Re: [boost] very poor thread performance

From: James Sutherland (James.Sutherland_at_[hidden])
Date: 2008-05-14 16:47:28

>> No, if you also increment itask in the outer loop, you are doing  
>> fewer
>> tasks when nThreads is smaller.  Although, what I said before about
>> creating more threads than you think was wrong (I misunderstood your
>> program).
>
> Also, your program appears to output the cpu time spent on the  
> program, not
> the real time it took.  What I see (after fixing the itask increment):
>
> $time ./a.out
> Executing 12 tasks using 1 threads.
> time: 9.64
>
> real    0m9.668s
> user    0m9.641s
> sys     0m0.004s
>
>
> $ time ./a.out
> Executing 12 tasks using 2 threads.
> time: 9.91
>
> real    0m4.991s
> user    0m9.909s
> sys     0m0.012s
>

You are right - here is the modified loop structure:

   int itask=0;
   while( itask<nTasks ){
     boost::thread_group threads;
     for( int i=0; i<nThreads; ++i ){
       threads.create_thread( MyStruct(itask++ + 100) );
     }
     threads.join_all();
   }

Any thoughts on what is chewing up the extra time then?  Isn't the  
total time the correct measure here?  Is the difference between the  
"real" time and the "system" time due to the join_all?

James