Re: [boost] very poor thread performance

From: Matt Doyle (mdoyle_at_[hidden])
Date: 2008-05-14 17:26:06

> [mailto:boost-bounces_at_[hidden]] On Behalf Of James Sutherland
> Sent: Wednesday, May 14, 2008 2:15 PM
> To: boost_at_[hidden]
> 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
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