPU clock speed, while Exynos 5433 and 7420 are manufactured using smaller leading edge processes at Samsung.

SoC          "big" CPU                    Arch     JPEG (int)  Lua (int)   Mandelb. (fp)
                                                   Comp. IPC         IPC         IPC
MT8173       2 x 1.6? GHz Cortex-A72      AArch32  1310  2.13  1380  2.10  1064  1.95
Exynos 5433  4 x 1.80 GHz Cortex-A57r1p0  AArch32  1456  2.10  1397  1.89  1174  1.91
Exynos 7420  4 x 1.97 GHz Cortex-A57r1p0  AArch64  1481  1.97  1409  1.74  1198  1.92

In this table, to determine the IPC index I have made an educated guess about the actual clock speed of MT8173 when running the benchmarks. Geekbench reports a 1.40 GHz clock speed (which probably applies to the Cortex-A53 cores), 1.6 GHz seems to be a good match, providing just a little better IPC than Cortex-A57. Note that Exynos 7420 runs in AArch64 mode, which skews direct IPC comparisons.

Practical implications unclear

Without knowing the exact clock speed of the Cortex-A72 cores, it is hard to draw conclusions about the actual IPC improvement over Cortex-A57. If the MT8173 uses a 28 nm process, the ability to approach the single-core performance of Samsung's Exynos 7420 manufactured using 14 nm FinFET process is impressive. However, although MediaTek demonstrated the MT8173 in an actual tablet at MWC, it is unclear what kind of device the Alps development board in the benchmark entries actually represents, so it remains to be seen whether the benchmarks actually reflect the power budget of a tablet.

The multi-core performance reported is not very impressive, as expected because of the relatively small number of CPU cores. The JPEG Compress multi-core score shows CPU scaling factor of 2.72, which is good and implies utilization of the Cortex-A53 cores. The Mandelbrot floating point benchmark shows similar scaling.

However, the Lua integer benchmark has a very low multi-core scaling factor of 1.41, which is lower than expected, even when allowing for the limited number of cores. For example, MediaTek's MT6795 achieves multi-core scaling of 7.5 in this benchmark, and the Exynos chips range from 3.9 to 5.0. Other chips with a low multi-core scaling factor for Geekbench's Lua subtest include Snapdragon 810 (Cortex-A57-based), MediaTek's MT6595 (Cortex-A17-based) and NVIDIA's Denver-based Tegra-K1 SoC. There are indications that this benchmark test heavily depends on on-chip cache (primarily L2 cache) size and speed.

GPU performance of MT8173's PowerVR GX6250 GPU improves on G6200

The MT8173 test device's GPU performance as shown in GFXBench results database is not overly impressive, but suitable for a mid-range chip and an improvement over the PowerVR G6200 GPU used in other MediaTek SoCs such as MT6595 and MT6795. In the T-Rex Offscreen benchmark, the MT8173 registers a score of 1487, higher than the 1311 of the MT6595 (G6200)-equipped Meizu MX4. In the GFXBench 3.0 low-level tests, alpha blending scores higher than the MT6595 while the other low-level scores are comparable.

Sources: ARM (Cortex-A57 announcement press release), AnandTech (MediaTek MT8173 article), MediaTek (MT8173 announcement), Geekbench Browser (MT8173 test device results), GFXBench (MT8173 test device result)

Updated 10 March 2015.

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