Devine, Karen D kddevin at sandia.gov
Tue Jan 9 12:15:56 EST 2018

The PHG partitioner is a hypergraph partitioner that can be used for graph partitioning.  
Before partitioning, it removes dense edges that contain more than 
PHG_EDGE_SIZE_THRESHOLD * total_number_of_vertices 
vertices, as these edges likely will incur high communication costs regardless of the partition.  

Your example is an extreme case.   You are doing graph partitioning (with number of vertices per edge always equal to 2) of a problem with total_number_of_vertices = 2.  Thus, the only edge has all the vertices and is, thus, dense.  There is no balanced partition that can avoid communication across that edge.  So we remove the edge.

I assume this test problem is not a real use case.  If you try a problem with nine elements, you should not see this warning.  Or you can set PHG_EDGE_SIZE_THRESHOLD to 1.0.

The Zoltan user guide has info on Zoltan parameters.  For this issue, see http://www.cs.sandia.gov/zoltan/ug_html/ug_alg_phg.html
From: Trilinos-Users <trilinos-users-bounces at trilinos.org> on behalf of Vishal Boddu <vishal.boddu at fau.de>
Sent: Tuesday, January 9, 2018 6:50 AM
To: trilinos-users at trilinos.org
Cc: Denis Davydov
Subject: [EXTERNAL] [Trilinos-Users] PHG_EDGE_SIZE_THRESHOLD is low

Dear all,

We are using Zoltan (Trilinos 12.12.1) to partition a graph and I don't
quite understand the following warning produced:

WARNING: PHG_EDGE_SIZE_THRESHOLD is low (0.250000), resulting in only 0

These were thrown when I tried to partition a graph (two quadrilaterals
sharing an edge) with two MPI processes. The only place which prints
such warnings is:

I appreciate any help with figuring out what this means and if this
warning is important or it could be ignored?

Kind regards,

Vishal Boddu M.Sc.(hons.)
Chair of Applied Mechanics
University of Erlangen-Nuremberg
Paul-Gordan-Straße 3, 91058 Erlangen
Tel: + 49-9131-85-64410, Fax: + 49-9131-85-28503
vishal.boddu at fau.de | www.ltm.tf.fau.de

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