[petsc-users] Mesh partitioning and MPI calls

[Tabrez Ali]

Matt you were right

I was stupidly passing the wrong array (column major order) to Metis. My 
code is in Fortran. I am surprised Metis didnt crash before.

Thanks anyway.

Tabrez


On 08/25/2011 11:49 PM, Matthew Knepley wrote:
> On Thu, Aug 25, 2011 at 9:15 PM, Tabrez Ali <stali at geology.wisc.edu 
> <mailto:stali at geology.wisc.edu>> wrote:
>
>     Hello
>
>     I have an unstructured FE mesh which I am partitioning using Metis.
>
>     In the first case I only use the element partitioning info and
>     discard the nodal partitioning info i.e., the original ordering is
>     same as petsc's global ordering. In the second case I do use the
>     nodal partitioning info and nodes are distributed accordingly.
>
>     I would expect that in the 2nd scenario the total number of MPI
>     messages (at the end of the solve) would be lower than the 1st.
>     However I see that opposite is true. See the plot at
>     http://stali.freeshell.org/mpi.png
>
>     The number on the y axis is the last column of the "MPI messages:"
>     field from the -log_summary output.
>
>     Any ideas as to why this is happening. Does relying on total
>     number of MPI messages as a performance measure even make sense.
>     Please excuse my ignorance on the subject.
>
>     Alternatively what is a good way to measure how good the Metis
>     partitioning is?
>
>
> The thing to do here is take a case like 2 proc that can be completely 
> understood, and get down to the details. I
> think there is a probably just a simple misunderstanding here.
>
> The first thing to check is that you are partitioning what you think. 
> By default, Metis partitions the vertices of a graph,
> not elements, Thus you usually have to give Metis the "dual" of your 
> finite element mesh. I would take a small (maybe
> 8 or 10 elements) mesh and look at the original and Metis partitions. 
> If Metis does not look better, something is wrong.
>
>     Matt
>
>     Thanks in advance
>
>     Tabrez
>
>
>
>
> -- 
> What most experimenters take for granted before they begin their 
> experiments is infinitely more interesting than any results to which 
> their experiments lead.
> -- Norbert Wiener

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