[FLASH-USERS] FLASH memory usage: Static, Dynamic and Machine Specific

[Tomasz Plewa]

Accuracy and functionality does not necessarily come together. If one 
really needs accurate solutions for general mass distributions, there is 
no more accurate method than the multigrid.

Tomek
--
On 08/11/16 13:00, Carlo Graziani wrote:
> Actually, the old solver is considerably less accurate than the new one,
> particularly for large multipole cutoff values.  In fact, the old solver -- the
> old algorithm, for that matter -- fails altogether to converge in l. The new
> solver does in fact converge.
>
> Furthermore, the optimization of the center of the expansion implemented in the new
> solver is designed to accelerate the convergence with l, so that it should not
> be necessary to go to lmax=60 in most cases.  All this is described in
> https://arxiv.org/abs/1307.3135.
>
> Note also that if you're running a 3D problem, the array sizes scale as (lmax+1)^2,
> so part of your problem may be due to choosing an excessive lmax.  You could
> try cutting it down a bit, to see if the problem fits.
>
> Carlo
>
> On 08/11/2016 11:47 AM, Tomasz Plewa wrote:
>> Why not to use the original multipole solver? It is not really any less
>> accurate.
>>
>> Tomek
>> --
>> On 08/11/16 12:41, Klaus Weide wrote:
>>> On Wed, 10 Aug 2016, Rahul Kashyap wrote:
>>>
>>>> Yes, I forgot to mention that I'm using new multipole implementation with
>>>> 60 poles.
>>>>
>>>> I have attached a small txt files with short summary on three runs which
>>>> very well describes my problem. 1024 proc have been used for all runs with
>>>> fixed lrefinemax and base blocks. I get three differenet error for three
>>>> different maxblocks value.
>>>>
>>>> My understanding was that reasonable use of maxblocks avoids any such
>>>> memory failures.
>>> Rahul,
>>>
>>> It appears that the total memory required by
>>>
>>>     PARAMESH Grid + multipole solver ( + Particles + other units )
>>>
>>> is just too much; I suspect that this is PRIMARILY due to the memory
>>> requirements of the Multipole(_new) solver.
>>>
>>> There are several large arrays allocated, see in particular statements
>>> like
>>>
>>>     allocate (gr_mpoleScratch (1:gr_mpoleMaxLM,1:gr_mpoleMaxQ  ),...)
>>>
>>> in gr_mpoleAllocateRadialArrays.F90, where gr_mpoleMaxQ may be very large
>>> and gr_mpoleMaxLM ~ gr_mpoleMaxL ** 2 ( == 60**2 in your case?).
>>>
>>> Unfortunately this memory is required for each process, you cannot reduce
>>> this by running on more procs.
>>>
>>> It makes sense in general to try to reduce the memory required by the
>>> PARAMESH Grid by lowering maxblocks, but this can go only so far;
>>> maxblocks has to leave room for "a few" more blocks than the number
>>> actually required by the distributed grid. These additional slots
>>> are needed for temporary storage during processing by some internal
>>> PARAMESH routines for things like block redistribution. I don't
>>> know of a reliable way to predict how low "a few" can go in a given
>>> case, so this has to be determined empirically. Apparently,
>>> this was too low in your maxblocks=5 case.
>>>
>>> It may be possible to tweak the maxblocks value further, possibly in
>>> connection with also modifying the values of maxblocks_alloc and
>>> maxblocks_tr (see amr_initialize.F90 and paramesh_dimensions.F90),
>>> in order to allow the Grid domain initialization to proceed with
>>> maxblocks < 10; but this may then still not give you enough free memory
>>> for the multipole solver (and particles, etc).
>>>
>>> So you should investigate ways to lower the memory requirements of the
>>> Poisson solver; you may have to lower the resolution (not sure which
>>> runtime parameters to change), or perhaps use a different implementation.
>>>
>>> Klaus
>

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