Next: 9.6.4 Concurrent Formulation Up: 9.6.3 proto-Cdyn - Simulation Previous: Template Structure

Problem Preformulation

In general, the ``optimal'' ordering for the equations of a dynamic simulation will in general be too difficult to establish, because of the NP-hard issues involved in structure selection. However, many important heuristics can be applied, such as those that precedence-order the nonlinear equations, and those that permute the Jacobian structure to a more nearly triangular or banded form [Duff:86a]. For the proto-Cdyn simulator, we skirt these issues entirely, because it proves easy to arrange a network of columns to produce a ``good structure''-a main block tridiagonal Jacobian structure with off-block-diagonal structure for the intercolumn connections, simply by taking the distillation columns with their states in tray-by-tray, top-down (or bottom-up) order.

Given a set of DAEs, and an ordering for the equations and states (i.e., rows and columns of the Jacobian, respectively), we need to partition these equations between the multicomputer nodes, according to a two-dimensional process grid of shape . The partitioning of the equations forms, in main part, the so-called concurrent database. This grid structure is illustrated in [Skjellum:90d, Figure 2.]. In proto-Cdyn, we utilize a single process grid for the entire Concurrent DASSL calculation. That is, we do not currently exploit the Concurrent DASSL feature which allows explicit transformations between the main calculational phases (see below). In each process column, the entire set of equations is to be reproduced, so that any process column can compute not only the entire residual vector for a prediction calculation, but also, any column of the Jacobian matrix.

A mapping between the global and local equations must be created. In the general case, it will be difficult to generate a closed-form expression for either the global-to-local mapping or its inverse (that also require storage). At most, we will have on a hand a partial (or weak) inverse in each process, so that the corresponding global index of each local index will be available. Furthermore, in each node, a partial global-to-local list of indices associated with the given node will be stored in global sort order. Then, by binary search, a weak global-to-local mapping will be possible in each process. That is, each process will be able to identify if a global index resides within it, and the corresponding local index. A strong mapping for row (column) indices will require communication between all the processes in a process row (respectively, column). In the foregoing, we make the tacit assumption that it is an unreasonable practice to use storage proportional to the entire problem size N in each node, except if this unscalability can be removed cheaply when necessary for large problems.

The proto-Cdyn simulator works with templates of specific structure-each template is a form of a distillation tray and generates the same number of integration states. It therefore skirts the need for weak distributions. Consequently, the entire row-mapping procedure can be accomplished using the closed-form general two-parameter distribution function family described in [Skjellum:90d], where the block size B is chosen as the number of integration states per template. The column-mapping procedure is accomplished with the one-parameter distribution function family also described in [Skjellum:90d]. The effects of row and column degree-of-scattering are described in [Skjellum:90d] with attention to linear algebra performance.

Next: 9.6.4 Concurrent Formulation Up: 9.6.3 proto-Cdyn - Simulation Previous: Template Structure

Guy Robinson
Wed Mar 1 10:19:35 EST 1995