[petsc-dev] Question about Binary-IO in READ mode with POSIX APIs

[Jed Brown]

Lisandro Dalcin <dalcinl at gmail.com> writes:

>>   I'm not sure of this suggested change, in that a
>> "bad for MPI-IO" workload (like each rank randomly seeking around a big
>> file) might not be better with rank 0 acting as a service rank.
>>
>
> Please note my main question is unrelated to MPI-IO. It is about the
> original POSIX-based implementation of binary viewers. For mode READ, all
> processes open the file (with the open() system call), but in the current
> implementation, only process zero ever reads the file (unless the user gets
> the file descriptor and start issuing low-level PetscBinaryRead() calls).
> So I do not see the point of opening the file on all processes (and then
> stress metadata servers on parallel filesystem), if we are not going to
> ever read from rank != 0. Let's just fix things to open the file at rank==0
> only. 

Okay, this much I agree on.  If a rank doesn't actually read, it
shouldn't open the file to read.