[MITgcm-support] MITgcm virtual memory usage

[Jordan Dawe]

>>> 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.
>>     
>
> I think you're missing the bigger picture.  VM OSes (eg. Linux) are
> generally allowed to overcommit even when there is no swap device.
> Therefor, you can have a program that allocates a larger chunk of
> memory than exists and, so long as parts of that memory are never
> actually paged in, then its fine--it will run.
>   
My apologies; you are correct.  I was confusing >2GB memory space 
allocation errors with being out of RAM.