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Subject: Re: [OMPI users] mpi functions are slow when first called and become normal afterwards
From: RightCFD (rightcfd_at_[hidden])
Date: 2009-11-12 00:04:18


>
> Date: Thu, 29 Oct 2009 15:45:06 -0400
> From: Brock Palen <brockp_at_[hidden]>
> Subject: Re: [OMPI users] mpi functions are slow when first called and
> become normal afterwards
> To: Open MPI Users <users_at_[hidden]>
> Message-ID: <890CC430-68B0-4307-8260-24A6FADAE319_at_[hidden]>
> Content-Type: text/plain; charset=US-ASCII; format=flowed; delsp=yes
>
> > When MPI_Bcast and MPI_Reduce are called for the first time, they
> > are very slow. But after that, they run at normal and stable speed.
> > Is there anybody out there who have encountered such problem? If you
> > need any other information, please let me know and I'll provide.
> > Thanks in advance.
>
> This is expected, and I think you can dig though the message archive
> to find the answer. OMPI does not wireup all the communication at
> startup, thus the first time you communicate with a host the
> connection is made, but after that it is fast because it is already
> open. This behavior is expected, and is needed for very large systems
> where you could run out of sockets for some types of communication
> with so many hosts.
>
> Brock Palen
> www.umich.edu/~brockp
> Center for Advanced Computing
> brockp_at_[hidden]
> (734)936-1985
>
> Thanks for your reply. I am surprised to know this is an expected behavior
> of OMPI. I searched the archival but did not find many relevant messages. I
> am wondering why other users of OMPI do not complain this. Is there a way to
> avoid this when timing an MPI program?