我可以使用 OpenMP 通过单个线程释放共享变量吗？
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【中文标题】我可以使用 OpenMP 通过单个线程释放共享变量吗？【英文标题】：Can I deallocate a shared variable by a single thread using OpenMP? 【发布时间】：2021-08-31 03:17:15 【问题描述】：我正在使用OpenMP 来并行化代码。根据我要问的问题，这是代码中最重要的部分：
!$OMP PARALLEL PRIVATE(num_thread) &
!$OMP SHARED(tasklist_GRAD,threads_list,threads_list_all,tasks_ready_master) &
!$OMP SHARED(threads_list_part1,nthreads)
num_thread=OMP_GET_THREAD_NUM() ! le rang du thread
nthreads=OMP_GET_NUM_THREADS() ! le nombre de threads
!Thread Application Master (numero 1)
if (num_thread==1) then
do ff=1,3 ! 3 tâches
if (associated(tasklist_GRAD(ff)%f_ptr) .eqv. .true. ) then ! Si tâche attribuée
tasks_ready_master(ff) = tasklist_GRAD(ff) ! égalité de pointeurs
tasks_ready_master(ff)%state=STATE_READY
end if
end do
end if
!$OMP BARRIER
!Thread Master (numero 0)
if (num_thread==0) then
allocate(threads_list(nthreads-2)) ! liste des threads workers
do ff=1,nthreads-2
threads_list(ff)=ff+1 ! 2,3,..,nombre de threads-2
end do
do ff=1,3,nthreads-2
if (tasks_ready_master(ff)%state==STATE_READY) then
threads_list_all(ff:ff+nthreads-3)=threads_list(:)
end if
end do
threads_list_part1=threads_list_all(1:3) ! 3 tâches
deallocate(threads_list)
end if
!$OMP BARRIER
如您所见，threads_list 是一个共享变量。我的问题很简单。
我是否有权通过 1 个单线程取消分配 shared 变量，或者我应该停用 if (num_thread==0) then 以便让所有线程完成此操作？
我之所以问这个问题，是因为我遇到了与内存泄漏有关的错误。
【问题讨论】：
我的第一个想法是，如果只有一个线程使用，为什么要共享threads_list？为什么不把它设为私有呢？将您的对象保密可以避免许多只有通过共享才能遇到的问题。 @IanBush 感谢您的评论，我将threads_list 和 threads_list_all 从 shared 变量更改为 private 变量。我获得了大约 60 秒。太棒了！
【参考方案1】：
要回答这个问题，因为共享变量只有一个实例化，它只需要被释放一次 - 但哪个线程无关紧要：
ijb@ijb-Latitude-5410:~/work/stack$ cat omp.f90
Program omp_alloc
Use omp_lib, Only : omp_get_num_threads, omp_get_thread_num
Implicit None
Integer, Dimension( : ), Allocatable :: a
Integer :: ith
Integer :: nth
Integer :: i
Allocate( a( 1:4 ) )
a = [ ( i, i = 1, 4 ) ]
!$omp parallel default( none ) shared( a ) private( ith, nth )
nth = omp_get_num_threads()
ith = omp_get_thread_num ()
Write( *, * ) ith, nth, 'a = ', a
! Barrier to make sure a is printed by all threads before deallocation
!$omp barrier
!$omp single
Write( *, * ) 'Thread ', ith, ' Deallocating'
Deallocate( a )
!$omp end single
!$omp end parallel
Write( *, * ) Allocated( a )
End Program omp_alloc
ijb@ijb-Latitude-5410:~/work/stack$ gfortran-11 --version
GNU Fortran (GCC) 11.1.0
Copyright © 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
ijb@ijb-Latitude-5410:~/work/stack$ gfortran-11 -Wall -Wextra -fcheck=all -O -g -std=f2018 -fopenmp omp.f90
ijb@ijb-Latitude-5410:~/work/stack$ export OMP_NUM_THREADS=4
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 4 a = 1 2 3 4
2 4 a = 1 2 3 4
3 4 a = 1 2 3 4
1 4 a = 1 2 3 4
Thread 3 Deallocating
F
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 4 a = 1 2 3 4
1 4 a = 1 2 3 4
3 4 a = 1 2 3 4
2 4 a = 1 2 3 4
Thread 0 Deallocating
F
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 4 a = 1 2 3 4
3 4 a = 1 2 3 4
1 4 a = 1 2 3 4
2 4 a = 1 2 3 4
Thread 0 Deallocating
F
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 4 a = 1 2 3 4
1 4 a = 1 2 3 4
3 4 a = 1 2 3 4
2 4 a = 1 2 3 4
Thread 0 Deallocating
F
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 4 a = 1 2 3 4
3 4 a = 1 2 3 4
2 4 a = 1 2 3 4
1 4 a = 1 2 3 4
Thread 0 Deallocating
F
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 4 a = 1 2 3 4
1 4 a = 1 2 3 4
3 4 a = 1 2 3 4
2 4 a = 1 2 3 4
Thread 0 Deallocating
F
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 4 a = 1 2 3 4
3 4 a = 1 2 3 4
1 4 a = 1 2 3 4
2 4 a = 1 2 3 4
Thread 0 Deallocating
F
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 4 a = 1 2 3 4
3 4 a = 1 2 3 4
1 4 a = 1 2 3 4
2 4 a = 1 2 3 4
Thread 2 Deallocating
F
如果您尝试通过多个线程解除分配，您会得到未定义的行为。在 gfortran 上，至少他通过在您尝试解除分配数组时看起来像是测试是否分配数组的竞争条件来表现出来：
ijb@ijb-Latitude-5410:~/work/stack$ cat omp.f90
Program omp_alloc
Use omp_lib, Only : omp_get_num_threads, omp_get_thread_num
Implicit None
Integer, Dimension( : ), Allocatable :: a
Integer :: ith
Integer :: nth
Integer :: i
Allocate( a( 1:4 ) )
a = [ ( i, i = 1, 4 ) ]
!$omp parallel default( none ) shared( a ) private( ith, nth )
nth = omp_get_num_threads()
ith = omp_get_thread_num ()
Write( *, * ) ith, nth, 'a = ', a
!$omp barrier
Deallocate( a )
!$omp end parallel
Write( *, * ) Allocated( a )
End Program omp_alloc
ijb@ijb-Latitude-5410:~/work/stack$ gfortran-11 -Wall -Wextra -fcheck=all -O -g -std=f2018 -fopenmp omp.f90
ijb@ijb-Latitude-5410:~/work/stack$ export OMP_NUM_THREADS=8
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 8 a = 1 2 3 4
7 8 a = 1 2 3 4
2 8 a = 1 2 3 4
4 8 a = 1 2 3 4
1 8 a = 1 2 3 4
6 8 a = 1 2 3 4
3 8 a = 1 2 3 4
5 8 a = 1 2 3 4
F
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
6 8 a = 1 2 3 4
0 8 a = 1 2 3 4
2 8 a = 1 2 3 4
5 8 a = 1 2 3 4
4 8 a = 1 2 3 4
3 8 a = 1 2 3 4
7 8 a = 1 2 3 4
1 8 a = 1 2 3 4
At line 21 of file omp.f90
Fortran runtime error: Attempt to DEALLOCATE unallocated 'a'
Error termination. Backtrace:
#0 0x7fe058962d01 in ???
#1 0x7fe058963849 in ???
#2 0x7fe058963ec6 in ???
#3 0x4012fa in MAIN__._omp_fn.0
at /home/ijb/work/stack/omp.f90:21
#4 0x7fe0587cb77d in ???
#5 0x7fe058732608 in start_thread
at /build/glibc-YbNSs7/glibc-2.31/nptl/pthread_create.c:477
#6 0x7fe058657292 in ???
#7 0xffffffffffffffff in ???
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 8 a = 1 2 3 4
3 8 a = 1 2 3 4
1 8 a = 1 2 3 4
6 8 a = 1 2 3 4
7 8 a = 1 2 3 4
2 8 a = 1 2 3 4
4 8 a = 1 2 3 4
5 8 a = 1 2 3 4
F
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
6 8 a = 1 2 3 4
0 8 a = 1 2 3 4
2 8 a = 1 2 3 4
1 8 a = 1 2 3 4
5 8 a = 1 2 3 4
4 8 a = 1 2 3 4
7 8 a = 1 2 3 4
3 8 a = 1 2 3 4
F
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 8 a = 1 2 3 4
4 8 a = 1 2 3 4
2 8 a = 1 2 3 4
1 8 a = 1 2 3 4
3 8 a = 1 2 3 4
5 8 a = 1 2 3 4
7 8 a = 1 2 3 4
6 8 a = 1 2 3 4
F
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 8 a = 1 2 3 4
3 8 a = 1 2 3 4
7 8 a = 1 2 3 4
2 8 a = 1 2 3 4
4 8 a = 1 2 3 4
6 8 a = 1 2 3 4
5 8 a = 1 2 3 4
1 8 a = 1 2 3 4
F
ijb@ijb-Latitude-5410:~/work/stack$ ./a.out
0 8 a = 1 2 3 4
1 8 a = 1 2 3 4
3 8 a = 1 2 3 4
2 8 a = 1 2 3 4
4 8 a = 1 2 3 4
7 8 a = 1 2 3 4
5 8 a = 1 2 3 4
6 8 a = 1 2 3 4
At line 21 of file omp.f90
Fortran runtime error: Attempt to DEALLOCATE unallocated 'a'
Error termination. Backtrace:
#0 0x7fef41e85d01 in ???
#1 0x7fef41e86849 in ???
#2 0x7fef41e86ec6 in ???
#3 0x4012fa in MAIN__._omp_fn.0
at /home/ijb/work/stack/omp.f90:21
#4 0x7fef41cee77d in ???
#5 0x7fef41c55608 in start_thread
at /build/glibc-YbNSs7/glibc-2.31/nptl/pthread_create.c:477
#6 0x7fef41b7a292 in ???
#7 0xffffffffffffffff in ???
【讨论】：
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