Hi, IMHO the problem is that the tasks are too simple and are executed too quickly when compared to thread creation time. A task, or several tasks, that spend less that one second of CPU time are not worth creating a new dedicated thread. This is actually why thread pools are created. In my experience it is much more efficient to put such short tasks into a queue and execute them sequentially. If you have many of such tasks (~hundreds and more) and You want to use multiple CPU's then create several worker threads at application startup, that pick the tasks from the queue and execute them. I didn't try to run Your example myself but I've seen things like this happening before. Maybe try to change the const int n = 100; to something like const int n = 10000; in the MyStruct::operator() and see what happens. However, it's also possible that I'm missing something :-) and there is indeed some issue with the implementation of Boost.Threads On Wed, May 14, 2008 at 9:31 PM, James Sutherland <James.Sutherland@utah.edu> wrote:
I have been testing thread performance on Linux and Mac. My Linux system has two dual-core processors and my Mac has one dual-core processor. Both are intel chips.
For the code snippet given below, the execution time should ideally decrease as the number of threads increases. However, the opposite trend is observed. For example, using -O3 flags on my Linux desktop produces the following timings: 1 Thread: 0.66 sec 2 Threads: 0.9 sec 3 Threads: 1.2 sec 4 Threads: 1.4 sec
I do not have a lot of experience with threads, and was wondering if this result surprises anyone?
James
----- #include <boost/thread.hpp> #include <iostream> #include <vector> #include <time.h>
struct MyStruct { explicit MyStruct(const int i) : tag(i) {} void operator()() const { const int n = 100; std::vector<int> nums(n,0); for( int j=0; j<1000000; ++j ) for( int i=0; i<n; ++i ) nums[i] = i+tag; } private: int tag; };
int main() { using namespace std; const int nTasks = 12; const int nThreads = 4; assert( nTasks%nThreads == 0 ); assert( nThreads<=nTasks ); cout << "Executing " << nTasks << " using " << nThreads << " threads." << endl; time_t t1 = clock(); for( int itask=0; itask<nTasks; ++itask ){ boost::thread_group threads; for( int i=0; i<nThreads; ++i ){ threads.create_thread( MyStruct(itask++ + 100) ); } threads.join_all(); } t2 = clock(); cout << "time: " << difftime( t2,t1)/CLOCKS_PER_SEC << endl; } ---- _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
-- ________________ ::matus_chochlik