Memory allocated by PETSC?

[Shi Jin]

Hi, 
I am curious how much extra memory PETSc allocates in
the background. Since my estimate of memory usage of
the code is much smaller than what I see when it runs.
So I did this simple test:
First I used PETSc to dump a matrix in binary format
into a file. The file has a size of 13MB. I assume
this should be the same size that is used to store the
matrix in memory. Then I wrote a simple code that does
nothing but to load this matrix from the file by
MatLoad(). However, I found that the code consumes
29MB of memory (VIRT=29M from top) using single
process. 
This is confirmed by the -malloc_log option where it
says
 Maximum memory PetscMalloc()ed 29246912 maximum size
of entire process 0
I've attached the output of the code with detailed
malloc information.
Could you please explain to me about the  difference
of  over two time?
I don't want to criticize anything but need an clear
idea of how much memory is needed so that I know
whether there is a chance for me to reduce the memory
usage of my production code.
Thank you very much.

Shi


 
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