Disable Eigen's parallelization with OpenMP (interferes with TBB)
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@ -129,7 +129,7 @@ endif()
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# Set platform / compiler specific compile flags and checks
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# Set platform / compiler specific compile flags and checks
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if(APPLE)
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if(APPLE)
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# Need to investigate how to reliably detect and use OpenMP on macOS...
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# Need to investigate how to reliably detect and use OpenMP on macOS...
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set(USE_OPENMP_DEFAULT OFF)
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# set(USE_OPENMP_DEFAULT OFF)
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# Among others, setting CMAKE_FIND_FRAMEWORK to LAST fixed issues
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# Among others, setting CMAKE_FIND_FRAMEWORK to LAST fixed issues
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# with installed Mono that contains old headers (libpng, ...).
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# with installed Mono that contains old headers (libpng, ...).
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@ -163,7 +163,7 @@ if(APPLE)
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endif()
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endif()
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elseif(UNIX)
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elseif(UNIX)
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set(USE_OPENMP_DEFAULT ON)
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# set(USE_OPENMP_DEFAULT ON)
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# assume libstdc++
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# assume libstdc++
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set(STD_CXX_FS stdc++fs)
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set(STD_CXX_FS stdc++fs)
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@ -186,13 +186,37 @@ endif()
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# OpenMP option and compile flags
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# OpenMP option and compile flags
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option(USE_OPENMP "Use OpenMP (e.g. for parallel computation in Eigen)" ${USE_OPENMP_DEFAULT})
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#
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if(USE_OPENMP)
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# Note: OpenMP and TBB don't mix well, so we disable Eigen's parallelization.
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message(STATUS "OpenMP Enabled")
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# It's trying to paralellize matrix products during SC, which we run in a parallel_reduce using TBB.
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set(BASALT_CXX_FLAGS "${BASALT_CXX_FLAGS} -fopenmp")
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# Turns out using OpenMP can slow down the computby factor 10-100x! So for now we discable it completely.
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else()
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# One place where Eigen's parallelization could still have been useful is the CG solver in the mapper.
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message(STATUS "OpenMP Disabled")
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# We could in the future investiagte other implementations (paralellized with TBB) or selectively enabling
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endif()
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# Eigen's parallelization just for CG, setting number of threads to 1 everywhere else.
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# Another way to ensure Eigen doesn't use OpenMP regardless of how it was built is setting the environment
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# variable OMP_NUM_THREADS=1 beofre running the application.
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#
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# See: https://eigen.tuxfamily.org/dox/TopicMultiThreading.html
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#
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# If we enable BLAS / LAPACK either directly or via thirdparty libs like ceres,
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# make sure to disable OpenMP for the linked BLAS library. In particular on Ubuntu it seems OpenBLAS is often installed,
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# and it can have similar issues in multithreaded applications if it's own parallelization with OpenMP is enabled.
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# You can set the environment varaibles OPENBLAS_NUM_THREADS=1 or OMP_NUM_THREADS=1. This is also mentioned in the ceres
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# installation documentation.
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#
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# See also: https://github.com/xianyi/OpenBLAS/wiki/faq#multi-threaded
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#
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# Set EIGEN_DONT_PARALLELIZE to be sure it doesn't use openmp,
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# just in case some dependency enables openmp without us noticing.
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set(BASALT_CXX_FLAGS "${BASALT_CXX_FLAGS} -DEIGEN_DONT_PARALLELIZE")
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#option(USE_OPENMP "Use OpenMP (e.g. for parallel computation in Eigen)" ${USE_OPENMP_DEFAULT})
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#if(USE_OPENMP)
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# message(STATUS "OpenMP Enabled")
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# set(BASALT_CXX_FLAGS "${BASALT_CXX_FLAGS} -fopenmp")
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#else()
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# message(STATUS "OpenMP Disabled")
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#endif()
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# setup combined compiler flags
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# setup combined compiler flags
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@ -201,6 +201,10 @@ class SparseHashAccumulator {
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VectorX res;
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VectorX res;
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if (iterative_solver) {
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if (iterative_solver) {
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// NOTE: since we have to disable Eigen's parallelization with OpenMP
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// (interferes with TBB), the current CG is single-threaded, and we
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// can expect a substantial speedup by switching to a parallel
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// implementation of CG.
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Eigen::ConjugateGradient<Eigen::SparseMatrix<double>,
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Eigen::ConjugateGradient<Eigen::SparseMatrix<double>,
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Eigen::Lower | Eigen::Upper>
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Eigen::Lower | Eigen::Upper>
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cg;
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cg;
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