Parallelizing general histogram application for CUDA architectures

Abstract

Histogramming is a tool commonly used in data analysis. Although its serial version is simple to implement, providing an efficient and scalable way to parallelize it can be challenging. This especially holds in case of platforms that contain one or several massively parallel devices like CUDA-capable GPUs due to issues with domain decomposition, use of global memory and similar. In this paper we compare two approaches for implementing general purpose histogramming on GPUs. The first algorithm is based on private copies of bin counters stored in shared memory for each block of threads. The second one uses the Thrust library to sort the input elements and then to search for upper bounds according to bin widths. For both algorithms we analyze how the speedup over the sequential version depends on the size of input collection, number of bins, and the type and distribution of input elements. We also implement overlapping of data transfers between host CPU and CUDA device with kernel execution. For both algorithms we analyze the pros and cons in detail. For example, privatization strategy can be up to 2x faster than sort-search with realistic inputs, but can only support a limited number of bins. On the other hand, sort-search strategy has about 50% higher speedup than privatization when we use characters as input and can support unlimited number of bins. Finally, we perform an exploration to determine the optimal algorithm depending on the characteristics and values of input parameters.