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Dmitriy Korchemkin
dc7a859752
As pointed out by several users, introduction of parallel operations on vectors severely impacts solver performance on small problems, with time consumption increasing with the number of threads. In order to minimize overhead of trying to execute small tasks using a large number of threads, task scheduling mechanism was changed to avoid scheduling all tasks at once. However, there is still a large difference in exectuion time because the main thread always launches the next thread before starting doing the work. This leads to several orders of magnitude slowdown when going from a single-threaded execution (which follows a fast-forward path to a single loop over all indices, without any synchronization involved) to a two-thread execution: /bin/parallel_vector_operations_benchmark ------------------------------------------- Benchmark Time ------------------------------------------- SetZero/128 12.8 ns SetZeroParallel/128/1 16.6 ns SetZeroParallel/128/2 2211 ns In order to eliminate this effect, we limit the block-size of parallel execution of vector operations to 2^16 elements (thus, starting parallel execution only for vectors of at least 2^17 elements). Threshold of 2^16 elements was choosen by evaluating thresholds from 2^10 to 2^20 (only powers of 2), with 2^14..2^20 significantly reducing worst-case runtime degradation. Details can be found in discussion of the issue at https://github.com/ceres-solver/ceres-solver/issues/1016 Change-Id: I555c882d63ee53323ceb426743b970f989b65503