[petsc-users] Memory Used When Reading petscrc

[David Scott]

Matt,

Thanks for the quick response.

Yes 1) is trivially true.

With regard to 2), from the SLURM output:
[0] Maximum memory PetscMalloc()ed 29552 maximum size of entire process 
4312375296
[1] Maximum memory PetscMalloc()ed 29552 maximum size of entire process 
4311990272
Yes only 29KB was malloced but the total figure was 4GB per process.

Looking at
  mem0 =    16420864.000000000
  mem0 =    16117760.000000000
  mem1 =    4311490560.0000000
  mem1 =    4311826432.0000000
  mem2 =    4311490560.0000000
  mem2 =    4311826432.0000000
mem0 is written after PetscInitialize.
mem1 is written roughly half way through the options being read.
mem2 is written on completion of the options being read.

The code does very little other than read configuration options. Why is 
so much memory used?

I do not understand what is going on and I may have expressed myself 
badly but I do have a problem as I certainly cannot use anywhere near 
128 processes on a node with 128GB of RAM before I get an OOM error. 
(The code runs successfully on 32 processes but not 64.)

Regards,

David

On 22/11/2024 16:53, Matthew Knepley wrote:
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> On Fri, Nov 22, 2024 at 11:36 AM David Scott <d.scott at epcc.ed.ac.uk> 
> wrote:
>
>     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
>     The University of Edinburgh is a charitable body, registered in
>     Scotland, with registration number SC005336. Is e buidheann
>     carthannais a th’ ann an Oilthigh Dhùn Èideann, clàraichte an
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>
>
>
> -- 
> What most experimenters take for granted before they begin their 
> experiments is infinitely more interesting than any results to which 
> their experiments lead.
> -- Norbert Wiener
>
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