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Subject: Re: [OMPI users] Communication Round-trip time
From: Elken, Tom (tom.elken_at_[hidden])
Date: 2012-10-09 11:13:57


I'll agree with Jeff that what you propose sounds right for avg. round-trip time.

Just thought I'd mention that when people talk about the ping-pong latency or MPI latency benchmarks, they are usually referring to 1/2 the round-trip time. So you compute everything the same as you did, and then divide by 2.

But if your professor clearly said round-trip, I'd agree that you need to get a clearer definition of what he's looking for. Maybe it's units. If he asked for round-trip time in microseconds, you have to adjust your formula.

-Tom

> -----Original Message-----
> From: users-bounces_at_[hidden] [mailto:users-bounces_at_[hidden]] On
> Behalf Of Jeff Squyres
> Sent: Tuesday, October 09, 2012 2:58 AM
> To: Open MPI Users
> Subject: Re: [OMPI users] Communication Round-trip time
>
> In general, what you said is "right"... for some definition of "right". :-)
>
> Usually, benchmarking programs start a timer, do the round trip sends N times,
> stop the timer, and then divide the total time by N (to get a smoother definition
> of "average").
>
> But keep in mind that there are many, many factors involved here. For example,
> it is also common to do "warmup" round trip communications -- do, say, 100
> round-trip communications before you start the timer. This allows MPI to
> establish connections and do any other startup processing that isn't a factor
> during steady-state communications.
>
> More specifically, the definitions of "right" and "wrong" very much depend on
> what is in your professor's head. Every benchmark is different; there are many
> similar-but-slightly-different benchmark definitions out there. You'll need to
> figure out exactly what he is looking for; sorry.
>
>
>
> On Oct 8, 2012, at 10:59 PM, huydanlin wrote:
>
> > I'm currently doing MPI project in NUS University. My professor require me
> make a monitoring tool to check the connection among processes in Cluster.
> And also measure communication round-trip time.
> > My solution is :
> > 1. At the source process : start MPI_Send
> > 2. At the destination process : start MPI_Recv to receive the message from
> source
> > 3. At the destination process : start MPI_Send to send immediately the same
> message to source
> > 4. At the source process : start MPI_Recv to receive the message from
> destination
> > I start timer( before (1) - t1=MPI_Wtime & after (4) - t2=MPI_Wtime )
> > then t2 - t1 is time for communication. I also do (1) to (4) for N times. then i
> calculate the avg round-trip time by (t2-t1)/N.
> > So is it right? Because he tells it 's wrong. And what exactly is the
> communication round-trip time ?
> > Regards and hope to see your reply soon.
> > _______________________________________________
> > users mailing list
> > users_at_[hidden]
> > http://www.open-mpi.org/mailman/listinfo.cgi/users
>
>
> --
> Jeff Squyres
> jsquyres_at_[hidden]
> For corporate legal information go to:
> http://www.cisco.com/web/about/doing_business/legal/cri/
>
>
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