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在编写多线程程序时，运行多少线程比较合适呢？线程并不是越多越好，理论上，硬件支持多少线程数，就开多少个线程比较合适，有的比如完成端口IOCP中建议开2倍线程数，因为考虑到有些线程可能会挂起等情况。但最重要的一条，首先要获取当前硬件支持的线程数，通常情况下为CPU核数。

std::thread::hardware_concurrency();    //获取当前CPU核心数量

代码示例：
以下代码为std::accumulate的简单并行版本实现，通过将大量的累加操作，分配给多个线程去计算，最后将各个线程计算的结果累加，得出最终结果。真正的并行计算任务分割是很麻烦的，这里并不需要考虑线程的同步等问题。

template<typename Iterator,typename T>
struct accumulate_block
{void operator()(Iterator first,Iterator last,T& result){result = std::accumulate(first,last,result);}
};template<typename Iterator,typename T>
T parallel_accumulate(Iterator first,Iterator last,T init)
{unsigned long const length=std::distance(first,last);if(!length)return init;unsigned long const min_per_thread=25;unsigned long const max_threads=(length+min_per_thread-1)/min_per_thread;   //获取最大线程数量unsigned long const hardware_threads=std::thread::hardware_concurrency();   //获取当前CPU核心数量unsigned long const num_threads=std::min(hardware_threads!=0?hardware_threads:2,max_threads);//运行线程数量unsigned long const block_size=length/num_threads;std::vector<T> results(num_threads);Iterator block_start=first;std::vector<std::thread> v_threads(num_threads-1);for(unsigned long i=0;i<num_threads-1;++i){Iterator block_end=block_start;std::advance(block_end,block_size);v_threads[i]=std::thread(accumulate_block<Iterator,T>(),block_start,block_end,std::ref(results[i]));block_start=block_end;}accumulate_block<Iterator,T>()(block_start,last,results[num_threads-1]);    //计算剩下的数,相当于在主线程中计算std::for_each(v_threads.begin(),v_threads.end(),std::mem_fn(&std::thread::join));//等待所有线程计算完成return std::accumulate(results.begin(),results.end(),init);
}int _tmain(int argc, _TCHAR* argv[])
{std::vector<int> v(100000);std::iota(v.begin(),v.end(),1);long long sum=parallel_accumulate(v.begin(),v.end(),0);cout<<"sum="<<sum<<endl;return 0;
}

相关STL源码：

//std::distance源码
template<class _BidIt,class _Diff> inlinevoid _Distance2(_BidIt _First, _BidIt _Last, _Diff& _Off,bidirectional_iterator_tag){   // add to _Off distance between bidirectional iterators (redundant)for (; _First != _Last; ++_First)++_Off;}template<class _InIt> inlinetypename iterator_traits<_InIt>::difference_typedistance(_InIt _First, _InIt _Last){   // return distance between iteratorstypename iterator_traits<_InIt>::difference_type _Off = 0;_Distance2(_First, _Last, _Off, _Iter_cat(_First));return (_Off);}

//std::advance源代码// TEMPLATE FUNCTION advance
template<class _InIt,class _Diff> inlinevoid _Advance(_InIt& _Where, _Diff _Off, input_iterator_tag){   // increment iterator by offset, input iterators#if _ITERATOR_DEBUG_LEVEL == 2if (_Off < 0)_DEBUG_ERROR("negative offset in advance");#endif /* _ITERATOR_DEBUG_LEVEL == 2 */for (; 0 < _Off; --_Off)++_Where;}template<class _FwdIt,class _Diff> inlinevoid _Advance(_FwdIt& _Where, _Diff _Off, forward_iterator_tag){   // increment iterator by offset, forward iterators#if _ITERATOR_DEBUG_LEVEL == 2if (_Off < 0)_DEBUG_ERROR("negative offset in advance");#endif /* _ITERATOR_DEBUG_LEVEL == 2 */for (; 0 < _Off; --_Off)++_Where;}template<class _BidIt,class _Diff> inlinevoid _Advance(_BidIt& _Where, _Diff _Off, bidirectional_iterator_tag){   // increment iterator by offset, bidirectional iteratorsfor (; 0 < _Off; --_Off)++_Where;for (; _Off < 0; ++_Off)--_Where;}template<class _RanIt,class _Diff> inlinevoid _Advance(_RanIt& _Where, _Diff _Off, random_access_iterator_tag){   // increment iterator by offset, random-access iterators_Where += _Off;}template<class _InIt,class _Diff> inlinevoid advance(_InIt& _Where, _Diff _Off){   // increment iterator by offset, arbitrary iterators_Advance(_Where, _Off, _Iter_cat(_Where));}

//获取硬件线程数量static unsigned int hardware_concurrency() _NOEXCEPT{   // return number of hardware thread contextsreturn (::Concurrency::details::_GetConcurrency());}