fortran& openmp:将多个do-s和section-s放在同一并行环境中 [英] fortran & openmp: put multiple do-s and section-s in the same parallel enviroment

问题描述

我有一些像这样的序列号:

I have some serial codes like this:

do i=1,N  
    ...  
end do

do j=1,M  
    ...  
end do

...(1)

...(2)

上面显示了三个带有两个dos和两个独立块的串行代码块.我想将其改编成并行代码.我知道这样做的一种方式是:

Above showed three blocks of serial codes with two do-s and two independent blocks. and I want to adapt it into parallel codes. One way I know of doing is:

!$omp parallel do   
do i ...  
!$omp end parallel  

!$omp parallel do  
do j ...  
!$omp end parallel  

!$omp parallel  
!$omp section ...(1)  
!$omp section ...(2)
!$omp end parallel

请注意，通过这种方式，我正在线程执行四次.作为非专家，我不确定这是否会导致额外的管理时间.是否有可能将所有内容都放在一个并行环境中，并会改善间接费用时间?

Notice that in doing this way, I am threading four times. As a non-expert, I am not sure if this will cause extra overhead time. Would it be possible to put everything in one parallel environment and would it improve the overhead time?

谢谢！

推荐答案

您可以并且可能应该通过放置所有并行构造来分摊创建和销毁OpenMP并行区域的运行时开销-在这种情况下，两个do循环和一对部分-在一个平行区域内.以下代码按预期编译并执行.

You can and probably should amortize the runtime overhead of creating and destroying an OpenMP parallel region by putting all of your parallel constructions - in this case, two do loops and a pair of sections - within a single parallel region. The following code compiles and executes as expected.

program main
implicit none
integer i
!$omp parallel
!$omp do   
do i = 1, 1000
    print*,'loop 1 ',i
enddo
!$omp do   
do i = 1, 1000
    print*,'loop 2 ',i
enddo
!$omp sections
!$omp section
print*,'section 1'
!$omp section
print*,'section 2'
!$omp end sections
!$omp end parallel
return
end program main

请注意，在此程序中，OpenMP线程池是在该行中构造的

Note that in this program, the OpenMP thread pool is constructed in the line

!omp parallel

并且直到被破坏

!omp end parallel

您可以添加"nowait"来减少与OpenMP并行构造结束时隐式屏障相关的运行时开销，但前提是要保留代码的正确性.

You can add "nowait" to reduce the runtime overhead associated with implicit barriers at the end of OpenMP parallel constructs, but only if it preserves the correctness of the code.

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