Add README.md

README.md

+#include <iostream>
+#include <vector>
+#include <thread>
+#include <mutex>
+#include <chrono>
+
+using namespace std;
+const int DIM = 1000;
+const int THREADS = 4;
+
+vector<vector<long>> a(DIM, vector<long>(DIM));
+vector<vector<long>> b(DIM, vector<long>(DIM));
+vector<vector<long>> c(DIM, vector<long>(DIM));
+
+mutex mtx;
+
+void init() {
+    for(int i=0; i<DIM; i++)
+        for(int j=0; j<DIM; j++) {
+            a[i][j] = i+j;
+            b[i][j] = i-j;
+            c[i][j] = 0;
+        }
+}
+
+void seq() {
+    for(int i=0; i<DIM; i++)
+        for(int j=0; j<DIM; j++) {
+            long s=0;
+            for(int k=0; k<DIM; k++)
+                s += a[i][k]*b[k][j];
+            c[i][j]=s;
+        }
+}
+
+void para_nosync(int id) {
+    int start=id*(DIM/THREADS);
+    int end=(id==THREADS-1)?DIM:start+(DIM/THREADS);
+    for(int i=start; i<end; i++)
+        for(int j=0; j<DIM; j++) {
+            long s=0;
+            for(int k=0; k<DIM; k++)
+                s += a[i][k]*b[k][j];
+            c[i][j]=s;
+        }
+}
+
+void para_sync(int id) {
+    int start=id*(DIM/THREADS);
+    int end=(id==THREADS-1)?DIM:start+(DIM/THREADS);
+    for(int i=start; i<end; i++)
+        for(int j=0; j<DIM; j++) {
+            long s=0;
+            for(int k=0; k<DIM; k++)
+                s += a[i][k]*b[k][j];
+            lock_guard<mutex> lock(mtx);
+            c[i][j]=s;
+        }
+}
+
+void reset() {
+    for(auto& row : c)
+        fill(row.begin(), row.end(), 0);
+}
+
+double time_func(void (*func)()) {
+    auto start = chrono::high_resolution_clock::now();
+    func();
+    auto end = chrono::high_resolution_clock::now();
+    return chrono::duration<double>(end-start).count();
+}
+
+int main() {
+    init();
+    
+    reset(); double t1 = time_func(seq); cout << "Seq: " << t1 << "s\n";
+    
+    reset(); double t2 = time_func([](){
+        thread(para_nosync,0).join();
+    }); cout << "1 thread: " << t2 << "s\n";
+    
+    reset(); double t3 = time_func([](){
+        thread t[THREADS];
+        for(int i=0;i<THREADS;i++) t[i]=thread(para_nosync,i);
+        for(int i=0;i<THREADS;i++) t[i].join();
+    }); cout << THREADS << " threads: " << t3 << "s\n";
+    
+    reset(); double t4 = time_func([](){
+        thread t[THREADS];
+        for(int i=0;i<THREADS;i++) t[i]=thread(para_sync,i);
+        for(int i=0;i<THREADS;i++) t[i].join();
+    }); cout << THREADS << " threads sync: " << t4 << "s\n";
+    
+    return 0;
+}
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