[mailto:boost-bounces@lists.boost.org] On Behalf Of James Sutherland Sent: Wednesday, May 14, 2008 2:15 PM To: boost@lists.boost.org Subject: Re: [boost] very poor thread performance
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?
I'm sorry to jump in the middle, but now you've lost me. The way I see the measurements going from 1 thread to 2 cut the over-all execution time roughly in half. User time is measuring the amount of work performed and that didn't change. Sorry if I missed something obvious here. Matt
James _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost