Hi Michal, I think that your reasoning is not fully correct: - Gecode's quicksort uses O(log n) stackspace: it will always push the larger array onto the stack, which will be at least n/2 long! That's O(log n). - There is no difference (in principle) whether you use a strict or non-strict ordering (as the complement will be the opposite).
The only problem I see is that 32 was not good enough. Could you be so kind and retry what happens with replacing 32 by "sizeof(int) * 64"? Then, how much memory does your machine have? Then, how deep does the stack grow for Quicksort and how many elements does the array have you try to sort? Thanks Christian -- Christian Schulte, www.it.kth.se/~cschulte/ -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Michal D. Sent: Saturday, December 13, 2008 7:03 PM To: [email protected] Subject: [gecode-users] Quicksort bug Hi All, I had one hell of a bug hunt recently. What started as a crash on winxp x64 ended up in a "true" being changed to a "false" on freebsd (so I could sanely debug into the Gecode code base). The culprit was FullTupleCompare (gecode/int/extensional/tuple-set.cc) but quicksort also got hit in the cross-fire. FullTupleCompare implements a less than or equal (<=) operator and not a strict less than (<) operator. In effect, for the case where both tuples are identical up the arity it returns true instead of false. This causes quicksort massive problems. See tuple-set.cc.diff for the fix. The gecode quicksort assumes that its stack will never exceed a depth of 32. This was getting exceeded when used on a large tupleset with the above comparison operator. However, in general this assumption is wrong. Stack usage for quicksort is worst case O(n) depending on the values that are chosen for the pivot! Exceeding the stack current results in an outright crash as an out of bounds pointer is dereferenced. I toyed with two fixes: 1) make push return a success/failure code and if the push fails then call quicksort recursively instead of pushing onto the stack. See sort.icc.diff. 2) start off with an static stack and expand it dynamically if it's exceeded. I'm not sure if Memory::malloc() / Memory::free() are the right ways to go about grabbing memory. There is no performance penalty if the stack depth does not exceed 32 entries. See sort.icc.diff2. Both solutions have the overhead of checking if the stack is going to exceed 32 entries on a push. This doesn't seem significant especially since most of the work should be happening in partition. My vote is for fix # 2 as it seems to be the more robust solution (no potential stack overflow). Cheers, Michal _______________________________________________ Gecode users mailing list [email protected] https://www.gecode.org/mailman/listinfo/gecode-users
