AMPL running cplex wont go past first node.

[Erin Mullin]

Im new to ampl and cplex, and I received a model from a colleague of mine. I cant get it to run through to completion though.  Additionally even though I've specified in the options for cplex to use all 32 cpu's its only using 2. 

Any advice would be greatly appreciated.

[Robert Fourer]

We ran out of memory trying to solve your example with the setting threads=32. However we were successful with threads=1 and threads=2 as you can see in the attached listing. You can see that threads=2 did not speed up the solution very much. Most of the computation was in root node processing, which is not readily parallelizable; the tree search, which is the part of the algorithm that lends itself to distribution over multiple threads, was easy for threads=1 and not even needed for threads=2.

Each extra thread does create a requirement for more memory, so in general it is a good idea to try threads=1, 2, 4, 8, ... rather than trying to use a very large number of threads from the beginning. Even for problems that require CPLEX to spend a substantial amount of time in the tree search, there tends to be limited benefit to adding threads past 8 or 16 -- see for example the mixed-integer linear programming benchmarks at http://plato.asu.edu/bench.html.

Bob Fourer
am...@googlegroups.com

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