Optimizing ARM cache usage for different arrays

Question

Hi Folks, I want to port a small piece of code on ARM Cortex A8 processor. Both L1 cache and L2 cache are very limited. There are 3 arrays in my program. Two of them are sequentially accessed(size> Array A: 6MB and Array B: 3MB) and the access pattern for the third array(size> Array C: 3MB) is unpredictable. Though the calculations are not very rigorous but there are huge cache misses for accessing array C. One solution that I thought would be to allocate more cache (L2) space for array C and less for Array A & B. But I'm not able to find any way to achieve this. I went through preload engine of ARM but could not find anything useful. I would be grateful if you could help me in this regard.

Thanks,

Answer 1

It would be a good idea to split the cache and allocate each array in a different part of it.

Unfortunately that is not possible. The caches of the CortexA8 just are not that flexible. The good old StrongArm had a secondary cache for exactly this splitting purpose, but it's not available anymore. We have L1 and L2 caches instead (overall a good change imho.)

However, there is a thing you can do:

The NEON SIMD unit of the CortexA8 lags behind the general purpose processing unit by around 10 processor cycles. With clever programming you can issue cache prefetches from the general purpose unit but do the accesses via NEON. The delay between the two pipelines gives the cache a bit of time to do the prefetches, so your average cache miss time will be lower.

The drawback is that if you must never move the result of a calculation back from NEON to the ARM unit. Since NEON lags behind this will cause a full CPU pipeline flush. Almost if not even more costly as a cache miss.

The difference in performance can be significant. Out of the blue I would expect something between 20% and 30% of speed improvement.

Answer 2

From what I could find via Google, it looks like ARMv7 (which is the version of the ISA that Cortex A8 supports) has cache-flush capability, though I couldn't find a clear reference on how to use it -- perhaps you can do better if you spend more time on it than the minute or two I spent typing "ARM cache flush" into a search box and reading the results.

In any case, you should be able to achieve an approximation of what you want by periodically issuing "flush" instructions to flush out the parts of A and B that you know you no longer need.