65

OpenMP 之间有什么区别：

#pragma omp parallel sections
{
    #pragma omp section
    {
       fct1();
    }
    #pragma omp section
    {
       fct2();
    }
}

和 ：

#pragma omp parallel 
{
    #pragma omp single
    {
       #pragma omp task
       fct1();
       #pragma omp task
       fct2();
    }
}

我不确定第二个代码是否正确...

4

2 回答 2

158

任务和部分之间的区别在于代码将执行的时间范围。部分包含在sections构造中，并且（除非nowait指定子句）线程不会离开它，直到所有部分都已执行：

                 [    sections     ]
Thread 0: -------< section 1 >---->*------
Thread 1: -------< section 2      >*------
Thread 2: ------------------------>*------
...                                *
Thread N-1: ---------------------->*------

这里N线程遇到一个sections有两个部分的构造，第二个比第一个花费更多的时间。前两个线程各执行一个部分。其他N-2线程只是在 section 构造末尾的隐式屏障处等待（此处显示为*）。

任务尽可能在所谓的任务调度点排队和执行。在某些情况下，可以允许运行时在线程之间移动任务，即使在它们的生命周期中也是如此。这样的任务被称为 untied 并且 untied 任务可能开始在一个线程中执行，然后在某个调度点它可能被运行时迁移到另一个线程。

尽管如此，任务和部分在许多方面都是相似的。例如，以下两个代码片段实现了基本相同的结果：

// sections
...
#pragma omp sections
{
   #pragma omp section
   foo();
   #pragma omp section
   bar();
}
...

// tasks
...
#pragma omp single nowait
{
   #pragma omp task
   foo();
   #pragma omp task
   bar();
}
#pragma omp taskwait
...

taskwait工作方式与任务非常相似barrier- 它确保当前执行流程将暂停，直到所有排队的任务都已执行。它是一个调度点，即它允许线程处理任务。需要该single构造以便仅由一个线程创建任务。如果没有single构造，每个任务都会获得创建num_threads时间，这可能不是人们想要的。构造中的nowait子句single指示其他线程不要等到single构造被执行（即在single构造结束时移除隐式屏障）。所以他们taskwait立即点击并开始处理任务。

taskwait为了清楚起见，这里显示了一个明确的调度点。还有隐式调度点，尤其是在屏障同步内部，无论是显式的还是隐式的。因此，上面的代码也可以简单地写成：

// tasks
...
#pragma omp single
{
   #pragma omp task
   foo();
   #pragma omp task
   bar();
}
...

如果存在三个线程，以下是一种可能发生的情况：

               +--+-->[ task queue ]--+
               |  |                   |
               |  |       +-----------+
               |  |       |
Thread 0: --< single >-|  v  |-----
Thread 1: -------->|< foo() >|-----
Thread 2: -------->|< bar() >|-----

此处显示的| ... |是调度点的操作（taskwait指令或隐式屏障）。基本上线程1并2暂停他们当时正在做的事情并开始处理队列中的任务。处理完所有任务后，线程将恢复其正常执行流程。请注意，线程1和可能在线程退出构造2之前到达调度点，因此左s 不需要对齐（如上图所示）。0single|

甚至在其他线程能够请求任务之前，线程也可能1完成处理foo()任务并请求另一个任务。所以两者都foo()可能bar()被同一个线程执行：

               +--+-->[ task queue ]--+
               |  |                   |
               |  |      +------------+
               |  |      |
Thread 0: --< single >-| v             |---
Thread 1: --------->|< foo() >< bar() >|---
Thread 2: --------------------->|      |---

如果线程 2 来得太晚，单出线程也可能执行第二个任务：

               +--+-->[ task queue ]--+
               |  |                   |
               |  |      +------------+
               |  |      |
Thread 0: --< single >-| v < bar() >|---
Thread 1: --------->|< foo() >      |---
Thread 2: ----------------->|       |---

在某些情况下，编译器或 OpenMP 运行时甚至可能完全绕过任务队列并串行执行任务：

Thread 0: --< single: foo(); bar() >*---
Thread 1: ------------------------->*---
Thread 2: ------------------------->*---

如果区域代码中不存在任务调度点，则 OpenMP 运行时可能会在它认为合适的时候启动任务。例如，所有任务都可能被推迟，直到parallel达到区域末端的障碍。

于 2012-12-09T16:08:37.933 回答
0

我不是 OpenMP 方面的专家，但尝试在我的机器上使用task和测试 fib 序列sections

部分

int fib(int n)
{
    int i, j;
    if (n < 2)
        return n;
    else
    {
#pragma omp parallel sections       
{
#pragma omp section             
{
                i = fib(n - 1);
            }
#pragma omp section             
{
                j = fib(n - 2);
            }
        }
        printf("Current int %d is on thread %d \n", i + j, omp_get_thread_num());
        return i + j;
    }
}

int main()
{
    int n = 10;

#pragma omp parallel shared(n)  {
#pragma omp single      {
            printf("%d\n", omp_get_num_threads());
            printf("fib(%d) = %d\n", n, fib(n));
        }
    }
}

任务

#include <stdio.h>
#include <omp.h>
int fib(int n)
{
  int i, j;
  if (n<2)
    return n;
  else
    {
       #pragma omp task shared(i) firstprivate(n)
       i=fib(n-1);

       #pragma omp task shared(j) firstprivate(n)
       j=fib(n-2);

       #pragma omp taskwait
    printf("Current int %d is on thread %d \n", i + j, omp_get_thread_num());
       return i+j;
    }
}

int main()
{
  int n = 10;

  #pragma omp parallel shared(n)
  {
    #pragma omp single
    {
    printf("%d\n", omp_get_num_threads());
        printf ("fib(%d) = %d\n", n, fib(n));
    }
  }
}

部分结果：

12
Current int 1 is on thread 0
Current int 2 is on thread 0
Current int 1 is on thread 0
Current int 3 is on thread 0
Current int 1 is on thread 0
Current int 2 is on thread 0
Current int 5 is on thread 0
Current int 1 is on thread 0
Current int 2 is on thread 0
Current int 1 is on thread 0
Current int 3 is on thread 0
Current int 8 is on thread 0
Current int 1 is on thread 0
Current int 2 is on thread 0
Current int 1 is on thread 0
Current int 3 is on thread 0
Current int 1 is on thread 0
Current int 2 is on thread 0
Current int 5 is on thread 0
Current int 13 is on thread 0
Current int 1 is on thread 0
Current int 2 is on thread 0
Current int 1 is on thread 0
Current int 3 is on thread 0
Current int 1 is on thread 0
Current int 2 is on thread 0
Current int 5 is on thread 0
Current int 1 is on thread 0
Current int 2 is on thread 0
Current int 1 is on thread 0
Current int 3 is on thread 0
Current int 8 is on thread 0
Current int 21 is on thread 0
Current int 1 is on thread 0
Current int 2 is on thread 0
Current int 1 is on thread 0
Current int 3 is on thread 0
Current int 1 is on thread 0
Current int 2 is on thread 0
Current int 5 is on thread 0
Current int 1 is on thread 0
Current int 2 is on thread 0
Current int 1 is on thread 0
Current int 3 is on thread 0
Current int 8 is on thread 0
Current int 1 is on thread 0
Current int 2 is on thread 0
Current int 1 is on thread 0
Current int 3 is on thread 0
Current int 1 is on thread 0
Current int 2 is on thread 0
Current int 5 is on thread 0
Current int 13 is on thread 0
Current int 34 is on thread 0
Current int 1 is on thread 0
Current int 2 is on thread 0
Current int 1 is on thread 0
Current int 3 is on thread 0
Current int 1 is on thread 0
Current int 2 is on thread 0
Current int 5 is on thread 0
Current int 1 is on thread 0
Current int 2 is on thread 0
Current int 1 is on thread 0
Current int 3 is on thread 0
Current int 8 is on thread 0
Current int 1 is on thread 0
Current int 2 is on thread 0
Current int 1 is on thread 0
Current int 3 is on thread 0
Current int 1 is on thread 0
Current int 2 is on thread 0
Current int 5 is on thread 0
Current int 13 is on thread 0
Current int 1 is on thread 0
Current int 2 is on thread 0
Current int 1 is on thread 0
Current int 3 is on thread 0
Current int 1 is on thread 0
Current int 2 is on thread 0
Current int 5 is on thread 0
Current int 1 is on thread 0
Current int 2 is on thread 0
Current int 1 is on thread 0
Current int 3 is on thread 0
Current int 8 is on thread 0
Current int 21 is on thread 0
Current int 55 is on thread 4
fib(10) = 55

任务结果：

12
Current int 1 is on thread 3
Current int 2 is on thread 3
Current int 1 is on thread 8
Current int 2 is on thread 8
Current int 1 is on thread 8
Current int 1 is on thread 4
Current int 1 is on thread 11
Current int 1 is on thread 11
Current int 2 is on thread 11
Current int 3 is on thread 11
Current int 1 is on thread 11
Current int 2 is on thread 11
Current int 1 is on thread 11
Current int 1 is on thread 11
Current int 2 is on thread 11
Current int 3 is on thread 11
Current int 1 is on thread 11
Current int 2 is on thread 11
Current int 1 is on thread 11
Current int 1 is on thread 11
Current int 2 is on thread 11
Current int 3 is on thread 11
Current int 5 is on thread 11
Current int 8 is on thread 11
Current int 1 is on thread 8
Current int 2 is on thread 8
Current int 3 is on thread 8
Current int 5 is on thread 8
Current int 13 is on thread 8
Current int 1 is on thread 7
Current int 2 is on thread 7
Current int 1 is on thread 7
Current int 1 is on thread 7
Current int 1 is on thread 0
Current int 1 is on thread 0
Current int 2 is on thread 0
Current int 3 is on thread 0
Current int 1 is on thread 1
Current int 1 is on thread 6
Current int 2 is on thread 6
Current int 1 is on thread 9
Current int 2 is on thread 9
Current int 1 is on thread 2
Current int 2 is on thread 7
Current int 3 is on thread 7
Current int 5 is on thread 7
Current int 2 is on thread 5
Current int 5 is on thread 5
Current int 1 is on thread 5
Current int 2 is on thread 5
Current int 1 is on thread 5
Current int 1 is on thread 5
Current int 2 is on thread 5
Current int 3 is on thread 5
Current int 1 is on thread 5
Current int 2 is on thread 5
Current int 1 is on thread 5
Current int 1 is on thread 5
Current int 2 is on thread 5
Current int 3 is on thread 5
Current int 5 is on thread 5
Current int 1 is on thread 5
Current int 2 is on thread 5
Current int 1 is on thread 11
Current int 2 is on thread 11
Current int 1 is on thread 8
Current int 2 is on thread 8
Current int 5 is on thread 8
Current int 3 is on thread 1
Current int 8 is on thread 1
Current int 21 is on thread 1
Current int 1 is on thread 10
Current int 3 is on thread 10
Current int 8 is on thread 0
Current int 1 is on thread 4
Current int 3 is on thread 4
Current int 1 is on thread 9
Current int 3 is on thread 9
Current int 8 is on thread 9
Current int 3 is on thread 2
Current int 5 is on thread 3
Current int 13 is on thread 3
Current int 5 is on thread 6
Current int 13 is on thread 7
Current int 8 is on thread 10
Current int 21 is on thread 10
Current int 34 is on thread 3
Current int 55 is on thread 1
fib(10) = 55

在分配计算资源时，似乎任务比部分更明智

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于 2020-07-17T17:14:11.640 回答