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Subject: Re: [OMPI users] Simple Question regarding MPI Scatterv
From: Ralph Castain (rhc_at_[hidden])
Date: 2014-04-08 10:45:38


I suspect it all depends on when you start the clock. If the data is sitting in the file at time=0, then the file I/O method will likely be faster as every proc just reads its data in parallel - no comm required as it is all handled by the parallel file system.

I confess I don't quite understand your reference to "shared memory" in the MPIO case, but I suspect what you really meant was just "parallel file system"?

On Apr 8, 2014, at 6:12 AM, Hamid Saeed <e.hamidsaeed_at_[hidden]> wrote:

> Can someone kindly reply?
>
>
>
> On Tue, Apr 8, 2014 at 1:01 PM, Hamid Saeed <e.hamidsaeed_at_[hidden]> wrote:
> Hello,
> I think that the MPI open its sockets even though the number of processor is only 1 on the same machine?
> regards.
>
>
> On Tue, Apr 8, 2014 at 9:43 AM, Hamid Saeed <e.hamidsaeed_at_[hidden]> wrote:
> Hello all,
>
> I have a very basic question regarding MPI communication.
>
> In my Task, what i am doing is..
> Comparing Scatterv and MPIO.
> 1) In scatterv, I scatter all the data to the other ranks and SCAN for the specific characters.
> MPI_Scatterv (chunk, send_counts, displacements, MPI_CHAR, copychunk, smallchunk_size, MPI_CHAR, 0, MPI_COMM_WORLD);
> &
> 2) On the other hand using MPIO, I have the data available in the shared memory and every rank searches in the specific chunk.
> MPI_File_open(MPI_COMM_WORLD, "170mb.txt", MPI_MODE_RDONLY, MPI_INFO_NULL, &in);
> here i assign every processor to search in a specific "chunk"
>
>
> My question is..
>
> Why MPI_Scatterv using 1 processor takes more time then MPI_File_open?
> How does MPI sending and receiving takes place?
>
> I think using 1 processor does not include physical sending and receiving. Then why it consumes more clock?
>
> In the attachment you can observe the plots in which i performed some tests using both algorithms.
>
> Kindly explain briefly the mpi communication using 1 processor and multiple processors.
>
>
> Thanks in advance.
>
> Regards
> Hamid
>
>
>
> --
> Hamid
>
>
>
>
> --
> _______________________________________________
> Hamid Saeed
> CoSynth GmbH & Co. KG
> Escherweg 2 - 26121 Oldenburg - Germany
> Tel +49 441 9722 738 | Fax -278
> http://www.cosynth.com
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