Local Search Techniques for Temporal Planning in LPG Paper by Gerevini, Serina, Saetti, Spinoni...
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Transcript of Local Search Techniques for Temporal Planning in LPG Paper by Gerevini, Serina, Saetti, Spinoni...
Local Search Techniques for Temporal Planning in LPG
Paper by Gerevini, Serina, Saetti, Spinoni
Presented by Alex
Overview of Problem
• What problem are they solving?– Temporal planning (duh)– More specifically? What kinds of action
restrictions?
Overview of Techniques
• In general, what techniques do they use?– Local search, variation of Walksat– Mix of Graphplan and POP– Complicated heuristics
Outline
• Search Space
• Local search and heuristics– Search operators– Various cost metrics
• Experiments
And TA-graphs
• What is a TA-graph?– Triple (LA-graph, Time-assign’t, Ordering Cs)
• What do the time assignments represent?– Earliest time a fact can become true– Earliest time an action can finish executing
How does it fit into Graphplan?
• Only permanent action mutexes – meaning?
• What good are action mutexes in an LA-graph?
• How much of Graphplan is left?
POP & LPG
• POP• Threats
– Promotion
– Demotion
• Causal Link• Least Commitment
• LPG• Mutexes
– LA-graph ordering
• Action-Effect edge• Topological Sort (as
opposed to total order of LA-graph)
Solution State
• What constraints does a TA-graph have to obey to be a solution?– Logical constraints (are goals and action
preconditions satisfied)– Temporal/Ordering constraints
• Time assign’ts consistent with orderings
• Ordering constraints imply mutex actions don’t co-occur
Local Search: Operators
• Every search operator fixes an “inconsistency” in the partial plan– What kinds of inconsistencies are there?
• How do you fix inconsistencies?
Local Search: Walksat
• Proposition with k clauses and n variables – With probability p, pick a variable at random
from an unsatisfied clause and flip its value– With prob. 1-p, flip value of variable that
maximizes number of satisfied clauses
WalkPlan
• For a given inconsistency, if there is a move that increases plan quality, do it.
• Otherwise, – with probability p, pick a move at random– With probability 1-p, pick best move
• Do random restarts after too many moves
Questions about WalkPlan
• How do they pick the inconsistency to fix?
• Why do they change the Walksat algorithm?
• Where do they start search after a random restart?
Relaxed Plan
• When adding (or removing) action a, find relaxed plan for unsupported preconditions of a (or the preconditions that used to be supported by a, if removing a).
• In what way(s) is it relaxed?
Experiments
• Impressive, no?
• Are there any criticisms? caveats?– Is it fair to compare separately along the time
and quality axes? I.e., is a quickly-generated but very crappy plan better than a more slowly-generated but decent plan?