SAT and CSP competitions & benchmark libraries: some lessons learnt?

1. SAT and CSP competitions & benchmark libraries:some lessons learnt? Toby Walsh NICTA & UNSW Sydney, Australia

2. Whats the best way to benchmark systems?

3. Outline • Benchmark libraries • Founding CSPLib.org • Competitions • SAT competition judge • TPTP competition judge • …

4. Why? • Why did I set up CSPLib.org • I needed problems against which to benchmark my latest inference techniques • Zebra and random problems don’t cut it! • I thought it would help unify and advance the CP community

5. Random problems • +ve • Easy to generate • Hard (if chosen from phase transition) • Impossible to cheat • You can solve 1000 variable random 3SAT problems at l/n=4.2, I’ll be impressed

6. Random problems • -ve • Lack structures found in real world • Unrepresentative • E.g. random 3SAT either have many solutions or none • Different methods work well on them • Random SAT: forward looking algorithms • Industrial SAT: backward looking algorithms

7. Why? • Thesis: every mature field has a benchmark library • Deduction started in 1960s • TPTP set up in 1993 • SAT started in 1960s • SAT DIMACS challenge in 1992 • SATLib set up in 1999 • CP started in 1970s • CSPLib set up in 1998

8. Why? • Thesis: every mature field has a benchmark library • Spatial and temporal reasoning started in early 80s (or before?) • It’s been approximately 30 years so it’s about time you guys set one up!

9. Benchmark libraries • CSPLib.org • Over 35k unique visitors • Still not everything I’d want it to be • But state of the art for experimentation is now much better than it was • I haven’t seen a zebra for a very long time

10. An ideal library • Desiderata taken from: • CSPLib: a benchmark library for constraints, Proc. CP-99

11. An ideal library • Location • On the web and easy to find • TPTP.org • CSPLib.org • SATLib.org • QBFLib.org • … • http://elib.zib.de/pub/mp-testdata/tsp/tsplib/tsplib.html • http://mat.gsia.cmu.edu/COLOR/instances.html

12. An ideal library • Easy to use • Tools to make benchmarking as painless as possible • tptp2X, … • Diverse • To help prevent over-fitting

13. An ideal library • Large • Growing continuously • Again helps to prevent over-fitting • Extensible • To new problems or domains

14. An ideal library • Complete • One stop for your problems • Topical • For instance, it should report current best solutions found

15. An ideal library • Independent • Not tied to a particular solver or proprietary input language • Mix of difficulties • Hard and easy problems • Solved and open problems • With perhaps even a difficulty index?

16. An ideal library • Accurate • It should be trusted • Used • A valued resource for the community

17. Problem format • Lo-tech or hi-tech?

18. Lo-tech formats • DIMACS format used in SATLib c a simple example p cnf 3 2 1 -1 0 1 2 3 0 This represents: x v -x, x or y or z

19. Lo-tech formats • DIMACS format used in SATLib • +ve • All programming languages can read integers! • Small amount of extensibility built in (e.g. QBF) • -ve • Larger extensions are problematic (e.g. beyond CNF to arbitrary Boolean circuits)

20. Hi-tech formats • CP competition <instance> <presentation name="4-queens" description="This problem involves placing 4 queens on a chessboard" nbSolutions="at least 1" format="XCSP1.1 (XML CSP Representation 1.1)" /> <domains nbDomains="1"> <domain name="dom0" nbValues="4" values="1..4" /> </domains> <variables nbVariables="4"> <variable name="X0" domain="dom0"/> … </variables> <relations nbRelations="3"> <relation name="rel0" domain="dom0 dom0” nbConflicts="10 conflicts="(1,1)(1,2)(2,1)(2,2)(2,3)(3,2)(3,3)(3,4)(4,3)(4,4)" /> … </relations > <constraints nbConstraints="6"> <constraint name="C0" scope="X0 X1" relation="rel0"/> …

21. Hi-tech formats • XML • +ve • Easy to extend • Parsing tools can be provided • -ve • Complex and verbose • Computers can parse terse structures easily

22. No-tech formats • CSPLib • Problems are specified in natural language • No agreement at that time for an input language • One focus was on how you model a problem • Today there is more consensus on modelling languages like Zinc

23. No-tech formats • CSPLib • Problems are specified in natural language • But you can still provide in one place • Input data • Results • Code • Parsers …

24. Getting problems • Submit them yourself • Initially, you must do this so library has some critical mass first time people look at it • But it becomes tiresome and unrepresentative to do so continually • Ask at every talk • Tried for several years but it (almost) never worked

25. Getting problems • Need some incentive • Offer money? • Price of entry for the competition? • If you have a competition, users will submit problems that their solver is good at?

26. Competitions

27. Libraries + Competitions • You can have a library without a competition • But you can’t have a competition without a library

28. Libraries + Competitions • Libraries then competition • TPTP then CASC • Easy and safe! • Libraries and competition • Planning • RoboCup • …

29. Increasing complexity • Constraints • 1st year, binary extensional • 2nd year, limited number of globals • 3rd year, unlimited • Planning • Increasing complexity • Time, metrics, uncertainty, …

30. Benefits • Gets ideas implemented • Rewards engineering • Progress needs both science and engineering! • Puts it all together

31. Benefits • Gives greater importance to important low-level issues • In SAT: • Watched literals • VSIDS • …

32. Benefits • Witness the progress in SAT • 1985, 10s vars • 1995, 100s vars • 2005, 1000s vars • … • Not just Moore’s law at play!

33. Pitfalls • Competitions require lots of work • Organizers get limited (academic) reward • One solution is to organize also competition special issues

34. Pitfalls • Competitions encourage incremental improvements • Don’t have them too often! • You may discover a local minimum • E.g. MDPs for speech recognition • Give out best new solver prize?

35. The Chaff story • Industrial problems, SAT & UNSAT instances • 2008, 1st MiniSAT (son of zChaff) • 2007, 1st RSAT (son of MiniSAT) • 2006, 1st MiniSAT • 2005, 1st SatELite GTI (MiniSAT+preprocessor) • 2004, 1st zChaff (Forklift from 2003 was better) • 2003, 1st Forklift • 2002, 1st zChaff

36. Other issues • Man-power • Organizers • One is not enough? • Judges • All rules need interpretation • Compute-power • Find a friendly cluster

37. Other issues • Multiple tracks • SAT/UNSAT • Random/industrial/crafted • … • Certificate/Uncertificated

38. Other issues • Holding problems back if possible • Release some problems so competitors can ensure solver compliance • But hold most back so competition is blind!

39. Other issues • Multiple phases • Too many solvers for all to compete with long timeouts • First phase to test correctness • Second phase to throw out the slow solvers (who cost you many timeouts) • Third phase to differentiate between better solvers

40. Other issues • Reward function • <#completed, average time, …> • solution purse + speed purse • Points for each problem divided between those solvers that solve it • Getting buy in from competitors • It will (and should) evolve over time!

41. Other issues • Prizes • Give out many! • Good for people’s CVs • Good motivator for future years

42. Other issues • Open or closed source? • Open to share progress • Closed to get the best • Last year’s winner • Condition of entry • To see progress is being made!

43. Other issues • Smallest unsolved problem • Give a prize! • Timing • Run during the conference • Creates a buzz so people enter next year • Get a slot in program to discuss results • Get a slot in banquet to give out prizes

44. Conclusions • Benchmark libraries • When an area is several decades old, why wouldn’t you have one? • Competitions • Designed well, held not too frequently, & with buy-in from the community, why wouldn’t you?

45. Questions • Disagreements • Other opinions • Different experiences • …