From: Wolfgang Bangerth Date: Fri, 18 Feb 2022 03:38:50 +0000 (-0700) Subject: Introduce free functions for consensus algorithms. X-Git-Tag: v9.4.0-rc1~259^2~1 X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=f77ac652e0d6bdaa92e4466e1461b310b36e5eeb;p=dealii.git Introduce free functions for consensus algorithms. --- diff --git a/include/deal.II/base/mpi_consensus_algorithms.h b/include/deal.II/base/mpi_consensus_algorithms.h index 03f7edbb57..af6fcee110 100644 --- a/include/deal.II/base/mpi_consensus_algorithms.h +++ b/include/deal.II/base/mpi_consensus_algorithms.h @@ -29,8 +29,74 @@ namespace Utilities namespace MPI { /** - * A namespace for consensus algorithms designed for dynamic-sparse - * communication patterns. + * A namespace for algorithms that implement the task of communicating + * in a dynamic-sparse way. In computer science, this is often called a + * consensus + * problem. + * + * The problem consensus algorithms are trying to solve is this: Let's + * say you have $P$ processes that work together via MPI. Each (or at + * least some) of these want to send information to some of the other + * processes, or request information from other processes. No process + * knows which other process wants to communicate with them. The challenge + * is to determine who needs to talk to whom and what information needs to + * be sent, and to come up with an algorithm that ensures that this + * communication happens. + * + * That this is not a trivial problem can be seen by an analogy of the + * postal service. There, some senders may request information from some + * other participants in the postal service. So they send a letter that + * requests the information, but the recipients do not know how many such + * letters they need to expect (or that they should expect any at all). + * They also do not know how long they need to keep checking their mailbox + * for incoming requests. The recipients can be considered reliable, + * however: We can assume that everyone who is sent a request puts a + * letter with the answer in the mail. This time at least the recipients + * of these answers know that they are waiting for these answers because + * they have previously sent a request. They do not know in advance, + * however, when the answer will arrive and how long to wait. The goal of + * a consensus algorithm is then to come up with a strategy in which every + * participant can say who they want to send requests to, what that + * request is, and is then guaranteed an answer. The algorithm will only + * return when all requests by all participants have been answered and the + * answer delivered to the requesters. + * + * The problem is generally posed in terms of *requests* and *answers*. + * In practice, either of these two may be empty messages. For example, + * processes may simply want to send information to others that they know + * these others need; in this case, the "answer" message may be empty + * and its meaning is simply an affirmation that the information was + * received. Similarly, in some cases processes simply need to inform + * others that they want information, but the destination process knows + * what information is being requested (based on where in the program + * the request happens) and can send that information without there be + * any identifying information in the request; in that case, the + * request message may be empty and simply serve to identify the + * requester. (Each message can be queried for its sender.) + * + * As mentioned in the first paragraph, the algorithms we are interested + * in are "dynamic-sparse": + * - Dynamic: By the time the algorithm is called, the other processes do + * not know yet that they have to answer requests. + * - Sparse: Each process only has to communicate with a small subset of + * processes of the MPI communicator. + * + * In order to run the communication algorithms, users of this class have + * to provide a number of pieces of information: + * - An MPI communicator. + * - On each process, a list of ranks of processes to communicate with. + * - Functionality to pack/unpack data to send as either the original + * request or as part of the answer. + * This information is typically either provided as direct objects (for + * the first two of the points above), or as function objects (for the + * third point above). In the latter case, the function objects are often + * simply lambda functions declared right in the context where one wants + * to run a consensus algorithm; these lambda functions may then reference + * variables that are active at the point of declaration of the lambda + * function, such as variables local to the surrounding function. + * + * This namespace provides several implementations of consensus algorithms, + * such as the nbx(), pex(), serial(), and selector() functions. * * @ingroup MPI */ @@ -124,69 +190,9 @@ namespace Utilities /** - * A base class for algorithms that implement the task of coming up with - * communication patterns to retrieve data from other processes in a - * dynamic-sparse way. In computer science, this is often called a - * consensus - * problem. - * - * The problem consensus algorithms are trying to solve is this: Let's - * say you have $P$ processes that work together via MPI. Each (or at - * least some) of these want to send information to some of the other - * processes, or request information from other processes. No process - * knows which other process wants to communicate with them. The challenge - * is to determine who needs to talk to whom and what information needs to - * be sent, and to come up with an algorithm that ensures that this - * communication happens. - * - * That this is not a trivial problem can be seen by an analogy of the - * postal service. There, some senders may request information from some - * other participants in the postal service. So they send a letter that - * requests the information, but the recipients do not know how many such - * letters they need to expect (or that they should expect any at all). - * They also do not know how long they need to keep checking their mailbox - * for incoming requests. The recipients can be considered reliable, - * however: We can assume that everyone who is sent a request puts a - * letter with the answer in the mail. This time at least the recipients - * of these answers know that they are waiting for these answers because - * they have previously sent a request. They do not know in advance, - * however, when the answer will arrive and how long to wait. The goal of - * a consensus algorithm is then to come up with a strategy in which every - * participant can say who they want to send requests to, what that - * request is, and is then guaranteed an answer. The algorithm will only - * return when all requests by all participants have been answered and the - * answer delivered to the requesters. - * - * The problem is generally posed in terms of *requests* and *answers*. - * In practice, either of these two may be empty messages. For example, - * processes may simply want to send information to others that they know - * these others need; in this case, the "answer" message may be empty - * and its meaning is simply an affirmation that the information was - * received. Similarly, in some cases processes simply need to inform - * others that they want information, but the destination process knows - * what information is being requested (based on where in the program - * the request happens) and can send that information without there be - * any identifying information in the request; in that case, the - * request message may be empty and simply serve to identify the - * requester. (Each message can be queried for its sender.) - * - * As mentioned in the first paragraph, the algorithms we are interested - * in are "dynamic-sparse": - * - Dynamic: By the time the algorithm is called, the other processes do - * not know yet that they have to answer requests. - * - Sparse: Each process only has to communicate with a small subset of - * processes of the MPI communicator. - * - * In order to run the communication algorithms, users of this class have - * to provide a number of pieces of information: - * - An MPI communicator. - * - On each process, a list of ranks of processes to communicate with. - * - Functionality to pack/unpack data to send as either the original - * request or as part of the answer. - * This information is encoded through the `virtual` functions of classes - * derived from ConsensusAlgorithm::Process, and the constructor of the - * current class receives an object of a type derived from that class as - * an argument. + * A base class for algorithms that implement consensus algorithms, + * see the documentation of the surrounding namespace for more + * information. * * This base class only introduces a basic interface to achieve * these goals, while derived classes implement different algorithms @@ -462,6 +468,34 @@ namespace Utilities clean_up_and_end_communication(const MPI_Comm &comm); }; + + /** + * This function implements a concrete algorithm for the + * consensus algorithms problem (see the documentation of the + * surrounding namespace), using only point-to-point + * communications and a single IBarrier. This algorithm is suitable + * for very large process counts because it does not require the + * allocation of arrays with size proportional to the number of processes. + * + * @note This class closely follows @cite hoefler2010scalable, but our + * implementation also deals with payloads. + * + * @tparam T1 The type of the elements of the vector to be sent. + * @tparam T2 The type of the elements of the vector to be received. + */ + template + std::vector + nbx(const std::vector &targets, + const std::function(const unsigned int)> + &create_request, + const std::function(const unsigned int, + const std::vector &)> + &answer_request, + const std::function &)> + & process_answer, + const MPI_Comm &comm); + + /** * This class implements a concrete algorithm for the * ConsensusAlgorithms::Interface base class, using a two step approach. @@ -610,6 +644,45 @@ namespace Utilities + /** + * This function implements a concrete algorithm for the + * consensus algorithms problem (see the documentation of the + * surrounding namespace), using a two step approach. + * In the first step the source ranks are determined and in the second + * step a static sparse data exchange is performed. This algorithm is most + * suitable for relatively small process counts -- say, less than 100. + * + * @note In contrast to NBX, this class splits the same + * task into two distinct steps. In the first step, all processes + * are identified who want to send a request to this process. In the + * second step, the data is exchanged. However, since - in the + * second step - now it is clear how many requests have to be answered, + * i.e. when this process can stop waiting for requests, no IBarrier is + * needed. + * + * @note Under the hood, this function uses + * Utilities::MPI::compute_point_to_point_communication_pattern() + * to determine the source processes, which itself is based on the + * NBX-algorithm from @cite hoefler2010scalable that is implemented + * in the ConsensusAlgorithms::NBX class (a sister class to the + * current one). + * + * @tparam T1 The type of the elements of the vector to be sent. + * @tparam T2 The type of the elements of the vector to be received. + */ + template + std::vector + pex(const std::vector &targets, + const std::function(const unsigned int)> + &create_request, + const std::function(const unsigned int, + const std::vector &)> + &answer_request, + const std::function &)> + & process_answer, + const MPI_Comm &comm); + + /** * A serial fall back for the above classes to allow programming * independently of whether MPI is used or not. @@ -658,6 +731,30 @@ namespace Utilities + /** + * This function implements a concrete algorithm for the + * consensus algorithms problem (see the documentation of the + * surrounding namespace), as a fall-back option for the case + * where the communicator provided has only one rank (or when + * MPI is simply not used at all). + * + * @tparam T1 The type of the elements of the vector to be sent. + * @tparam T2 The type of the elements of the vector to be received. + */ + template + std::vector + pex(const std::vector &targets, + const std::function(const unsigned int)> + &create_request, + const std::function(const unsigned int, + const std::vector &)> + &answer_request, + const std::function &)> + & process_answer, + const MPI_Comm &comm); + + + /** * A class which delegates its task to other * ConsensusAlgorithms::Interface implementations depending on the number @@ -725,6 +822,35 @@ namespace Utilities + /** + * This function implements a concrete algorithm for the + * consensus algorithms problem (see the documentation of the + * surrounding namespace). In particular, it delegates its work + * to one of the other functions in this namespace depending on the number + * of processes in the MPI communicator. For a small number of processes + * it uses pex() and for a large number of processes nbx(). The threshold + * depends if the program is compiled in debug or release mode, but the + * goal is to always use the most efficient algorithm for however many + * processes participate in the communication. + * + * @tparam T1 The type of the elements of the vector to be sent. + * @tparam T2 The type of the elements of the vector to be received. + */ + template + std::vector + selector( + const std::vector &targets, + const std::function(const unsigned int)> + &create_request, + const std::function(const unsigned int, + const std::vector &)> + &answer_request, + const std::function &)> + & process_answer, + const MPI_Comm &comm); + + + /** * This class implements Utilities::MPI::ConsensusAlgorithms::Process, * using user-provided function wrappers. @@ -797,6 +923,81 @@ namespace Utilities }; +#ifndef DOXYGEN + // Implementation of the functions in this namespace. + + template + std::vector + nbx(const std::vector &targets, + const std::function(const unsigned int)> + &create_request, + const std::function(const unsigned int, + const std::vector &)> + &answer_request, + const std::function &)> + & process_answer, + const MPI_Comm &comm) + { + return NBX().run( + targets, create_request, answer_request, process_answer, comm); + } + + + + template + std::vector + pex(const std::vector &targets, + const std::function(const unsigned int)> + &create_request, + const std::function(const unsigned int, + const std::vector &)> + &answer_request, + const std::function &)> + & process_answer, + const MPI_Comm &comm) + { + return PEX().run( + targets, create_request, answer_request, process_answer, comm); + } + + + + template + std::vector + serial(const std::vector &targets, + const std::function(const unsigned int)> + &create_request, + const std::function(const unsigned int, + const std::vector &)> + & answer_request, + const std::function &)> &process_answer, + const MPI_Comm & comm) + { + return Serial().run( + targets, create_request, answer_request, process_answer, comm); + } + + + + template + std::vector + selector( + const std::vector &targets, + const std::function(const unsigned int)> + &create_request, + const std::function(const unsigned int, + const std::vector &)> + &answer_request, + const std::function &)> + & process_answer, + const MPI_Comm &comm) + { + return Selector().run( + targets, create_request, answer_request, process_answer, comm); + } + + template AnonymousProcess::AnonymousProcess( @@ -859,6 +1060,7 @@ namespace Utilities function_read_answer(other_rank, recv_buffer); } +#endif } // namespace ConsensusAlgorithms diff --git a/include/deal.II/fe/fe_tools_extrapolate.templates.h b/include/deal.II/fe/fe_tools_extrapolate.templates.h index 8eb09d6928..3168ffe92e 100644 --- a/include/deal.II/fe/fe_tools_extrapolate.templates.h +++ b/include/deal.II/fe/fe_tools_extrapolate.templates.h @@ -1177,12 +1177,11 @@ namespace FETools Assert(answer.size() == 0, ExcInternalError()); }; - Utilities::MPI::ConsensusAlgorithms::Selector().run( - destinations, - create_request, - answer_request, - read_answer, - communicator); + Utilities::MPI::ConsensusAlgorithms::selector(destinations, + create_request, + answer_request, + read_answer, + communicator); } diff --git a/source/base/mpi.cc b/source/base/mpi.cc index 481102eeb6..3e5fb1a930 100644 --- a/source/base/mpi.cc +++ b/source/base/mpi.cc @@ -432,7 +432,7 @@ namespace Utilities if (Utilities::MPI::min((my_destinations_are_unique ? 1 : 0), mpi_comm) == 1) { - return ConsensusAlgorithms::NBX().run( + return ConsensusAlgorithms::nbx( destinations, {}, {}, {}, mpi_comm); } // If that was not the case, we need to use the remainder of the code @@ -588,8 +588,8 @@ namespace Utilities if (Utilities::MPI::min((my_destinations_are_unique ? 1 : 0), mpi_comm) == 1) { - return ConsensusAlgorithms::NBX() - .run(destinations, {}, {}, {}, mpi_comm) + return ConsensusAlgorithms::nbx( + destinations, {}, {}, {}, mpi_comm) .size(); } else diff --git a/source/grid/grid_tools.cc b/source/grid/grid_tools.cc index 9cd09727d8..272e1133ac 100644 --- a/source/grid/grid_tools.cc +++ b/source/grid/grid_tools.cc @@ -6019,7 +6019,7 @@ namespace GridTools } }; - Utilities::MPI::ConsensusAlgorithms::Selector().run( + Utilities::MPI::ConsensusAlgorithms::selector( potential_owners_ranks, create_request, answer_request, diff --git a/source/grid/tria_description.cc b/source/grid/tria_description.cc index c65388a3ae..f7ce80c9f4 100644 --- a/source/grid/tria_description.cc +++ b/source/grid/tria_description.cc @@ -143,12 +143,12 @@ namespace TriangulationDescription }; - dealii::Utilities::MPI::ConsensusAlgorithms::Selector() - .run(relevant_processes, - create_request, - answer_request, - process_answer, - comm); + dealii::Utilities::MPI::ConsensusAlgorithms::selector( + relevant_processes, + create_request, + answer_request, + process_answer, + comm); } /**