Main file - Exercise 5* (or Ex6)

Sheet5/mainEx6.cpp

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+// HOW TO COMPILE ON MAC (paste these lines in terminal):
+// export CPPFLAGS="-I/opt/homebrew/opt/libomp/include"
+// export LDFLAGS="-L/opt/homebrew/opt/libomp/lib"
+// clang++ -std=c++17 -O3 -Xpreprocessor -fopenmp $CPPFLAGS mainEx6.cpp bench_funcs.cpp $LDFLAGS -lomp -o Ex6
+// ./Ex6
+//
+// You can set OMP_NUM_THREADS in the shell to control threads.
+
+#include <iostream>
+#include <vector>
+#include <chrono>
+#include <omp.h>
+#include "bench_funcs.h"
+
+using namespace std;
+using namespace std::chrono;
+
+int main()
+{
+    size_t n = 200000; 
+    size_t maxit = 5000;
+    double omega = 1.0;
+    double tol = 1e-8;
+
+    // show threads
+    #pragma omp parallel
+    {
+        #pragma omp single
+        cout << "Using " << omp_get_num_threads() << " OpenMP threads\n";
+    }
+
+    CSR K_atomic, K_noatom;
+    vector<double> f;
+
+    cout << "\n--- Build with ATOMICS (parallel over elements with atomic adds) ---\n";
+    auto t0 = high_resolution_clock::now();
+    build_fem_system_atomic(n, K_atomic, f);
+    auto t1 = high_resolution_clock::now();
+    cout << "Assembly (atomic) time = " << duration<double>(t1 - t0).count() << " s\n";
+
+    cout << "\nBuild with NO ATOMICS (grouped / per-row accumulation)\n";
+    t0 = high_resolution_clock::now();
+    build_fem_system_no_atomic(n, K_noatom, f);
+    t1 = high_resolution_clock::now();
+    cout << "Assembly (no-atomic) time = " << duration<double>(t1 - t0).count() << " s\n";
+
+    // quick check: compare val arrays (they should be equal)
+    bool same = true;
+    if (K_atomic.val.size() == K_noatom.val.size()) {
+        for (size_t i = 0; i < K_atomic.val.size(); ++i) {
+            if (std::fabs(K_atomic.val[i] - K_noatom.val[i]) > 1e-12) {
+                same = false; break;
+            }
+        }
+    } else same = false;
+
+    cout << "\nMatrix equality check (atomic vs no-atomic): " << (same ? "OK" : "DIFFER") << "\n";
+
+    // Run Jacobi solver on the no-atomic assembled matrix
+    cout << "\nRun Jacobi on NO-ATOMIC matrix\n";
+    vector<double> u;
+    t0 = high_resolution_clock::now();
+    jacobi_csr_parallel(K_noatom, f, u, maxit, omega, tol);
+    t1 = high_resolution_clock::now();
+    cout << "Jacobi time = " << duration<double>(t1 - t0).count() << " s\n";
+
+    cout << "\nDone.\n";
+    return 0;
+}