57 lines
1.6 KiB
C++
57 lines
1.6 KiB
C++
template<typename Iterator,typename T>
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struct accumulate_block
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{
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T operator()(Iterator first,Iterator last)
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{
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return std::accumulate(first,last,T());
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}
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};
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template<typename Iterator,typename T>
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T parallel_accumulate(Iterator first,Iterator last,T init)
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{
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unsigned long const length=std::distance(first,last);
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if(!length)
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return init;
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unsigned long const min_per_thread=25;
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unsigned long const max_threads=
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(length+min_per_thread-1)/min_per_thread;
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unsigned long const hardware_threads=
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std::thread::hardware_concurrency();
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unsigned long const num_threads=
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std::min(hardware_threads!=0?hardware_threads:2,max_threads);
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unsigned long const block_size=length/num_threads;
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std::vector<std::future<T> > futures(num_threads-1);
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std::vector<std::thread> threads(num_threads-1);
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Iterator block_start=first;
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for(unsigned long i=0;i<(num_threads-1);++i)
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{
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Iterator block_end=block_start;
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std::advance(block_end,block_size);
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std::packaged_task<T(Iterator,Iterator)> task(
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accumulate_block<Iterator,T>());
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futures[i]=task.get_future();
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threads[i]=std::thread(std::move(task),block_start,block_end);
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block_start=block_end;
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}
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T last_result=accumulate_block()(block_start,last);
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std::for_each(threads.begin(),threads.end(),
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std::mem_fn(&std::thread::join));
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T result=init;
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for(unsigned long i=0;i<(num_threads-1);++i)
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{
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result+=futures[i].get();
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}
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result += last_result;
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return result;
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}
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