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Subject: Re: [OMPI users] Simple Question regarding MPI Scatterv
From: Ralph Castain (rhc_at_[hidden])
Date: 2014-04-08 11:35:24


On Apr 8, 2014, at 8:05 AM, Hamid Saeed <e.hamidsaeed_at_[hidden]> wrote:

> Yes i meant Parallel file system.
>
> And can you kindly explain what exactly happens if
> the RANK0 want to send to RANK0?

It goes thru the "self" BTL, which is pretty fast but does require a little time You also have the collective operation overhead in the scatterv algorithm.

>
> Why does MPIO is different in time consumption than RANK0 to RANK0 communication?

Again, it depends on how you did the test. MPIO on a single node is just a file read operation - depending on the parallel file system, that can be heavily optimized with pre-fetch and memory caching.

>
>
> On Tue, Apr 8, 2014 at 4:45 PM, Ralph Castain <rhc_at_[hidden]> wrote:
> 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
>> _______________________________________________
>> _______________________________________________
>> users mailing list
>> users_at_[hidden]
>> http://www.open-mpi.org/mailman/listinfo.cgi/users
>
>
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>
> --
> _______________________________________________
> Hamid Saeed
> CoSynth GmbH & Co. KG
> Escherweg 2 - 26121 Oldenburg - Germany
> Tel +49 441 9722 738 | Fax -278
> http://www.cosynth.com
> _______________________________________________
> _______________________________________________
> users mailing list
> users_at_[hidden]
> http://www.open-mpi.org/mailman/listinfo.cgi/users