C++ openmp 并行 for 循环有两个或多个减少
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openmp parallel for loop with two or more reductions
提问by pyCthon
Hi just wondering if this is the right way to go going about having a regular for loop but with two reductions , is this the right approach below? Would this work with more then two reductions as well. Is there a better way to do this? also is there any chance to integrate this with an MPI_ALLREDUCE command?
嗨，只是想知道这是否是进行常规 for 循环但有两次减少的正确方法，这是下面的正确方法吗？这是否也适用于两次以上的减少。有一个更好的方法吗？还有机会将它与 MPI_ALLREDUCE 命令集成吗？
heres the psuedo code
#pragma omp parallel for \
default(shared) private(i) \
//todo first reduction(+:sum)
//todo second reduction(+:result)
for loop i < n; i ++; {
y = fun(x,z,i)
sum += fun2(y,x)
result += fun3(y,z)
}
回答by devil
You can do reduction by specifying more than one variable separated by a comma, i.e. a list:
您可以通过指定多个以逗号分隔的变量来进行归约，即一个列表：
#pragma omp parallel for default(shared) reduction(+:sum,result) ...
#pragma omp parallel for default(shared) reduction(+:sum,result) ...
Private thread variables will be created for sumand resultthat will be combined using +and assigned to the original global variables at the end of the thread block.
私有线程变量将被创建，sum并将在线程块末尾result使用+并分配给原始全局变量。
Also, variable yshould be marked private.
此外，变量y应标记为私有。
回答by Azmisov
You can simply add another reductionclause:
您可以简单地添加另一个reduction子句：
#include <iostream>
#include <cmath>
int main(){
double sum_i = 0, max_i = -1;
#pragma omp parallel for reduction(+:sum_i) reduction(max:max_i)
for (int i=0; i<5000; i++){
sum_i += i;
if (i > max_i)
max_i = i;
}
std::cout << "Sum = " << sum_i << std::endl;
std::cout << "Max = " << max_i << std::endl;
return 0;
}
From OpenMP 4.5 Complete Specifications (Nov 2015)
来自OpenMP 4.5 完整规范（2015 年 11 月）
Any number of reduction clauses can be specified on the directive, but a list item can appear only once in the reduction clauses for that directive.
可以在指令上指定任意数量的归约子句，但列表项在该指令的归约子句中只能出现一次。
The same works on Visual C++ that uses oMP v2.0: reduction VC++
同样适用于使用 oMP v2.0 的 Visual C++：reduction VC++
