OpenMP編程學習筆記五

critical 使用：

如果程式碼片段只需要一個thread執行，可以使用single標明。如果程式碼片段需要ID為0的thread執行，使用master標明。

那麼，如果程式碼片段需要各個CPU互斥執行，也就是要求每個CPU都執行一次，但任何時候只用一個CPU在執行。這種

情況可以使用critical。從宏觀上看，該程式碼片段被依次在各個CPU上被執行，各個CPU在執行該代碼的時序上沒有重疊。

測試代碼：

void testCritical()
{
    omp_set_num_threads( 4 );
    clock_t t1 = clock();
#pragma omp parallel
    {
       
        printf("test OpenMP/n");
#pragma omp critical
        {
            clock_t t11 = clock();
            printf("test OpenMP critical/n");
            printf("execute thread ID is %d/n", omp_get_thread_num());
                for (int i = 0; i < 100000000; i++)
             {
                 int a = i+1;
             }

            clock_t t22 = clock();
            printf("thread time = %d/n", t22 - t11);
        }
    }
    clock_t t2 = clock();
    printf("total time = %d/n", t2 - t1);
}

運行結果：

test OpenMP
test OpenMP
test OpenMP
test OpenMP
test OpenMP critical
execute thread ID is 0
thread time = 291
test OpenMP critical
execute thread ID is 2
thread time = 290
test OpenMP critical
execute thread ID is 1
thread time = 294
test OpenMP critical
execute thread ID is 3
thread time = 286
total time = 1164

可以看出，整個啟動並執行時間為1164ms,四個thread不是同步執行critical程式碼片段，而是依次執行的。

為了對比，注釋掉critical,

void testCritical()
{
    omp_set_num_threads( 4 );
    clock_t t1 = clock();
#pragma omp parallel
    {
       
        printf("test OpenMP/n");
//#pragma omp critical
        {
            clock_t t11 = clock();
            printf("test OpenMP critical/n");
            printf("execute thread ID is %d/n", omp_get_thread_num());
                for (int i = 0; i < 100000000; i++)
             {
                 int a = i+1;
             }

            clock_t t22 = clock();
            printf("thread time = %d/n", t22 - t11);
        }
    }
    clock_t t2 = clock();
    printf("total time = %d/n", t2 - t1);
}

再運行，結果為：

test OpenMP
test OpenMP critical
execute thread ID is 0
test OpenMP
test OpenMP critical
execute thread ID is 2
test OpenMP
test OpenMP critical
execute thread ID is 3
test OpenMP
test OpenMP critical
execute thread ID is 1
thread time = 224
thread time = 228
thread time = 237
thread time = 248
total time = 250

此時，total time基本上等於一個thread的運行所耗時間。