Specifies how much fill should be generated for ILU. The default is 10.
Recent optimizations have provided more room for tradeoffs in performance between the quality of the preconditioner and the expense of computing the preconditioner. The total time for a iterative linear solve is the time to compute/apply the preconditioner plus the number of iterations needed for accelerator times the time per iteration. The default fill provides more space to compute a good preconditioner at the expense of more memory usage and more time to compute/apply the preconditioner. It can be reduced, but the number of accelerator iterations may begin to climb and the overall time for a linear solve may increase.
The SLES linear solver will print iteration counts, total flops, and memory usage (memory usage is somewhat misleading because it does not include all the space used by linear solver). The best performance is typically achieved if the accelerator iterations is kept small (less than 3) instead of allowing it to creep into the 15-50 range.
It is highly recommended to tune the fill parameter to the model and dimensionality of the mesh if a large number of runs will be made using the model with similar parameters and similar initial conditions. There can easily be a factor of 2 improvement with an optimal fill.