[gecode-users] Quicksort bug

[Michal D.]

Here's another nugget of information. These are traces of the
variables l and r  between lines 135 and 136 of sort.icc. As you can
see l stays constant while r is jumping all over the place. Compiled
using the <= comparison with maxsize set to "sizeof(int) * 64".

34533544336 34533544656
34533544336 34533544624
34533544336 34533544640
34533544336 34533544608
34533544336 34533544624
34533544336 34533544592
34533544336 34533544608
34533544336 34533544576
34533544336 34533544592
34533544336 34533544560
34533544336 34533544576
34533544336 34533544544
34533544336 34533544560
34533544336 34533544528
34533544336 34533544544
34533544336 34533544512
34533544336 34533544528
34533544336 34533544496
34533544336 34533544512

Michal

On Mon, Dec 15, 2008 at 4:32 PM, Christian Schulte <cschulte at kth.se> wrote:
> 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: users-bounces at gecode.org [mailto:users-bounces at gecode.org] On Behalf
> Of Michal D.
> Sent: Saturday, December 13, 2008 7:03 PM
> To: users at gecode.org
> 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
>
>