问题描述
我正在尝试使用 openmp 通过 std::set 对循环进行多线程处理.当我编写以下代码时 -
I'm trying to use openmp to multithread a loop through std::set. When I write the following code -
#pragma omp parallel for
for (std::set<A>::const_iterator i = s.begin(); i != s.end(); ++i) {
const A a = *i;
operate(a);
}
我收到此错误:
error: invalid type for iteration variable 'i'
error: invalid controlling predicate
error: invalid increment expression.
是否有另一种正确的方法来使用 openmp 迭代 std 容器?我知道我可以使用 int i 并从 0 迭代到 s.size() 和迭代器或 operator[] 在循环体中，但这看起来不那么干净.
Is there an another, correct way to iterate through std containers using openmp?
I know I can use int i and iterate from 0 to s.size() and an iterator or operator[] in the loop body, but this looks much less clean.
推荐答案
stl 迭代器的循环并行化仅适用于 OpenMP 3.0 以来，并且仅适用于随机访问迭代器(例如 vector 和 deque).你应该能够做这样的事情:
Loop parallelization for stl iterators only works since OpenMP 3.0, and only for random access iterators (e.g. vector and deque). You should be able to do something like this:
#pragma omp parallel {
for (std::set<A>::const_iterator i = s.begin(); i != s.end(); ++i) {
#pragma omp single nowait {
operate(*i);
}
}
}
开销相当大，因为每个线程遍历整个序列(但只对其中的一些执行operate).您使用 int i 的方法效率更高.
Overhead is quite big though because each thread iterates over the whole sequence (but only executes operate on some of it). Your method using an int i is more efficient.
作为替代，看看 GCC 对 std 的并行实现::for_each.看我的评论.
As an alternative take a look at GCC's parallel implementation of std::for_each. See my comment.
编辑:STL Parallism TS，它将很可能是 C++17 的一部分，将来可能是迭代标准容器的不错选择.
EDIT: The STL Parallism TS, which will most likely be part of C++17, might be a good option in the future for iterating over standard containers.
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