[boost] very poor thread performance

From: James Sutherland (James.Sutherland_at_[hidden])
Date: 2008-05-14 15:31:32

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;
}
----