MPI I/O, mix of single- and multiple-process output

Question

I need an MPI C code to write data to a binary file via MPI I/O. I need process 0 to write a short header, then I need the whole range of processes to write their own pieces of

Answer 1

Your approach is fine and if you need something right now to put bits in a file, go ahead and call yourself done.

Here are some suggestions for more efficiency:

• You can consult the status object for how many bytes were written, instead of getting the position and translating into bytes.

• If you have the memory to hold all the data before you write, you could describe your I/O with an MPI datatype (admittedly, one that might end up being a pain to create). Then all processes would issue a single collective call.

• You should use collective I/O instead of independent I/O. A "quality library" should be able to give you equal if not better performance (and if not, you could raise the issue with your MPI implementation).

• If the processes have different amounts of data to write, MPI_EXSCAN is a good way to collect who has what data. Then you can call MPI_FILE_WRITE_AT_ALL to the correct offset in the file.