Why does those Google image processing sample Renderscript runs slower on GPU in Nexus 5

Question

I\'d like to thank Stephen for the very quick reply in a previous post. This is a follow up question for this post Why very simple Renderscript runs 3 times slower in GPU than i

Answer 1

There are really two answers to this question.

1: Don't believe the hype regarding GPUs. For some workloads they are faster. However, for many workloads, the difference is small or negative. You have at least 2 different processor types, don't worry about which one get used, only worry if the performance is what you want.

2: For performance tuning I would really focus on the algorithm and avoiding slow operations. Examples:

• Prefer float to double when float provides adequate precision.

• Use RS_FP_RELAXED when you don't need IEEE-754 compliance

• Prefer multiplication to division

• use native_* (ex: native_powr) in place of the full precision routines where the precision is adequate

• Use rsGetElementAt_* over rsSample or rsGetElementAt. The typed version of get are faster that the general get and much faster than rsSample in many cases.

• loads from script globals are typically faster than loads from an rs_allocation. Prefer global for kernel constants.

3: There are some performance issues with global loads today on the Nexus (4,5,7v2) GPU path. These will be improved with updates.