From: Wolfgang Bangerth Date: Tue, 16 Sep 2014 20:38:32 +0000 (-0500) Subject: Rewrite following the discussion on the pull request. X-Git-Tag: v8.2.0-rc1~145^2~1 X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=86ed179cf683a22d1c0ea8ab7db82d42d04fc87c;p=dealii.git Rewrite following the discussion on the pull request. --- diff --git a/doc/news/changes.h b/doc/news/changes.h index c6978f040e..108f02204d 100644 --- a/doc/news/changes.h +++ b/doc/news/changes.h @@ -40,6 +40,20 @@ inconvenience this causes.

    +
  1. Removed: The constructor of the Utilities::MPI::MPI_InitFinalize + class used to interpret a last argument equal to numbers::invalid_unsigned_int + as "create as many threads as there are processor cores on the current + system" for each MPI process. If there were multiple MPI processes on a + given node, this would lead to (sometimes massive) overallocation of resources + because every MPI process would create as many threads as there are cores. + This has now been changed: an argument equal to numbers::invalid_unsigned_int + is now interpreted as "subdivide the available cores between all MPI + processes running on the current system and let each process create as many + threads as cores were allocated to it". +
    + (Wolfgang Bangerth, 2014/09/16) +
    2. +
  2. Removed: CMake's configure phase no longer calls back into testsuite subprojects. If a build directory is reconfigured the testsuite has to be regenerated by hand using make regen_tests. diff --git a/include/deal.II/base/mpi.h b/include/deal.II/base/mpi.h index 038c5f5571..2f7762d59e 100644 --- a/include/deal.II/base/mpi.h +++ b/include/deal.II/base/mpi.h @@ -258,42 +258,6 @@ namespace Utilities class MPI_InitFinalize { public: - /** - * An enumeration data type that is used in one - * of the constructors. It determines how many threads - * the current process should be using. The options are: - * - * - @p one_thread_per_process : Each MPI process will be - * run as a single-threaded process. In other words, deal.II - * will not support running multiple tasks in parallel. This - * is often a useful default if you start as many MPI processes - * per node as there are processor cores. In this case, running - * each process with multiple threads would oversubscribe the - * available resources. - * - @p one_thread_per_core : This is the default behavior of - * sequential programs, where you want to run as many threads - * in parallel as there are cores on your machine. It would - * also be the appropriate behavior if you started only one - * MPI process per node of your cluster, even if these nodes - * have multiple cores. - * - @p optimal_number_of_threads : This selects the number of - * threads to runon this MPI process in such a way that all of - * the cores in your node are spoken for. In other words, if you - * have started one MPI process per node, this option is equivalent - * to @p one_thread_per_core. If you have started as many MPI - * processes per node as there are cores on each node, then - * this is equivalent to @p one_thread_per_process. On the - * other hand, if, for example, you start 4 MPI processes - * on each 16-core node, then this option will start 4 worker - * threads for each node. If you start 3 processes on an 8 core - * node, then they will start 3, 3 and 2 threads, respectively. - */ - enum ThreadsPerMPIProcess - { - one_thread_per_process, - one_thread_per_core, - optimal_number_of_threads - }; /** * Constructor. Takes the arguments from the command line (in case of @@ -310,55 +274,43 @@ namespace Utilities /** * Initialize MPI (and, if deal.II was configured to use it, PETSc) * and set the number of threads used by deal.II (via the underlying - * Threading Building Blocks library) to the given parameter. If set to - * numbers::invalid_unsigned_int, the number of threads is determined by - * TBB. When in doubt, set this value to 1 since MPI jobs are typically - * run in a way where one has one MPI process per available processor - * core and there will be little CPU resources left to support multithreaded - * processes. - * - * This function calls MultithreadInfo::set_thread_limit() - * unconditionally with @p max_num_threads . That function in turn also - * evaluates the environment variable DEAL_II_NUM_THREADS and the number - * of threads to be used will be the minimum of the argument passed here - * and the environment (if both are set). + * Threading Building Blocks library) to the given parameter. * * @param[in,out] argc A reference to the 'argc' argument passed to main. This * argument is used to initialize MPI (and, possibly, PETSc) as they * read arguments from the command line. * @param[in,out] argv A reference to the 'argv' argument passed to main. * @param[in] max_num_threads The maximal number of threads this MPI process - * should utilize. - */ - MPI_InitFinalize (int &argc, - char ** &argv, - const unsigned int max_num_threads); - - /** - * Initialize MPI (and, if deal.II was configured to use it, PETSc) - * and set the number of threads used by deal.II (via the underlying - * Threading Building Blocks library) to using the policy described by - * the last argument. - * - * This function calls MultithreadInfo::set_thread_limit() - * unconditionally with @p max_num_threads . That function in turn also - * evaluates the environment variable DEAL_II_NUM_THREADS and the number - * of threads to be used will be the minimum of the number determined - * by the policy selected via the last argument and the environment - * (if both are set). + * should utilize. If this argument is set to + * numbers::invalid_unsigned_int, the number of threads is determined by + * automatically in the following way: the number of + * threads to run on this MPI process is set in such a way that all of + * the cores in your node are spoken for. In other words, if you + * have started one MPI process per node, setting this argument is + * equivalent to setting it to the number of cores present in the node + * this MPI process runs on. If you have started as many MPI + * processes per node as there are cores on each node, then + * this is equivalent to passing 1 as the argument. On the + * other hand, if, for example, you start 4 MPI processes + * on each 16-core node, then this option will start 4 worker + * threads for each node. If you start 3 processes on an 8 core + * node, then they will start 3, 3 and 2 threads, respectively. * - * @param[in,out] argc A reference to the 'argc' argument passed to main. This - * argument is used to initialize MPI (and, possibly, PETSc) as they - * read arguments from the command line. - * @param[in,out] argv A reference to the 'argv' argument passed to main. - * @param[in] threads_per_process A policy that describes how the number - * of threads this MPI process should use is determined. See the - * documentation of the ThreadsPerMPIProcess enum for a discussion of the - * possible options. + * @note This function calls MultithreadInfo::set_thread_limit() + * with either @p max_num_threads or, following the discussion above, a + * number of threads equal to the number of cores allocated to this + * MPI process. However, MultithreadInfo::set_thread_limit() in turn also + * evaluates the environment variable DEAL_II_NUM_THREADS. Finally, the worker + * threads can only be created on cores to which the current MPI process has + * access to; some MPI implementations limit the number of cores each process + * has access to to one or a subset of cores in order to ensure better cache + * behavior. Consequently, the number of threads that will really be created + * will be the minimum of the argument passed here, the environment variable + * (if set), and the number of cores accessible to the thread. */ MPI_InitFinalize (int &argc, char ** &argv, - const ThreadsPerMPIProcess threads_per_process); + const unsigned int max_num_threads); /** * Destructor. Calls MPI_Finalize() in case this class owns the diff --git a/source/base/mpi.cc b/source/base/mpi.cc index f4de7f984f..4d87cd9da7 100644 --- a/source/base/mpi.cc +++ b/source/base/mpi.cc @@ -321,38 +321,15 @@ namespace Utilities { do_init(argc, argv); - // set maximum number of threads (also respecting the environment - // variable that the called function evaluates) - multithread_info.set_thread_limit(max_num_threads); - } - - - - MPI_InitFinalize::MPI_InitFinalize (int &argc, - char ** &argv, - const ThreadsPerMPIProcess threads_per_process) - : - owns_mpi (true) - { - do_init(argc, argv); - - // set maximum number of threads (also respecting the environment - // variable that the called function evaluates) - switch (threads_per_process) - { - case one_thread_per_process: + if (max_num_threads != numbers::invalid_unsigned_int) { - multithread_info.set_thread_limit(1); - break; + // set maximum number of threads (also respecting the environment + // variable that the called function evaluates) based on what + // the user asked + multithread_info.set_thread_limit(max_num_threads); } - case one_thread_per_core: - { - // choose the maximal number of threads possible - multithread_info.set_thread_limit(numbers::invalid_unsigned_int); - break; - } - - case optimal_number_of_threads: + else + // user wants automatic choice { // we need to figure out how many MPI processes there // are on the current node, as well as how many CPU cores @@ -407,11 +384,6 @@ namespace Utilities // finally set this number of threads multithread_info.set_thread_limit(n_threads); - break; - } - - default: - Assert (false, ExcNotImplemented()); } }