sym_dynamic_var_ordering - control the application of dynamic variable ordering


sym_dynamic_var_ordering [-d] [-e <method>] [-f <method>] [-h]

Control the application of dynamic variable ordering to the flattened network. Dynamic ordering is a technique to reorder the MDD variables to reduce the size of the existing MDDs. When no options are specified, the current status of dynamic ordering is displayed. At most one of the options -e, -f, and -d should be specified. The commands flatten_hierarchy and static_order must be invoked before this command.

Dynamic ordering may be time consuming, but can often reduce the size of the MDDs dramatically. The good points to invoke dynamic ordering explicitly (using the -f option) are after the commands build_partition_mdds and print_img_info. If dynamic ordering finds a good ordering, then you may wish to save this ordering (using write_order) and reuse it (using static_order -s) in the future. A common sequence used to get a good ordering is the following:

  init_verify
  print_img_info
  dynamic_var_ordering -f sift
  write_order 
  flatten_hierarchy
  static_order -s input_and_latch -f 
  build_partition_mdds
  print_img_info
  dynamic_var_ordering -f sift
  
Command options:

-d
Disable dynamic ordering from triggering automatically.

-e <method>
Enable dynamic ordering to trigger automatically whenever a certain threshold on the overall MDD size is reached. "Method" must be one of the following:

window: Permutes the variables within windows of three adjacent variables so as to minimize the overall MDD size. This process is repeated until no more reduction in size occurs.

sift: Moves each variable throughout the order to find an optimal position for that variable (assuming all other variables are fixed). This generally achieves greater size reductions than the window method, but is slower.

The following methods are only available if VIS has been linked with the Bdd package from the University of Colorado (cuBdd).

random: Pairs of variables are randomly chosen, and swapped in the order. The swap is performed by a series of swaps of adjacent variables. The best order among those obtained by the series of swaps is retained. The number of pairs chosen for swapping equals the number of variables in the diagram.

random_pivot: Same as random, but the two variables are chosen so that the first is above the variable with the largest number of nodes, and the second is below that variable. In case there are several variables tied for the maximum number of nodes, the one closest to the root is used.

sift_converge: The sift method is iterated until no further improvement is obtained.

symmetry_sift: This method is an implementation of symmetric sifting. It is similar to sifting, with one addition: Variables that become adjacent during sifting are tested for symmetry. If they are symmetric, they are linked in a group. Sifting then continues with a group being moved, instead of a single variable.

symmetry_sift_converge: The symmetry_sift method is iterated until no further improvement is obtained.

window{2,3,4}: Permutes the variables within windows of n adjacent variables, where "n" can be either 2, 3 or 4, so as to minimize the overall MDD size.

window{2,3,4}_converge: The window{2,3,4} method is iterated until no further improvement is obtained.

group_sift: This method is similar to symmetry_sift, but uses more general criteria to create groups. group_sift_converge: The group_sift method is iterated until no further improvement is obtained.

annealing: This method is an implementation of simulated annealing for variable ordering. This method is potentially very slow.

genetic: This method is an implementation of a genetic algorithm for variable ordering. This method is potentially very slow.

-f <method>
Force dynamic ordering to be invoked immediately. The values for method are the same as in option -e.

-h
Print the command usage.

Last updated on 980624 22h11