开始为OpenMP的每一个内循环线程 [英] Starting a thread for each inner loop in OpenMP

问题描述

我是相当新的OpenMP和我试图启动一个单独的线程在一个二维数组来处理每一个项目。

I'm fairly new to OpenMP and I'm trying to start an individual thread to process each item in a 2D array.

所以基本上，这样的：

for (i = 0; i < dimension; i++) {
    for (int j = 0; j < dimension; j++) {
        a[i][j] = b[i][j] + c[i][j];

我在做什么是这样的：

What I'm doing is this:

#pragma omp parallel for shared(a,b,c) private(i,j) reduction(+:diff) schedule(dynamic)
    for (i = 0; i < dimension; i++) {
        for (int j = 0; j < dimension; j++) {
            a[i][j] = b[i][j] + c[i][j];

这是否实际上开始对每个2D项目或没有主题？我将如何测试？如果是错的，什么是做了正确的方法是什么？谢谢！

Does this in fact start a thread for each 2D item or no? How would I test that? If it is wrong, what is the correct way to do it? Thanks!

注：code已被大大简化了。

Note: The code has been greatly simplified

推荐答案

只有外环您code样品平行。您可以通过打印 omp_get_thread_num（）在内部循环测试，你会看到，对于一个给定的 I 中，螺纹num是相同的（当然，这个测试是示范，而不是明确的，因为不同的运行会得出不同的结果）。例如，对于

Only the outer loop is parallel in your code sample. You can test by printing omp_get_thread_num() in the inner loop and you will see that, for a given i, the thread num is the same (of course, this test is demonstrative rather than definitive since different runs will give different results). For example, with:

#include <stdio.h>
#include <omp.h>
#define dimension 4

int main() {
    #pragma omp parallel for
    for (int i = 0; i < dimension; i++)
        for (int j = 0; j < dimension; j++)
            printf("i=%d, j=%d, thread = %d\n", i, j, omp_get_thread_num());
    }

我得到：

i=1, j=0, thread = 1
i=3, j=0, thread = 3
i=2, j=0, thread = 2
i=0, j=0, thread = 0
i=1, j=1, thread = 1
i=3, j=1, thread = 3
i=2, j=1, thread = 2
i=0, j=1, thread = 0
i=1, j=2, thread = 1
i=3, j=2, thread = 3
i=2, j=2, thread = 2
i=0, j=2, thread = 0
i=1, j=3, thread = 1
i=3, j=3, thread = 3
i=2, j=3, thread = 2
i=0, j=3, thread = 0

至于你的code的休息，你可能想要把更多的细节在一个新的问题（这是很难由小样本告诉），但例如，你不能把私人（J）在Ĵ只有可能在以后宣布。它是在我上面的例子自动私有的。我猜差异是，我们不能在样品中看到一个变量。此外，循环变量 I 自动私有的（从版本2.5规格 - 在3.0规范相同）

As for the rest of your code, you might want to put more details in a new question (it's difficult to tell from the small sample), but for example, you can't put private(j) when j is only declared later. It is automatically private in my example above. I guess diff is a variable that we can't see in the sample. Also, the loop variable i is automatically private (from the version 2.5 spec - same in the 3.0 spec)

在循环迭代变量
  for循环的一个为或平行于
  构建在私有
  构建。

The loop iteration variable in the for-loop of a for or parallel for construct is private in that construct.

编辑：以上全部是你和我已经证明了code正确的，但你可能有兴趣在下面。对于OpenMP 3.0版有一个<$（如在 gcc版本4.4 的，但不是4.3版本） C $ C>崩溃 where子句中，你有你可以写code，但与
的#pragma OMP并行的崩溃（2）并行既为循环（见的规范）。

All of the above is correct for the code that you and I have shown, but you may be interested in the following. For OpenMP Version 3.0 (available in e.g. gcc version 4.4, but not version 4.3) there is a collapse clause where you could write the code as you have, but with #pragma omp parallel for collapse (2) to parallelize both for loops (see the spec).

修改：OK，我下载了GCC 4.5.0，并运行上述code，但使用崩溃（2）来获得下面的输出，示出了内环现在并行

Edit: OK, I downloaded gcc 4.5.0 and ran the above code, but using collapse (2) to get the following output, showing the inner loop now parallelized:

i=0, j=0, thread = 0
i=0, j=2, thread = 1
i=1, j=0, thread = 2
i=2, j=0, thread = 4
i=0, j=1, thread = 0
i=1, j=2, thread = 3
i=3, j=0, thread = 6
i=2, j=2, thread = 5
i=3, j=2, thread = 7
i=0, j=3, thread = 1
i=1, j=1, thread = 2
i=2, j=1, thread = 4
i=1, j=3, thread = 3
i=3, j=1, thread = 6
i=2, j=3, thread = 5
i=3, j=3, thread = 7

评论这里 （搜索解决办法）也为相关版本的变通办法2.5如果要并行两个回路，但2.5版规范上面引用是非常明确（见不合格的部分实例的 A.35 ）。

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