
Domain # Solved # Steps # Best **Execution Time # Best** DriverLog 14 1.04 8 1.**50 0** ZenoTravel 13 1.04 10 1.**54 0** Satellite 17 1.04 9 2.**51 0**

Rovers 7 1.04 5 1.**39 0**

Table 12: Same information as in Table 10, but for domains with durative actions.

While VHPOP was a top performer at IPC3 in terms of plan quality, it was far **from**
the top in terms of planning time. VHPOP **was typically orders of magnitude slower than** the fastest planner. The high planning times for VHPOP **can in part be attributed to**
implementation details. Improvements to the code (e.g. using pointer comparison instead of string comparison whenever possible) since the planning **competition has resulted in 10 to** 20 percent lower planning times when using ground actions and when using lifted actions the planner is more than twice as fast as before. The reachability analysis is still a bottleneck, however, and further improvements could definitely be made there. It is important to remember, though, that we basically run four planners at once by using four flaw selection strategies concurrently. Table 14 shows the average relative performance of VHPOP at IPC3 compared to the performance of VHPOP **using only the best flaw selection strategy** for each problem. VHPOP **with the best flaw selection strategy is on average two to three**
times faster than VHPOP **with four concurrent strategies. Using several flaw selection**
strategies simultaneously helps us solve more problems, but the price is reduced speed. By more intelligently scheduling the different flaw selection strategies depending on domain and problem features, and not just using a fixed schedule for all problems, we could potentially increase planner efficiency significantly.

| Planner   | Depots   | DriverLog   | ZenoTravel   | Satellite   | Rovers   | Total   |
|-----------|----------|-------------|--------------|-------------|----------|---------|
| LPG       | 20       | 20          | 20           | 20          | 12       | 92      |
| MIPS      | 11       | 15          | 14           | 9           | 9        | 58      |
| TP4       | 1        | 2           | 5            | 3           | 4        | 15      |
| TPSYS     | 0        | 2           | 2            | 2           | 4        | 10      |
| VHPOP     | 3        | 14          | 13           | 17          | 7        | 54      |

## 6. Discussion

McDermott (2000) finds the absence of POCL planners at the first planning competition in 1998 striking, as such planners had been dominating planning research just a few years earlier. "It seems doubtful that the arguments in [POCL planners'] favor were all wrong, and it would be interesting to see partial-order planners compete in future competitions", McDermott writes. After two competitions without POCL planners, **we believe that** VHPOP's performance at IPC3 demonstrates that POCL planning—at least with ground actionscan be competitive with CSP-based and heuristic state space **planning.** VHPOP **also shows**