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Subject: [OMPI users] Simple Question regarding MPI Scatterv
From: Hamid Saeed (e.hamidsaeed_at_[hidden])
Date: 2014-04-08 03:43:33


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




karp_heterogeneous_Mean_MPIO_scatterv_using_170mb.jpg