[FLASH-USERS] How memory requirement calculations work ?

[Anshu Dubey]

Latif,

You should certainly be able to run 512^3 problem on your machine
unless you have an exorbitant number of variables. Uniform Grid will
be significantly cheaper than AMR memory wise, especially if you are
using 8^3 blocks. The simplest calculation of memory per variable is
as follows.

The amount of memory per  block is 16^3 words, where only 8^3 words
have the interior data, the remaining cells are for guard cells.
Effectively, for a 512^3 of actual data, 262144 blocks  or 262144*8^3
cells need to be stored per variable. If this same space was covered
with 32^3 blocks, then the numbers reduce to 4096 blocks and 4096*40^3
cells per variable. If you were to run the problem as a single block
then the number of cells is 520^3 words per variable.

AMR has several other memory overheads as well because a lot more
metadata and buffer storage is required. So if your mesh ends up being
uniform, you should absolutely use the UG implementation instead of
paramesh for your simulation.

Anshu

On Tue, Sep 15, 2009 at 9:02 AM, Latif <latife at astro.rug.nl> wrote:
> Hi
> Dear All,
> I am trying to run 512^3 ( which is equivalent to lrefinemax=7 in 3-D)
> uniform grid cosmological simulations (including gravity, particles,
> hydro,EOS). I am running FLASH on 250 GB internal memory machine.  I was not
> able to able to run simulations because i received the message that there is
> not enough virtual memory available. Will using uniform grid (UG) will
> reduce memory requirements. Can any one explain to me, how memory
>  requirement calculation is done in FLASH. I am really interested in knowing
> about. Some guy was arguing with me that he can run run 512^3 sph
> simulations on 30 GB machine why you can't. Memory requirement in AMR codes
> is generally high. Which part of FLASH is expensive in terms of memory? How
> can i know how much memory i will be required for X^3 grid calculations?
> Cheers
> Latif
>
>