add: ex_01 domain to solver,

add: MPI examples

Exercise_01/Makefile

@@ -12,9 +12,8 @@ mpi: main.cpp Matrix.h Solver.h
 
 all: seriel mpi
 
-test: seriel mpi
+test: seriel
 	./$(OUT)_seriel 120 10
-	mpirun -n 4 ./$(OUT)_mpi 120 10
 
 clean:
 	rm -f *.o main $(OUT)_seriel $(OUT)_mpi

Exercise_01/Matrix.h

@@ -2,6 +2,7 @@
 
 #include <stddef.h>
 #include <ostream>
+#include <utility>
 #include <vector>
 #define _USE_MATH_DEFINES   /* enables math constants from cmath */
 #include <cmath>
@@ -17,14 +18,28 @@ enum Norm { Frob, Max };
 template <typename T>
 class Matrix {
 public:
-    Matrix(size_t nrow, size_t ncol) : _nrow{nrow}, _ncol{ncol} {
+    // Default Constructor
-        _data.reserve(nrow * ncol);
+    Matrix() = default;
-    };
+    // Creation Constructor (0 Matrix)
-    Matrix(size_t nrow, size_t ncol, T elem) : Matrix(nrow, ncol) {
+    Matrix(size_t nrow, size_t ncol) :
-        for (size_t i = 0; i < nrow * ncol; ++i) {
+        _nrow{nrow},
-            _data[i] = elem;
+        _ncol{ncol},
-        }
+        _data(nrow * ncol) { };
-    };
+    // Creation Constructor with default Element (for example all elements 1)
+    Matrix(size_t nrow, size_t ncol, const T& elem) :
+        _nrow{nrow},
+        _ncol{ncol},
+        _data(nrow * ncol, elem) { };
+    // Copy Constructor
+    Matrix(const Matrix<T>& A) :
+        _nrow{A._nrow},
+        _ncol{A._ncol},
+        _data(A._data) { };
+    // // Move constructor // TODO:
+    // Matrix(Matrix<T>&& A) :
+    //     _nrow{A._nrow},
+    //     _ncol{A._ncol},
+    //     _data(std::move(A._data)) { };
 
     size_t nrow() const { return _nrow; };
     size_t ncol() const { return _ncol; };

Exercise_01/Solver.h

@@ -16,7 +16,10 @@ struct Stencil {
 
 class Solver {
 public:
-    Solver(const size_t nx, const size_t ny, const double h, const double k,
+    Solver(const size_t nx, const size_t ny,
+        const double xmin, const double xmax,
+        const double ymin, const double ymax,
+        const double h, const double k,
         std::function<double(double, double)> fun,
         std::function<double(double)> gN,
         std::function<double(double)> gE,
@@ -24,8 +27,9 @@ public:
         std::function<double(double)> gW
     ) :
         _iter{0},
-        _nx{nx},
+        _nx{nx}, _ny{ny},
-        _ny{ny},
+        _xmin{xmin}, _xmax{xmax},
+        _ymin{ymin}, _ymax{ymax},
         _stencil{ 4.0 / (h * h) + 4.0 * k * k,  /* Center */
                  -1.0 / (h * h),                /* Bottom */
                  -1.0 / (h * h),                /* Left */
@@ -58,12 +62,14 @@ public:
         for (size_t y = 0; y < ny; ++y) { _sol(0, y) = _tmp(0, y) = gW(Y(y)); }
     };
 
-    /* Maps x/y index to x/y position in the [0, 1] x [0, 1] domain */
+    /* Maps x/y index to x/y position in the [xmin, xmax] x [ymin, ymax] domain */
     double X(size_t x) const {
-        return static_cast<double>(x) / static_cast<double>(_nx - 1);
+        double ratio = static_cast<double>(x) / static_cast<double>(_nx - 1);
+        return _xmin + ratio * (_xmax - _xmin);
     }
     double Y(size_t y) const {
-        return static_cast<double>(y) / static_cast<double>(_ny - 1);
+        double ratio = static_cast<double>(y) / static_cast<double>(_ny - 1);
+        return _ymin + ratio * (_ymax - _ymin);
     }
 
     enum class Dir {
@@ -84,6 +90,11 @@ public:
         }
     };
 
+    /** Solution getter */
+    Matrix<double>& solution() {
+        return _sol;
+    }
+
     /**
      * Performs a single Jacobian Iteration
      */
@@ -107,6 +118,10 @@ private:
     size_t _iter;           /*< Iteration count */
     const size_t _nx;       /*< Number of X-axis grid points */
     const size_t _ny;       /*< Number of Y-axis grid points */
+    const double _xmin;     /*< Domain X-min (west border pos) */
+    const double _xmax;     /*< Domain X-max (east border pos) */
+    const double _ymin;     /*< Domain Y-min (south border pos) */
+    const double _ymax;     /*< Domain Y-max (north border pos) */
     const Stencil _stencil; /*< Simple, + shaped stencil */
     Matrix<double> _rhs;    /*< Grid evaluated RHS of the PDE = f(x, y) */
     Matrix<double> _sol;    /*< Solution after _iter iterations */

Exercise_01/examples/MPI_Cart_coords.cpp

@@ -0,0 +1,91 @@
+
+#include <iostream>
+#include <initializer_list>
+#include <mpi.h>
+
+/* Quick and dirty primality test */
+bool is_prime(int num) {
+    if (num == 2 || num == 3) { return true; }
+    if ((num % 2) == 0 || (num % 3) == 0) { return false; }
+
+    for (int div = 5; div * div <= num; div += 6) {
+        if ((num % div) == 0 || ((num + 2) % div) == 0) { return false; }
+    }
+
+    return true;
+}
+
+int main(int argn, char* argv[]) {
+
+    MPI_Init(nullptr, nullptr);
+
+    int mpi_size;
+    MPI_Comm_size(MPI_COMM_WORLD, &mpi_size);
+
+    // Check if a 2D grid is (more or less) possible
+    if ((mpi_size > 2) && is_prime(mpi_size)) {
+        // Print warning only once (by rank 0)
+        int mpi_rank;
+        MPI_Comm_rank(MPI_COMM_WORLD, &mpi_rank);
+        if (mpi_rank == 0) {
+            std::cout << "Warning: 2D grid degenerates to 1D." << std::endl;
+        }
+    }
+
+    // Group processes into a cartesian communication topology
+    int mpi_dims[2] = {0, 0};
+    MPI_Dims_create(mpi_size, 2, mpi_dims);
+
+    // Setup a cartesian topology communicator (NON-cyclic)
+    const int mpi_periods[2] = {false, false};
+    MPI_Comm mpi_comm_grid;
+    MPI_Cart_create(
+        MPI_COMM_WORLD,     // Old Communicator
+        2,                  // number of dimensions
+        mpi_dims,           // grid dimensions
+        mpi_periods,        // grid periodicity
+        true,               // allow process reordering
+        &mpi_comm_grid
+    );
+
+    // Get rank with respect to the grid communicator
+    int mpi_rank;
+    MPI_Comm_rank(mpi_comm_grid, &mpi_rank);
+
+    // Get coordinates with respect to the grid communicator
+    int mpi_coords[2];
+    MPI_Cart_coords(mpi_comm_grid, mpi_rank, 2, mpi_coords);
+
+    // Get direct neightbours in the communication grid
+    struct { int north; int east; int south; int west; } mpi_neighbours;
+    // Get X-direction (dim 0) neightbours
+    MPI_Cart_shift(
+        mpi_comm_grid,              // grid communicator
+        0,                          // axis index (0 <-> X)
+        1,                          // offset
+        &(mpi_neighbours.west),     // negated offset neightbour
+        &(mpi_neighbours.east)      // offset neightbour
+    );
+    // Get Y-direction (dim 1) neightbours
+    MPI_Cart_shift(
+        mpi_comm_grid,
+        1,                          // axis index (1 <-> X)
+        1,
+        &(mpi_neighbours.south),
+        &(mpi_neighbours.north)
+    );
+
+    // Print grid communicator location and with direct neighbors (offset +-1)
+    std::cout << "Rank " << mpi_rank << ": Coords ("
+        << mpi_coords[0] << ", "
+        << mpi_coords[1] << ") - Neighbours: ";
+    if (mpi_neighbours.west  != MPI_PROC_NULL) { std::cout << mpi_neighbours.west  << ' '; }
+    if (mpi_neighbours.east  != MPI_PROC_NULL) { std::cout << mpi_neighbours.east  << ' '; }
+    if (mpi_neighbours.south != MPI_PROC_NULL) { std::cout << mpi_neighbours.south << ' '; }
+    if (mpi_neighbours.north != MPI_PROC_NULL) { std::cout << mpi_neighbours.north << ' '; }
+    std::cout << std::endl;
+
+    MPI_Finalize();
+
+    return 0;
+}

Exercise_01/examples/MPI_RowSums.cpp

@@ -0,0 +1,288 @@
+/**
+ * Computes row sums of a matrix containing (0-indexed) consecutive numbers
+ * in row major order (yes, useless but illustrativ)
+ *
+ * Example:
+ *     matrix (11 x 7):                          row_sums (11):
+ *        0   11   22   33   44   55   66   ->      231
+ *        1   12   23   34   45   56   67   ->      238
+ *        2   13   24   35   46   57   68   ->      245
+ *        3   14   25   36   47   58   69   ->      252
+ *        4   15   26   37   48   59   70   ->      259
+ *        5   16   27   38   49   60   71   ->      266
+ *        6   17   28   39   50   61   72   ->      273
+ *        7   18   29   40   51   62   73   ->      280
+ *        8   19   30   41   52   63   74   ->      287
+ *        9   20   31   42   53   64   75   ->      294
+ *       10   21   32   43   54   65   76   ->      301
+ *
+ * Each worker process computes one row sum at a time. Therefore, the scheduler
+ * process (rank 0) sends one row to the worker (rank > 0) using a
+ * MPI_Type_vector representing a sparse row layout which is reseaved by the
+ * worker in a dense layout (MPI_DOUBLE)
+ *
+ * Send/Recv Example:
+ *     Data send (entries are the array indices):
+ *        -    -    -    -    -    -    -
+ *        -    -    -    -    -    -    -
+ *        -    -    -    -    -    -    -
+ *        -    -    -    -    -    -    -
+ *        -    -    -    -    -    -    -
+ *        5   16   27   38   49   60   71
+ *        -    -    -    -    -    -    -
+ *        -    -    -    -    -    -    -
+ *        -    -    -    -    -    -    -
+ *        -    -    -    -    -    -    -
+ *        -    -    -    -    -    -    -
+ *
+ *     Receved (indices 0, 1, 2, 3, 4, 5, 6):
+ *      5   16   27   38   49   60   71
+ *
+ * Usage:
+ *      mpirun -n <nproc> MPI_RowSums [<nrow> [<ncol>]]
+ *
+ *      <nproc> must be at least 2 
+ *      <nrow> number of rows, between 1 and 1024 defaults to 11
+ *      <ncol> number of cols, between 1 and 1024 defaults to 7
+ *
+ *      on parse error; <nrow>, <ncol> are set to there defaults.
+ *
+ * Compile:
+ *      mpic++ -Wall -Wpedantic -pedantic MPI_RowSums.cpp -o MPI_RowSums
+ *
+ * Interesting Parameters:
+ *      # Single Worker Process
+ *      mpirun -n 2 MPI_RowSums 1 10
+ *      mpirun -n 2 MPI_RowSums 20 10
+ *      # Less Rows than workers (some processes don't get any work at all)
+ *      mpirun -n 4 MPI_RowSums 2 10
+ *      # Classic Example (bunch of work and some workers)
+ *      mpirun -n 4 MPI_RowSums 100 42
+ */
+
+#include <iostream>
+#include <vector>
+#include <mpi.h>
+
+int min(int a, int b) { return a < b ? a : b; }
+
+int main(int argn, char* argv[]) {
+
+    // Build a (simple and barebones, row major) matrix model, it's just a
+    // vector with external row/col count.
+    int nrow = 11;  // defaults
+    int ncol = 7;   // defaults
+
+    // Parse arguments to set nrow, ncol (sloppy, but thats not the point of
+    // this example)
+    if (argn > 1) {
+        nrow = atoi(argv[1]);
+        if (nrow < 1 || nrow > 1024) {
+            nrow = 11;
+        }
+    }
+    if (argn > 2) {
+        ncol = atoi(argv[2]);
+        if (ncol < 1 || ncol > 1024) {
+            ncol = 7;
+        }
+    }
+
+    // Initialize MPI (always required)
+    MPI_Init(nullptr, nullptr);
+
+    // Allocate MPI Settings
+    int mpi_size;       /*< Number of processes */
+    int mpi_rank;       /*< This process rank (a.k.a. the MPI process ID) */
+    // Set/Get MPI Settings
+    MPI_Comm_size(MPI_COMM_WORLD, &mpi_size);
+    MPI_Comm_rank(MPI_COMM_WORLD, &mpi_rank);
+
+    // Check if there is at least a single worker, otherwise abort right away
+    if (mpi_size < 2) {
+        std::cerr
+            << "Nr. processes must be at least 2! (No workers -> No work done)"
+            << std::endl;
+        MPI_Finalize();
+        return -1;
+    }
+
+    // Setup shutdown tag (tags must be non-negative)
+    int shutdown = nrow + 1;    // unreachable row index
+
+    // Sparce row data type, elements are strided in the matrix
+    MPI_Datatype mpi_type_row;
+    MPI_Type_vector(ncol, 1, nrow, MPI_DOUBLE, &mpi_type_row);
+    MPI_Type_commit(&mpi_type_row);
+
+    // Distinguish between workers (rank > 0) and scheduler (rank = 0)
+    if (mpi_rank == 0) {
+        // Create row sums result array
+        std::vector<double> row_sums(nrow);
+
+        // Construct a nrow x ncol matrix (and enumerate elems)
+        std::vector<double> matrix(nrow * ncol);
+        for (size_t i = 0; i < matrix.size(); ++i) {
+            matrix[i] = static_cast<double>(i);
+        }
+
+        // tracks processed rows
+        int row_counter = 0;
+
+        // Start by sending to all workers some data
+        for (int rank = 1; rank < min(mpi_size, nrow + 1); ++rank) {
+            // Send rows
+            MPI_Send(
+                matrix.data() + row_counter,    // Pos of first row elem
+                1,                              // Send one row
+                mpi_type_row,                   // row datatype, (sparce layout)
+                rank,                           // target worker process
+                row_counter,                    // tag = row index
+                MPI_COMM_WORLD
+            );
+
+            // Increment processed row count
+            row_counter++;
+        }
+
+        // In case of less work than workers (nrow < mpi_size) send remaining
+        // workers home (all ranks with mpi_rank >= nrow get shutdown signal)
+        for (int rank = min(mpi_size, nrow + 1); rank < mpi_size; ++rank) {
+            // Empty workload with shutdown tag
+            MPI_Send(
+                nullptr,                        // no data
+                0,                              // no data
+                MPI_CHAR,                       // something
+                rank,                           // ranks without work
+                shutdown,                       // tag
+                MPI_COMM_WORLD
+            );
+        }
+
+        // Repeat till all rows are processed
+        while (row_counter < nrow) {
+            double row_sum;
+
+            // First listen for any worker process to respond (rank finished)
+            MPI_Status mpi_status;
+            MPI_Recv(
+                &row_sum,                       // responding rank result
+                1,                              // row sum is a scalar
+                MPI_DOUBLE,                     // and has type double
+                MPI_ANY_SOURCE,                 // listen for everything
+                MPI_ANY_TAG,                    // unknown who finishes first
+                MPI_COMM_WORLD,
+                &mpi_status
+            );
+
+            // Write result to row sums
+            row_sums[mpi_status.MPI_TAG] = row_sum;
+
+            // Send the next row to process
+            MPI_Send(
+                matrix.data() + row_counter,
+                1,
+                mpi_type_row,
+                mpi_status.MPI_SOURCE,          // responding rank gets new work
+                row_counter,
+                MPI_COMM_WORLD
+            );
+
+            // Increment processed row count
+            row_counter++;
+        }
+
+        // Now collect remaining results and send a "shutdown" message
+        for (int rank = 1; rank < min(mpi_size, nrow + 1); ++rank) {
+            double row_sum;
+
+            // First listen for any rank to respond (a rank finished working)
+            MPI_Status mpi_status;
+            MPI_Recv(
+                &row_sum,
+                1,
+                MPI_DOUBLE,
+                MPI_ANY_SOURCE,
+                MPI_ANY_TAG,
+                MPI_COMM_WORLD,
+                &mpi_status
+            );
+
+            // Write result to row sums
+            row_sums[mpi_status.MPI_TAG] = row_sum;
+
+            // Send rank shutdown message (work done)
+            MPI_Send(
+                nullptr,                        // no data
+                0,                              // no data
+                MPI_CHAR,                       // something
+                mpi_status.MPI_SOURCE,          // responding rank gets shutdown
+                shutdown,                       // tag
+                MPI_COMM_WORLD
+            );
+        }
+
+        // Report final result (row sums)
+        std::cout << "Rank 0: Done.\n\033[1m";
+        for (double& val : row_sums) {
+            std::cout << val << ' ';
+        }
+        std::cout << "\033[0m\nCheck result with the following R code:\n"
+            << "    rowSums(matrix(seq(0, len = " << (nrow * ncol) << "), "
+            << nrow << ", " << ncol << "))" << std::endl;
+    } else {
+        // Dense row representation, NO stride
+        std::vector<double> row(ncol);
+        // Counts the number of processed rows (for analytic purposes)
+        int work_count = 0;
+
+        // Repeate till a shutdown signal is send (shutdown tag)
+        while (true) {
+
+            // Receive new work
+            MPI_Status mpi_status;
+            MPI_Recv(
+                row.data(),         // Raw row vector data
+                ncol,               // nr of row elments
+                MPI_DOUBLE,         // simple double data type (dense layout)
+                0,                  // Listen to main rank (rank 0)
+                MPI_ANY_TAG,        // tag (at this point) unknown
+                MPI_COMM_WORLD,
+                &mpi_status
+            );
+
+            // check shutdown -> go home, all work done
+            if (mpi_status.MPI_TAG == shutdown) {
+                break;
+            }
+
+            // Process new work (compute row sum)
+            double sum = 0;
+            for (double& val : row) {
+                sum += val;
+            }
+
+            // Increment work count (track number of completed jobs)
+            work_count++;
+
+            // Send result back to scheduler
+            MPI_Send(
+                &sum,                           // processing result
+                1,                              // single scalar
+                MPI_DOUBLE,
+                0,                              // target scheduler (rank 0)
+                mpi_status.MPI_TAG,             // row index
+                MPI_COMM_WORLD
+            );
+        }
+
+        // Report shutdown (worker goes home)
+        std::cout << "Rank " << mpi_rank << ": processed "
+            << work_count << " rows done -> shutdown" << std::endl;
+    }
+
+    // Shutdown MPI (always required)
+    MPI_Finalize();
+
+    return 0;
+}

Exercise_01/main.cpp

@@ -62,33 +62,24 @@ int main(int argn, char* argv[]) {
     /** East, South and West boundary conditions are simply = 0 */
     std::function<double(double)> g0 = [k](double) { return 0.0; };
 
-    // Instanciate solver
-    Solver solver(nx, ny, h, k, fun, gN, g0, g0, g0);
-
-    // Set Diriclet boundary conditions (East, South and West to 0)
-    for (auto dir : { Solver::Dir::E, Solver::Dir::S, Solver::Dir::W }) {
-        MatrixView<double> boundary = solver.boundary(dir);
-        for (size_t i = 0; i < boundary.size(); ++i) {
-            boundary(i) = 0.0;
-        }
-    }
-    // The North boundary condition is g(x) = sin(2 pi x) sinh(2 pi)
-    {
-        MatrixView<double> boundary = solver.boundary(Solver::Dir::North);
-        for (size_t i = 0; i < boundary.size(); ++i) {
-            double x = static_cast<double>(i) / static_cast<double>(nx - 1);
-            boundary(i) = sin(M_2_PI * x) * sinh(M_2_PI);
-        }
-    }
-
     /******************************* Solve PDE ********************************/
-    // Run solver iterations
+    Matrix<double> solution;
-    for (size_t iter = 0; iter < iterations; ++iter) {
+    {
-        solver.iterate();
+        // Instanciate solver (local instance)
+        Solver solver(nx, ny, 0., 1., 0., 1., h, k, fun, gN, g0, g0, g0);
+
+        // Run solver iterations
+        for (size_t iter = 0; iter < iterations; ++iter) {
+            solver.iterate();
+        }
+
+        // extract solution
+        solution = std::move(solver.solution());
     }
 
     /****************************** Tests/Report ******************************/
 
+    std::cout << solution << std::endl;
 
     // MPI shutdown/cleanup
 #ifdef USE_MPI