On Sun, Nov 28, 2010 at 10:28 PM, Michael Hope <michael.h...@linaro.org> wrote: > I sat down and measured the power consumption of the NEON unit on an > OMAP3. Method and results are here: > https://wiki.linaro.org/MichaelHope/Sandbox/NEONPower > > The board takes 2.37 W and the NEON unit adds an extra 120 mW. > Assuming the core takes 1 W, then the code needs to run 12 % faster > with NEON on to be a net power win. > > Note that the results are inaccurate but valid enough.
Just to play devil's advocate... the results will differ, perhaps significantly, between SoCs of course. In terms of the amount of energy required to perform a particular operation (i.e., at the microbenchmark level) I agree with your conclusion. However, in practice I suspect this isn't enough. I'm not familiar with exactly when NEON is likely to get turned on and off, but you need to factor in the behaviour of the OS--- if you accelerate a DSP operation which is used a few dozen times per timeslice, NEON will be used for only a tiny proportion of the time it is used, because once NEON is on, it probably stays on at least until the interrupt, and probably until the next task switch. With the kernel configured for dynamic timer tick, this can get even more exaggerated, since the rescheduling frequency may drop. The real benefits, in performance and power, therefore come in operations which dominate the run-time of a particular process, such as intensive image handling or codec operations. NEON in widely-dispersed but sporadically used features (such as general-purpose library code) could be expected to come at a net power cost. If you use NEON for memcpy for example, you will basically never be able to turn the NEON unit off. That's unlikely to be a win overall, since even if you now optimise all the code in the system for NEON, you're unlikely to see a significant performance boost-- NEON simply isn't designed for accelerating general-purpose code. The correct decision for how to optimise a given piece of code seems to depend on the SoC and the runtime load profile. And while you can usefully predict that at build-time for a media player or dedicated media stack components, it's pretty much impossible to do so with general-purpose libraries... unless there's a cunning strategy I haven't thought of. Ideally, processes whose load varies significantly over time and between different use cases (such as Xorg) would be able to select between NEON-ised and non-NEON-ised implementations dynamically, based on the current load. But I guess we're some distance away from being able to achieve that... ? Cheers ---Dave _______________________________________________ linaro-dev mailing list linaro-dev@lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-dev
