Re: [boost] very poor thread performance

From: James Sutherland (James.Sutherland_at_[hidden])
Date: 2008-05-14 17:14:47

>> 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?
>
> There is no extra time, or not much.  You seem to be misinterpreting  
> the
> output of the "time" command.  The real time is the elapsed time in  
> the real
> world.  The sys plus user time is the time the all cpus spent  
> executing the
> program in total, which you would expect to remain roughly constant.
>
> If 2 cpus each spent 1 second running the program in parallel, the  
> real time
> would be 1 second and the system plus user time would be 2 seconds.
>
> If one cpu each spent 2 seconds running the program, the real time  
> would be 2
> seconds and the system plus user time would still be 2 seconds.
>
> Oh, I also made my version loop 10 times as long, which is why it  
> takes almost
> 10 seconds.
>
Okay, then I am interpreting things correctly, and my internal timer  
seems to be consistent with the output of the "time" command that you  
refer to.  To restate, the sum of "real" and "system" time is the  
pertinent measure.  The results you posted then indicate that there is  
no speedup associated with increasing the number of threads.  I am now  
seeing the same thing...

So any thoughts as to why there is no speedup?

James