如何获得整个程序执行过程中可能创建的最大OpenMP线程数? [英] How can I get the maximum number of OpenMP threads that may be created during the whole execution of the program?

问题描述

我想创建一个对象的全局数组(每个可能的线程由OpenMP生成一个对象)并在整个程序中重用它.每个线程将使用omp_get_thread_num读取其编号，并将其用于索引数组.

I want to create one global array of objects (One object per possible thread spawned by OpenMP) and reuse it throughout the program. Each thread will read its number using omp_get_thread_num and use it to index into the array.

如何获取整个程序执行期间可能创建的最大OpenMP个线程?

How can I get the maximum number of OpenMP threads that may be created during the whole execution of the program?

omp_get_max_threads 的文档说返回一个特定于调用它的特定并行区域的值

The documentation of omp_get_max_threads says that this function is specified to return a value which is specific to the particular parallel region where it was invoked

omp_get_max_threads –并行区域的最大线程数

omp_get_max_threads – Maximum number of threads of parallel region

描述:返回不使用子句num_threads的当前并行区域使用的最大线程数.

Description: Return the maximum number of threads used for the current parallel region that does not use the clause num_threads.

MSDN文档的措辞暗示由omp_get_max_threads在并行区域之外的值与在其他任何点处返回的值相同.

Whereas the wording of MSDN documentation implies that the value returned by omp_get_max_threads outside a parallel region is the same value that would be returned in any other point.

omp_get_max_threads

返回一个整数，该整数等于或大于如果在代码中的那个点定义了没有num_threads的并行区域时可用的线程数.

Returns an integer that is equal to or greater than the number of threads that would be available if a parallel region without num_threads were defined at that point in the code.

其中哪一个是正确的?

推荐答案

没有最大数量.

从技术上讲，OpenMP定义了称为nthreads-var的内部控制变量(请参见 OpenMP 4.5 2.3.3)，它是默认的线程数.您可以使用omp_get_max_threads进行阅读，使用omp_set_num_threads进行设置(不幸的命名错误)，然后使用显式的num_threads子句覆盖它.

Technically OpenMP defines an internal control variable called nthreads-var (see OpenMP 4.5 2.3.3) that is sort of the default number of threads. You read it with omp_get_max_threads, you set it with omp_set_num_threads (an unfortunate naming glitch), and you override it with an explicit num_threads clause.

因此，您将必须编写代码，使其能够处理意外数量的线程，例如通过将数组预定义为omp_get_num_threads()并在有更多线程到达时延迟调整大小.或者进行合理的猜测并检查每次访问的索引范围.

So you will have to write your code such that it can cope with an unexpectedly number of threads, e.g. by predefining the array up to omp_get_num_threads() and lazily resizing it if more threads arrive. Or take a reasonable guess and check the index bounds on each access.

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