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Using OpenMP, I multithreaded functions that were major bottlenecks in the optical flow computation.
Good news: Computation time may be reduced by up to 50%.
Bad news: The performance scales poorly with the number of cores. This seems to be because these functions intensively write out to memory, causing false sharing.
In my Linux environment with 32 cores, the best performance was achieved by limiting OpenMP to use just two threads (by setting the environment variable OMP_NUM_THREADS) and then make use of other cores by multiprocessing over frames.