The impact of grain size on the efficiency of embedded SIMD image processing architectures

Pixel-per-processing element (PPE) ratio—the amount of image data directly mapped to each processing element—has a significant impact on the area and energy efficiency of embedded SIMD architectures for image processing applications. This paper quantitatively evaluates the impact of PPE ratio on system performance and efficiency for focal-plane SIMD image processing architectures by comparing throughput, area efficiency, and energy efficiency for a range of common application kernels using architectural and workload simulation. While the impact of grain size is affected by the mix of executed instructions within an application program, the most efficient PPE ratio often does not occur at PE grain size extremes (i.e., one pixel per processor or one processor per image). In this study, a set of four image processing application tasks is implemented on eight different SIMD configurations. Each configuration has a different PPE ratio and a different amount of local memory. Cycle accurate simulation and analytical technology modeling allows assessment of execution performance, area efficiency, and energy efficiency for each configuration. Results show the highest area and energy efficiency are achieved at PPE ratios between 16 and 256. Using these evaluation techniques (application grain size retargeting combined with area and energy technology modeling), a new class of efficient, embedded SIMD architectures for image processing can be designed.