[英]Why is my parallelized for loop giving a different output?
我這樣聲明我的線程:
for (thread_num = 0; thread_num < NUM_THREADS; thread_num++) //for each thread do
pthread_create(&thread_handles[thread_num], NULL, gemver_default, (void*)thread_num); //create and run the thread. The thread will run the gemver_default. The thread_num will be passed as input to the gemver_default().
for (thread_num = 0; thread_num < NUM_THREADS; thread_num++) //for each thread do
pthread_join(thread_handles[thread_num], NULL); //wait for the thread to finish
然后我的 pthread 循環:
unsigned short int gemver_default(void * thread_num) {
long int my_thread_num = (long int)thread_num; //store the input of the function to my_thread_num
int local = P / NUM_THREADS; //the number of array elements that each thread must compute their sqrt
int starting_element = my_thread_num * local; //first array element to be computed by this thread
int ending_element = starting_element + local - 1; //last array element to be computed by this thread
for (i = starting_element; i < ending_element; i++)
for (j = 0; j < local; j++)
A2[i][j] += u1[i] * v1[j] + u2[i] * v2[j];
}
然后我原來的循環:
unsigned short int gemver_default() {
//this is the loop to parallelize
for (int i = 0; i < P; i++)
for (int j = 0; j < P; j++)
A2[i][j] += u1[i] * v1[j] + u2[i] * v2[j];
return 0;
}
我不明白為什么輸出不同?
我已經創建了線程,引用了我想要處理的 function,並將其實現到我的舊循環中。
我目前在您的代碼中看到兩個小問題:
1.您正在設置ending_element = starting_element + local - 1
,但在循環中條件是i < ending_element
。
您應該將其更改為ending_element = starting_element + local
,或將循環中的 ondition 更改為i <= ending_element
。
2. 如果 P 可以被NUM_THREADS
整除而沒有余數,使用P / NUM_THREADS
可以正常工作,但如果不是,那么您的線程將不會覆蓋從 0 到 P 的所有索引。例如,如果P = 14
和NUM_THREADS = 5
,那么P / NUM_THREADS = 2
,您的線程將只處理索引 0 到 9,忽略索引 10 到 13。
解決這個問題:可以設置local = P / NUM_THREADS + 1
,將循環中的條件從i < ending_element
為(i < ending_element) && (i < P)
。
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