matrix memory allocation

[Matt Funk]

Maybe i should clarify a little more.
I guess what i am looking for is something that lets me split up the memory 
allocation within a given processor. 

The example you guys often show (for example on MatCreateMPIAIJ manual page) 
is where the local process owns 3 rows. 

But consider for example if the local process owns 1000 rows. Right now (as i 
can see it) i have two options. Do not allocate memory at all and use 
MatSetValues and allocate memory per row. This is very slow as you guys state 
on your manual pages.
The other option is to allocate all 1000 rows at a time which means extra 
overhead.

However, what i would like to be able to (as an example), make 2 calls for an 
allocation of 200 rows and a second call for an allocation of the remaining 
800 rows.

I do not know if this is possible as i don't know whether all the local stuff 
is stored in on continuous array?

Anyway, i hope this clarifies my question a little more.

thank
mat


On Wednesday 26 March 2008 14:31, Matt Funk wrote:
> Hi,
>
> in order to create a sparse MPIMatrix (with preallocated memory) it is
> necessary to give it the number of nonzero entries per row for the local
> and off-diagonal submatrix.
>
> The way i do things right now is that i need to allocate the entire matrix.
> However, my domain is decomposed into boxes where one or more boxes reside
> on the current processor.
>
> So, i was wondering if it is possible to allocate the matrix memory per
> box. Right now what i do is this:
> 1) iterate over the whole domain (every single box) and extract the info i
> need from each box for memory allocation.
> 2) call MatCreateMPIAIJ to create the matrix/allocate memory.
> 3) iterate over the whole domain again to get the info to insert the values
> into the matrix.
>
> Is it possible to instead do something like:
> 1) get info for values and memory allocation needed per box
> 2) allocate memory for x number of rows in global matrix and insert values
> for them (via MatSetValues)
> 3) start at 1) till every box in the domain is covered.
>
> The problem in 2) is that i can insert the values, but there is no memory
> preallocated for it, so it would be slow. If i could allocate it, i think
> it would solve my problem.
>
> I hope i am making sense. By the way, i cannot use the DA construct because
> i have overlapping values.
>
> thanks
> mat