[CIG-SHORT] Updated PyLith development priorities

[Brad Aagaard]

On Thursday 22 May 2008, Eric Andreas Hetland wrote:
> 1) in the flowchart, "improved PC for kinematic fault condition" leads to
> wave propagation, but in the description it seems that it is needed for
> fault friction models.

The kinematic fault implementation with cohesive cells uses Lagrange 
multipliers which results in a saddle point problem. Traditional wave 
propagation implementations use a lumped (i.e., diagonal) mass matrix. The 
combination of a diagonal matrix and Lagrange multipliers requires special 
considerations for efficiently solving the linear system at each time step 
(i.e., we need a special preconditioner for the system). Dynamic (fault 
friction) with cohesive cells will not use Lagrange multipliers, so we 
shouldn't end up with a saddle point problem.

> 2) restart files is indicated as "Difficult" & "computer science (new
> feature)", while adaptive time stepping is "Easy" & "Geophysics (new
> feature)". This seems backward to me, for instance, I know Geofest has both
> features, but the adaptive time stepping is not really that robust. Are
> there standard algorithms for adaptive time stepping (including non-linear
> rheologies) out there? On the other hand, the restart files work extremely
> well in geofest (also commercial codes I have used), so I thought this was
> a solved problem. When I was at MIT, there was a rule against running any
> program on the cluster that did not support checkpointing.

The green circle, blue square, and black diamond describe the level of effort 
required. In some cases this is the level of difficulty but in other cases 
(e.g., restart files) it is in the amount of work involved.

Checkpointing is absolutely necessary for very long jobs or jobs on large 
clusters. It is less important for jobs that run for less than about 8 hrs 
and clusters with less than ~100 cores.

I hope these explanations help clarify the listed features and the color 
scales.

Brad