Friday, 12 September 2014

Characterising the TegraK1 Cortex A15

A Benchmark Characterisation of the Cortex-A15-r3p3

I have run HPL and Coremark from the lowest frequency (204 MHz) to the highest (2.3 GHz). At first I thought the performance per Watt would be interesting but as expected the lower the frequency the lower the power consumption so it only shows that the efficiency is best at 204 MHz. A much more interesting value is the Performance/Watt X Performance. This essentially shows us at which frequency the CPU maximises both the performance and the efficiency simultaneously.

I have done this in the past with the Cubieboard2, Wandboard and Odroid (Cortex-A7-r0p4, Cortex-A9-r2p2, Cortex-A15-r3p2) but I only did it with HPL. I was asked if I had tried this with Coremark. It was simple enough but I wanted to see if I got the same profile shape as HPL. The question arose "how do I compare HPL and Coremark together?". Obviously a direct comparison is not possible as they are fundamentally different benchmarks but we can compare the shapes of the graphs. This can be done by normalising the results so that the area under each graph is equal to 1. The units on the y axis are then expressed in inverse frequency (MHz­­­-1). What we get is shown below.

This is really interesting. This shows that the approach of using the Performance/Watt X Performance does indeed describe the characteristic of the CPU. It also is nice to see that it is similar for both benchmarks. We see for both cases that the optimum frequency is at 1.73 GHz to maximise both Performance and Efficiency. We are in the process of putting together a full set of results into a paper. I will add a link as soon as its finished.

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