Multithreading 使用openmp和private子句的梯形规则集成

我正在更改串行执行的代码，并将其调整为并行执行（openmp），但得到的结果（pi值）的近似值不正确。我在下面显示这两个代码

有什么不对劲吗

program trap
use omp_lib 
implicit none
double precision::suma=0.d0 ! sum is a scalar
double precision:: h,x,lima,limb
integer::n,i, istart, iend, thread_num, total_threads=4, ppt
integer(kind=8):: tic, toc, rate
double precision:: time
double precision, dimension(4):: pi= 0.d0

call system_clock(count_rate = rate)
call system_clock(tic)

lima=0.0d0; limb=1.0d0; suma=0.0d0; n=10000000
h=(limb-lima)/n

suma=h*(f(lima)+f(limb))*0.5d0 !first and last points

ppt= n/total_threads
!$ call omp_set_num_threads(total_threads)

!$omp parallel private (istart, iend, thread_num, i)
  thread_num = omp_get_thread_num()
  !$ istart = thread_num*ppt +1
  !$ iend = min(thread_num*ppt + ppt, n-1)
do i=istart,iend ! this will control the loop in different images
  x=lima+i*h
  suma=suma+f(x) 
  pi(thread_num+1)=suma
enddo
!$omp end parallel

suma=sum(pi) 
suma=suma*h

print *,"The value of pi is= ",suma ! print once from the first image
!print*, 'pi=' , pi
call system_clock(toc)
time = real(toc-tic)/real(rate)
print*, 'Time ', time, 's'

contains

double precision function f(y)
double precision:: y
f=4.0d0/(1.0d0+y*y)
end function f

end program trap

!----------------------------------------------------------------------------------
program trap
implicit none
double precision::sum ! sum is a scalar
double precision:: h,x,lima,limb
integer::n,i
integer(kind=8):: tic, toc, rate
double precision:: time

call system_clock(count_rate = rate)
call system_clock(tic)

lima=0.0d0; limb=1.0d0; sum=0.0d0; n=10000000
h=(limb-lima)/n

sum=h*(f(lima)+f(limb))*0.5d0 !first and last points

do i=1,n-1 ! this will control the loop in different images
  x=lima+i*h
  sum=sum+f(x)
enddo

sum=sum*h

print *,"The value of pi is (serial exe)= ",sum ! print once from the first image

call system_clock(toc)
time = real(toc-tic)/real(rate)
print*, 'Time serial execution', time, 's'

contains

double precision function f(y)
double precision:: y
f=4.0d0/(1.0d0+y*y)
end function f

end program trap

使用以下方式编译：

$gfortran-fopenmp-Wall-Wextra-O2-Wall-o prog.exe test.f90
$./prog.exe

及

在串行执行中，我得到了pi（3.1415）的良好近似值，但使用并行执行，我得到了（我显示了几个并行执行）：

---

openmp并行语句中存在问题。 您不断地在变量

suma

上累加。 因此，您需要指定一个

reduce

语句。 此外，您没有将变量

x

指定为私有

我还修改了你代码的其他部分

• 您明确地告诉每个线程应该使用哪个索引范围。大多数情况下，编译器可以自己更有效地解决这个问题。为此，我将

  parallel

  更改为

  parallel do

• 将openmp并行区域中的变量属性设置为

  default（none）

  ，这是一种很好的做法。您需要显式地设置每个变量属性

$ gfortran -Wall -Wextra -O2 -Wall -o prog.exe testserial.f90 
$ ./prog.exe

 The value of pi is=    3.6731101425922810     

 Time    3.3386986702680588E-002 s

-------------------------------------------------------

 The value of pi is=    3.1556004791445953     

 Time    8.3681479096412659E-002 s

------------------------------------------------------

 The value of pi is=    3.2505952856717966     

 Time    5.1473543047904968E-002 s

program trap
  use omp_lib
  implicit none
  double precision   :: suma,h,x,lima,limb, time
  integer            :: n, i
  integer, parameter :: total_threads=5
  integer(kind=8)    :: tic, toc, rate

  call system_clock(count_rate = rate)
  call system_clock(tic)

  lima=0.0d0; limb=1.0d0; suma=0.0d0; n=10000000
  h=(limb-lima)/n

  suma=h*(f(lima)+f(limb))*0.5d0 !first and last points

  call omp_set_num_threads(total_threads)
  !$omp parallel do default(none) private(i, x) shared(lima, h, n)  reduction(+: suma)
  do i = 1, n
    x=lima+i*h
    suma=suma+f(x)
  end do
  !$omp end parallel do

  suma=suma*h

  print *,"The value of pi is= ", suma ! print once from the first image
  call system_clock(toc)
  time = real(toc-tic)/real(rate)
  print*, 'Time ', time, 's'

contains

  double precision function f(y)
    double precision:: y
    f=4.0d0/(1.0d0+y*y)
  end function

end program