[MITgcm-support] MITgcm virtual memory usage

[jdawe at eos.ubc.ca]

> Yes, you can reduce the overall memory footprint by reducing the size
> of the domain and/or by turning off packages or reducing the size of
> the buffers within some packages.  The prime candidates are timeave
> and diagnostics which have significant work arrays.

Ok, I'll try turning things off in those packages.

> But, really, who cares what the virtual memory usage is?  What
> difference does it make?  The main thing that matters is whether
> your model fits in memory or not.  If it spills over into swap then
> you have a huge performance hit.  But if you can hold the entire
> problem in RAM (without digging into swap on a continual basis) then
> what difference does it make whether you use a few more MB of RAM or
> not?

It matters because I want to upscale this run to a cluster with slave
nodes that have 2GB of RAM and no swap.  If I can get rid of the 60% of
allocated ram that MITgcm doesn't use, I can double the vertical
resolution of the model.  That matters to me.

> Oh, and don't place too much faith in any numbers you get from ps,
> top, or their ilk.  They're quite approximate.  See:
>
> http://virtualthreads.blogspot.com/2006/02/understanding-memory-usage-on-linux.html
>
> for a quick intro.

I am well aware of this.  For a array-intensive program like MITgcm,
however, ps gives a fair indication of memory usage.  Examining pmap's
output, for instance, suggests that 1602 of the 1617 MB of MITgcm's
process are unshared.

Jordan