[mpich-discuss] 1. Single file reading by all processors in MPI. 2.Difference between blocking and non-blocking call of MPI_neighbor_alltoallw

[hritikesh semwal]

On Sun, May 10, 2020 at 4:18 AM Jeff Hammond <jeff.science at gmail.com> wrote:

>
>
> On Sat, May 9, 2020 at 10:10 AM hritikesh semwal via discuss <
> discuss at mpich.org> wrote:
>
>> Hello,
>>
>> I have following two questions with MPI,
>>
>> 1. Do I need to give a separate input file to each processor in a group
>> or all of them can read the input from a single file? All data inside the
>> file should be read by every processor.
>>
>
> You can read the input file from every process but it isn't a good idea.
> Read the input file from rank 0 and broadcast the contents.  This assumes
> that your input file is small.  If your input file is huge, then you should
> consider designing it for parallel reading with MPI_File_* functions.
>

Thanks for your response. I am reading two input files in my parallel CFD
solver at different places of the code. The first input file has 7 integer
values and second input file has 15 integer values. Is it large to read it
through conventional C functions for file reading i.e. fscanf() and is
broadcast good for this amount of data?


>
>
>> 2. Can you please tell me what is the difference between
>> MPI_Neighbor_alltoallw and MPI_Ineighbor_alltoallw? As I read in MPI 3.0
>> document that MPI_Neighbor_alltoallw also used non-blocking send and
>> receive inside its execution.
>>
>>
> MPI_Neighbor_alltoallw is a blocking function while
> MPI_Ineighbor_alltoallw is not.  Using non-blocking send and receive inside
> its execution does not mean that MPI_Neighbor_alltoallw doesn't block - if
> that is how it's implemented, then there will be the equivalent of a
> MPI_Waitall before the function returns.  But in any case, just because the
> MPI document suggests a possible implementation does not mean that is how
> all implementations work.
>

Yes, there is MPI_Waitall inside the MPI_Neighbor_alltoallw before it
returns.


>
> Jeff
>
>
>> Thank you.
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>
>
> --
> Jeff Hammond
> jeff.science at gmail.com
> http://jeffhammond.github.io/
>
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