## After CSC16 (cont’)

Umit V. Çatalyürek presented our work on the directed acyclic graph (DAG) partitioning problem. In this problem, we are given a DAG $G=(V,E)$ and an integer $k\geq 2$. The aim is to partition the vertex set $V$ into $k$ parts $V_1,\ldots, V_k$ in such a way that the parts have (almost) equal weight and the sum of the costs of all those arcs having their endpoint in different parts minimized. Vertices can have weights, and the edges can have costs. Up to now, all is standard. What is not standard is that the quotient graph of the parts should be acyclic. In other words, the directed graph $G'=(V', E')$, where $V'=\{V_1,\ldots,V_k\}$ and $V_i\rightarrow V_j\in E'$ iff $v_i\rightarrow v_j\in E$ for some $v_i\in V_i$ and $v_j\in V_j$, should be acyclic.

John R. Gilbert wanted to understand the complexity of the problem, with respect to the undirected version. He is an expert on the subject matter (see, e.g., [2]). He asked what happens if we orient the edges of the $n\times n$ model problem. If you are not familiar with this jargon, it is the $n\times n$ mesh with each node being connected to its immediate neighbor in the four main directions, if those neighbors exist. See the small image for an $8\times 8$ example.

Partitioning these kind of meshes is a very hard problem. Gary Miller had mentioned their optimal partitioning in his invited talk (about something else). Rob Bisseling [Ch. 4, 1] has a great text about partitioning these meshes and their cousins in 3D. I had surveyed known methods in a paper with Anaël Grandjean [3]. In particular, Anaël found about discrete isoperimetric problems, showed that the shape of an optimal partition at a corner, or inside the mesh was discussed in the related literature. He also showed that the Cartesian partitioning is optimal for edge cut.  Anaël also proposed efficient heuristics which produced connected components. See the cited papers for nice images. Our were based on our earlier work with Umit [4].

Anyhow,  let’s return back to acyclic partitioning of DAGs, and John’s question. He suggested that we should look at the electrical spiral heater to get an orientation. This orientation results in a total order of the vertices. The figures below show the ordering of the $8\times 8$ and the $16\times 16$ meshes. Only some of the edges are shown; all edges of the mesh, including those that are not shown are from the lower numbered vertex to the higher numbered one.

As seen in the figures above, the spiral ordering is a total order and there is only one way to cut the meshes into two parts with the same number of vertices; blue and red show the two parts.

Theorem: Consider the $n\times n$ mesh whose edges are oriented following the electrical spiral heater ordering. The unique acyclic cut with $n^2/2$ vertices in each side has $n^2-4n+3$ edges in cut, for $n\geq 8$.

The theorem can be proved by observing that the blue vertices in the border (excluding the corners) has one arc going to a red vertex; those in the interior, except the one labeled $n^2/2$ has 2 such arcs;  the vertex labeled $n^2/2$ has three such arcs. The condition $n\geq 8$ comes from the fact that we assumed that there are blue vertices in the interior of the mesh. This is a lot of edges to cut!

Later, John said that he visited the Maxwell Museum of Anthropology at UNM after the CSC16 workshop, and saw that similar designs by the original native New Mexicans.

### References

1. Rob H. Bisseling, Parallel Scientific Computation: A Structured Approach using BSP and MPI, 1st ed, Oxford University Press, 2004.
2. John R. Gilbert, Some nested dissection order is nearly optimal. Inf. Process. Lett. 26(6): 325–328 (1988).
3. Anaël Grandjean and Bora Uçar, On partitioning two dimensional finite difference meshes for distributed memory parallel computers. PDP 2014: 9–16.
4. Bora Uçar and Umit V. Çatalyürek, On the scalability of hypergraph models for sparse matrix partitioning. PDP 2014: 593–600.
5. Da-Lun Wang and Ping Wang, Discrete isoperimetric problems, SIAM Journal on Applied Mathematics, 32(4):860–870 (1977).

## After CSC16

CSC16 was held last week. Kudos to Assefaw and Erik as the chair of the workshop.

There are so much to talk about. We will have a series of posts about the workshop and related things. Here are some bits and pieces.

The workshop had three invited talks, 19 contributed talks, and eight posters, and attended by 60+ people. There will be a proceedings with 11 papers. The proceedings will be published by SIAM and will be hosted at its publication platform.

We had also celebrated the 60th birthdays of Alex Pothen and Rob Bisseling.

There was a best paper award. It went to Fredrik Manne, Md. Naim, Håkon Lerring, and Mahantesh Halappanavar for their paper titled On Stable Marriages and Greedy Matching. Congratulations. The citation by the best paper award committee (Uwe Naumann, Alex Pothen, and Sivan Toledo) reads as:

for the way the paper brings together several decades of work on stable marriages with the more recent work on approximation algorithms for weighted matchings, and the consequences for the average case complexity of the latter algorithms.

A heads up: the CSC18 meeting will likely be in Bergen, Norway. Erik cracked a joke about this in saying that the best paper awardees should take on organizing the next meeting.