openMP嵌套并行for循环与内部并行for [英] openMP nested parallel for loops vs inner parallel for
问题描述
如果我使用嵌套并行for循环,像这样:
#pragma omp parallel for schedule(dynamic,1)
for(int x = 0; x #pragma omp parallel for schedule(dynamic,1)
for(int y = 0; y< y_max; ++ y){
//并行化此代码
}
//重要信息:此处没有代码
}
这相当于:
for 0; x #pragma omp parallel for schedule(dynamic,1)
for(int y = 0; y //并行化此代码
}
//重要信息:此处没有代码
}
$ b
如果你创建一个新的任务,编译器支持OpenMP 3.0,可以使用 collapse
子句:
pragma omp parallel for schedule(dynamic,1)collapse(2)
for(int x = 0; x < x_max; ++ x){
for(int y = 0; y //并行化此代码
}
//重要:代码在这里
}
如果没有(例如只支持OpenMP 2.5) ,有一个简单的解决方法:
#pragma omp parallel for schedule(dynamic,1)
for = 0; xy int x = xy / y_max;
int y = xy%y_max;
//并行化此代码
}
omp_set_nested(1);
并且您的嵌套 omp parallel for
代码将起作用,但这可能不是最好的想法。 / p>
顺便说一下,为什么要动态调度?每个循环迭代是否在非恒定时间内进行计算?
If I use nested parallel for loops like this:
#pragma omp parallel for schedule(dynamic,1)
for (int x = 0; x < x_max; ++x) {
#pragma omp parallel for schedule(dynamic,1)
for (int y = 0; y < y_max; ++y) {
//parallelize this code here
}
//IMPORTANT: no code in here
}
is this equivalent to:
for (int x = 0; x < x_max; ++x) {
#pragma omp parallel for schedule(dynamic,1)
for (int y = 0; y < y_max; ++y) {
//parallelize this code here
}
//IMPORTANT: no code in here
}
Is the outer parallel for doing anything other than creating a new task?
If your compiler supports OpenMP 3.0, you can use the collapse
clause:
#pragma omp parallel for schedule(dynamic,1) collapse(2)
for (int x = 0; x < x_max; ++x) {
for (int y = 0; y < y_max; ++y) {
//parallelize this code here
}
//IMPORTANT: no code in here
}
If it doesn't (e.g. only OpenMP 2.5 is supported), there is a simple workaround:
#pragma omp parallel for schedule(dynamic,1)
for (int xy = 0; xy < x_max*y_max; ++xy) {
int x = xy / y_max;
int y = xy % y_max;
//parallelize this code here
}
You can enable nested parallelism with omp_set_nested(1);
and your nested omp parallel for
code will work but that might not be the best idea.
By the way, why the dynamic scheduling? Is every loop iteration evaluated in non-constant time?
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