CUDA Pro Tip: Occupancy API Simplifies Launch Configuration

CUDA programmers often need to decide on a block size to use for a kernel launch. For key kernels, its important to understand the constraints of the kernel and the GPU it is running on to choose a block size that will result in good performance. One common heuristic used to choose a good block size is to aim for high occupancy, which is the ratio of the number of active warps per multiprocessor to the maximum number of warps that can be active on the multiprocessor at once. Higher occupancy does not always mean higher performance, but it is a useful metric for gauging the latency hiding ability of a kernel.

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Before CUDA 6.5, calculating occupancy was tricky. It required implementing a complex computation that took account of the present GPU and its capabilities (including register file and shared memory size), and the properties of the kernel (shared memory usage, registers per thread, threads per block). Implementating the occupancy calculation is difficult, so very few programmers take this approach, instead using the occupancy calculator spreadsheet included with the CUDA Toolkit to find good block sizes for each supported GPU architecture.

CUDA 6.5 includes several new runtime functions to aid in occupancy calculations and launch configuration. The core occupancy calculator API, cudaOccupancyMaxActiveBlocksPerMultiprocessor produces an occupancy prediction based on the block size and shared memory usage of a kernel. This function reports occupancy in terms of the number of concurrent thread blocks per multiprocessor. Note that this value can be converted to other metrics. Multiplying by the number of warps per block yields the number of concurrent warps per multiprocessor; further dividing concurrent warps by max warps per multiprocessor gives the occupancy as a percentage. Continue reading


CUDA Pro Tip: Profiling MPI Applications

When I profile MPI+CUDA applications, sometimes performance issues only occur for certain MPI ranks. To fix these, it’s necessary to identify the MPI rank where the performance issue occurs. Before CUDA 6.5 it was hard to do this because the CUDA profiler only shows the PID of the processes and leaves the developer to figure out the mapping from PIDs to MPI ranks. Although the mapping can be done manually, for example for OpenMPI via the command-line option --display-map, it’s tedious and error prone. A solution which solves this for the command-line output of nvprof is described here . In this post I will describe how the new output file naming of nvprof to be introduced with CUDA 6.5 can be used to conveniently analyze the performance of a MPI+CUDA application with nvprof and the NVIDIA Visual Profiler (nvvp).

Profiling MPI applications with nvprof and nvvp

Collecting data with nvprof

nvprof supports dumping the profile to a file which can be later imported into nvvp. To generate a profile for a MPI+CUDA application I simply start nvprof with the MPI launcher and up to CUDA 6 I used the string “%p” in the output file name. nvprof automatically replaces that string with the PID and generates a separate file for each MPI rank. With CUDA 6.5, the string “%q{ENV}” can be used to name the output file of nvprof. This allows us to include the MPI rank in the output file name by utilizing environment variables automatically set by the MPI launcher (mpirun or mpiexec). E.g. for OpenMPI OMPI_COMM_WORLD_RANK is set to the MPI rank for each launched process.

$ mpirun -np 2 nvprof -o simpleMPI.%q{OMPI_COMM_WORLD_RANK}.nvprof ./simpleMPI
Running on 2 nodes
==18811== NVPROF is profiling process 18811, command: ./simpleMPI
==18813== NVPROF is profiling process 18813, command: ./simpleMPI
Average of square roots is: 0.667279
==18813== Generated result file: simpleMPI.1.nvprof
==18811== Generated result file: simpleMPI.0.nvprof

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