# Fast assembly via hierarchical matrices

Assembling boundary integral operators is expensive since the matrices are generally dense. If $\displaystyle N$ is the number of elements in the grid, the overall complexity of the assembly grows like $\displaystyle \mathcal{O}(N^3)$ and the cost of a matrix-vector product grows like $\displaystyle \mathcal{O}(N^2)$. This makes large problems infeasible on standard hardware.

In recent years two techniques have been established to speed-up the assembly of boundary integral operators: fast multipole methods (FMM) and hierarchical matrix (H-matrix) techniques. Bempp currently implements the latter one. It brings down the cost of assembly and matrix-vector products for integral operators to $\displaystyle \mathcal{O}(N\log N)$, making even larger problems with hundreds of thousands of degrees of freedom possible on a single workstation.

## Introduction to hierarchical matrices

Hierarchical matrices are a complex topic. For a good introduction we recommend
the MPI lecture notes by Börm, Grasedyck & Hackbusch
and the book Hierarchical Matrices by Hackbusch.

## Using hierarchical matrix assembly

Fast H-matrix assembly of boundary and potential operators is enabled by default in Bempp. The corresponding options are bempp.api.global_parameters.assembly.boundary_operator_assembly_type and bempp.api.global_parameters.assembly.potential_operator_assembly_type. By default both are set to hmat, which enables H-matrix assembly. For very small problems it is advisable to change the boundary operator assembly type to dense as there is a larger overhead of H-matrix techniques for small problems.

A range of parameters controls the H-matrix assembly itself. In the following, we summarise all parameters that control the assembly.

The most important parameter is the accuracy parameter bempp.api.global_parameters.hmat.eps. This specifies the accuracy of the H-matrix approximation, and by default is set to 1E-3. The best value is problem dependent.

For very large problems, it is advisable to increase the value of bempp.api.global_parameters.hmat.max_block_size. As default it is set to 2048. This restricts the largest size an admissible matrix block can have that will be low-rank approximated. Choosing a very large value can have negative effect on the load-balancing between the different cores during assembly. Choosing a too small value can lead to a significant increase in overall computational complexity.

By default, Bempp uses a coarsening strategy to post-process the hierarchical matrix assembly and further reduce the amount of memory an H-matrix requires. This post-processing is based an randomised low-rank approximations. In most of our experiments, it increased the assembly time by around 10% and often reduced the memory consumption by close to 50%. However, if it is not desired then the parameter bempp.api.global_parameters.hmat.coarsening should be set to False.

The accuracy of the coarsening strategy is by default the same as the assembly. Other values can be set by modifying bempp.api.global_parameters.hmat.coarsening_accuracy.

## Querying hierarchical matrices

Operators assembled via H-matrices mostly behave in the same way as operators assembled using dense matrix techniques and the interface is identical. However, it is often desirable to query advanced information from H-matrices. This is implemented in the module bempp.api.hmatrix_interface.

Let us assemble a simple H-matrix operator over a regular sphere.

import bempp.api
grid = bempp.api.shapes.regular_sphere(4)
space = bempp.api.function_space(grid, “DP”, 0)
discrete_operator = bempp.api.operators.boundary.laplace.single_layer(
space, space, space).weak_form()

Since H-matrix assembly is automatically active we do not have to do anything else. We can now collect some information on the H-matrix.

To find out the memory consumption in kB we can use the following command.

print(bempp.api.hmatrix_interface.mem_size(discrete_operator))
9560.0

This will show the amount of storage that the H-matrix data needs (not that it does not count storage of administrational data such as the underlying tree structure).

The total number of blocks in the H-matrix is given by

print(bempp.api.hmatrix_interface.number_of_blocks(discrete_operator))
5536

Similar commands exist for the number of dense blocks and the number of low-rank blocks.

Bempp also gives access to the complete underlying tree structure. To obtain the block cluster tree the following command can be used

tree = bempp.api.hmatrix_interface.block_cluster_tree(discrete_operator)

To plot the tree call:

tree.plot() This command requires that PyQt4 is installed.

An iterator over all leaf nodes of the tree is given by tree.leaf_nodes. The root node of the tree is obtained by:

root = tree.root

From the root node, the tree can be traversed hierarchically. The children of a node are available through an iterator. Hence, to iterate through the four children of root use:

for child in root.children:
print((child.row_cluster_range, child.column_cluster_range))
((0, 1024), (0, 1024))
((0, 1024), (1024, 2048))
((1024, 2048), (0,1024))
((1024,2048), (1024, 2048))

This prints out the ranges of the four subnodes of root. Subnodes are traversed in C-style order. Hence, if at the first level the H-matrix has the form $A = \begin{bmatrix} A_{11} & A_{12}\\ A_{21} & A_{22} \end{bmatrix},$

the children iterator returns the nodes in the order $\displaystyle A_{11}$, $\displaystyle A_{12}$, $\displaystyle A_{21}$, $\displaystyle A_{22}$.

To get from the leaves of the block cluster tree to the data associated with the leaves the function bempp.api.hmatrix_interface.data_block can be used.

For example, the following code makes a histogram plot of the ranks of the data for all admissible nodes in the above created H-matrix.

import matplotlib.pyplot as plt 