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Subject: Re: [OMPI users] OpenMPI load data to multiple nodes
From: Reuti (reuti_at_[hidden])
Date: 2010-07-13 07:55:29


Am 13.07.2010 um 04:50 schrieb Bowen Zhou:

> Since each node has its own memory in a distributed memory system,
> there is no such thing as "global variable" that can be accessed by all
> processes. So you need to use MPI to scatter the input from rank 0
> process to all the other processes explicitly.

I think options might be GlobalArrays or HPF:

http://www.emsl.pnl.gov/docs/global/

http://en.wikipedia.org/wiki/High_Performance_Fortran

-- Reuti

>
> From: dtustudy68_at_[hidden]
> To: users_at_[hidden]
> Date: Mon, 12 Jul 2010 20:33:04 -0600
> Subject: Re: [OMPI users] OpenMPI load data to multiple nodes
>
> thanks very much !!!
>
> May I use global variable to do that ?
>
> It means that all nodes have the same global variable,
> such as globalVector.
>
> In the initialization, only node 0 load data from files and assign values to
> the globalVector.
>
> After that, all other nodes can get the same data by accessing the globalVector.
>
> Does it make sense ?
>
> Any help is appreciated.
>
> Jack
>
> July 12 2010
>
> > Date: Mon, 12 Jul 2010 21:44:34 -0400
> > From: gus_at_[hidden]
> > To: users_at_[hidden]
> > Subject: Re: [OMPI users] OpenMPI load data to multiple nodes
> >
> > Hi Jack/Jinxu
> >
> > Jack Bryan wrote:
> > > Dear All,
> > >
> > > I am working on a multi-computer Open MPI cluster system.
> > >
> > > If I put some data files in /home/mypath/folder, is it possible that all > > non-head nodes can access the files in the folder ?
> &g t; >
> >
> > Yes, possible, for instance, if the /home/mypath/folder directory is
> > NFS mounted on all nodes/computers.
> > Otherwise, if all disks and directories are local to each computer,
> > you need to copy the input files to the local disks before you
> > start, and copy the output files back to your login computer after the
> > program ends.
> >
> > > I need to load some data to some nodes, if all nodes can access the
> > > data, I do not need to load them to each node one by one.
> > >
> > > If multiple nodes access the same file to get data, is there conflict ?
> > >
> >
> > To some extent.
> > The OS (on the computer where the file is located)
> > will do the arbitration on which process gets the hold of the file at
> > each time.
> > If you have 1000 processes, this means a lot of arbitration,
> > and most likely contention.
> > Even for two processes ! only, if the processes are writing data to a
> > single file, this won't ensure that they write
> > the output data in the order that you want.
> >
> > > For example,
> > >
> > > fopen(myFile) by node 1, at the same time fopen(myFile) by node 2.
> > >
> > > Is it allowed to do that on MPI cluster without conflict ?
> > >
> >
> > I think MPI won't have any control over this.
> > It is up to the operational system, and depends on
> > which process gets its "fopen" request to the OS first,
> > which is not a deterministic sequence of events.
> > That is not a clean technique.
> >
> > You could instead:
> >
> > 1) Assign a single process, say, rank 0,
> > to read and write data from/to the file(s).
> > Then use, say, MPI_Scatter[v] and MPI_Gather[v],
> > to distribute and collect the data back and forth
> > between that process (rank 0) and all other processe! s.
> >
> > That is an old fashioned but very robust technique .
> > It avoids any I/O conflict or contention among processes.
> > All the data flows across the processes via MPI.
> > The OS receives I/O requests from a single process (rank 0).
> >
> > Besides MPI_Gather/MPI_Scatter, look also at MPI_Bcast,
> > if you need to send the same data to all processes,
> > assuming the data is being read by a single process.
> >
> > 2) Alternatively, you could use the MPI I/O functions,
> > if your files are binary.
> >
> > I hope it helps,
> > Gus Correa
> >
> > > Any help is appreciated.
> > >
> > > Jinxu Ding
> > >
> > > July 12 2010
> > >
> > > ------------------------------------------------------------------------
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> > > with Hotmail. Get busy.
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> > >
> > >
> > > ------------------------------------------------------------------------
> > >
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