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

[Jeff Hammond]

> On May 9, 2020, at 8:34 PM, hritikesh semwal <hritikesh.semwal at gmail.com> wrote:
> 
> 
> 
> 
>> 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?

It depends on your filesystem details but I’d expect the cutoff where parallel reading of input to be necessary is greater than 1 MB and potentially much more than that. Anything less than 4K (or whatever the filesystem page size is if not 4K) is going to be dominated by block read latency, which can’t be reduced with parallelism.

Jeff

>>> 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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