Hello,
I am using the options mechanism of PETSc to configure my CFD code. I
have introduced options describing the size of the domain etc. I have
noticed that this consumes a lot of memory. I have found that the amount
of memory used scales linearly with the number of MPI processes used.
This restricts the number of MPI processes that I can use.
There are two statements:
1) The memory scales linearly with P
2) This uses a lot of memory
Let's deal with 1) first. This seems to be trivially true. If I want every process to have
access to a given option value, that option value must be in the memory of every process.
The only alternative would be to communicate with some process in order to get values.
Few codes seem to be willing to make this tradeoff, and we do not offer it.
Now 2). Looking at the source, for each option we store a PetscOptionItem, which I count
as having size 37 bytes (12 pointers/ints and a char). However, there is data behind every
pointer, like the name, help text, available values (sometimes), I could see it being as large
as 4K. Suppose it is. If I had 256 options, that would be 1M. Is this a large amount of memory?
The way I read the SLURM output, 29K was malloced. Is this a large amount of memory?
I am trying to get an idea of the scale.
Thanks,
Matt
Is there anything that I can do about this or do I need to configure my
code in a different way?
I have attached some code extracted from my application which
demonstrates this along with the output from a running it on 2 MPI
processes.
Best wishes,
David Scott
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