The first combination of options, denoted "standard" or S1, assumes LAMG code version 1.
The second combination of options, denoted "fast" or F, also assumes LAMG code version 1.
This more recent LAMG version includes all the functionality and performance of the earlier version but also includes the processor reduction algorithm described earlier as well as other small performance improvements.
For LAMG one can observe a slow increase in iteration count as the problem size is increased.
A comparison of the iteration counts for LAMG, JCG and AMGIR6 is given in Figure 5.
Significantly, for LAMG and for AMG1R6, the growth rate of the iteration counts as the problem size is increased is roughly the same, indicating the same basic algorithmic scaling behavior of the two codes.
Significantly, the maximum average nonzeros per row, which is a measure of matrix sparsity and its impact on communication for the parallel case, is strongly bounded for LAMG, showing very small growth with problem size, while for AMG1R6 this quantity grows very rapidly.
It should also be noted that the number of levels for LAMG is much less than for AMGIR6.
For these experiments we use LAMG with settings S2 applied to 3-D Poisson problems on 1, 8, 27 and 64 processors.