Bounding the effect of partition camping in GPU kernels

Current GPU tools and performance models provide some common architectural insights that guide the programmers to write optimal code. We challenge and complement these performance models and tools, by modeling and analyzing a lesser known, but very severe performance pitfall, called Partition Camping, in NVIDIA GPUs. Partition Camping is caused by memory accesses that are skewed towards a subset of the available memory partitions, which may degrade the performance of GPU kernels by up to seven-fold. There is no existing tool that can detect the partition camping effect in GPU kernels. Unlike the traditional performance modeling approaches, we predict a performance range that bounds the partition camping effect in the GPU kernel. Our idea of predicting a performance range, instead of the exact performance, is more realistic due to the large performance variations induced by partition camping. We design and develop the prediction model by first characterizing the effects of partition camping with an indigenous suite of micro-benchmarks. We then apply rigorous statistical regression techniques over the micro-benchmark data to predict the performance bounds of real GPU kernels, with and without the partition camping effect. We test the accuracy of our performance model by analyzing three real applications with known memory access patterns and partition camping effects. Our results show that the geometric mean of errors in our performance range prediction model is within 12% of the actual execution times. We also develop and present a very easy-to-use spreadsheet based tool called CampProf, which is a visual front-end to our performance range prediction model and can be used to gain insights into the degree of partition camping in GPU kernels. Lastly, we demonstrate how CampProf can be used to visually monitor the performance improvements in the kernels, as the partition camping effect is being removed.