Hi, that sounds ugly ;-) Before I start with rambling about cloning, a tip: Gecode supports unary branchings (with a single alternatiuve only). So you could have a unary branching and don't have to have a mock-up empty branch. The search engines even optimize that case. If you are looking for an example, check the assign function in gecode/int.hh for assigning variables.
Okay, cloning it is. So, there are two stages: copying and updating. During copying you first create a new space and copy all actors. While copying all actors you copy each variable and its domain used by an actor and collect a list of all these variables (actually a list for each variable type). The only thing that you do not do is copying the dependency array of a variable (the array of pointers that point to propagators that depend on a variable). Then in updating, you update the dependency arrays (and also deal with advisors). Updating a dependency array just copies for each old entry the new entry (stored via a forwarding pointer in each actor) and restore some information for variables that has been stored in the copy of the dependency array. After that you cleanup the space, the actors, and you are done. In VarImp<VIC>::enter, the --idx[0] makes room for an additional entry. Then you keep moving actors to a free entry and by this create a new free entry for the propagation condition i. When the loop is done, you know that there is a free entry for propagation condition pc and that's where the propagator is entered. Your guess about x->idx[0] in VarImp<VIC>::update is correct: this actually stores the forwarding pointer for a variable implementation. Also your suspicion about ActorLink::prev is correct: there the forwarding pointer for actors is stored. Let me come up with some idea where the trouble could be: - you forget to update a variable or view at some place. Either in your branching, propagator, or model. Then you can see an invalid pointer in the dependency array. You might want to change the default constructor of VarBase in gecode/kernel/var.icc and the default constructor of VarViewBase in gecode/kernel/view.icc so that they set varimp to NULL (actually, I just did that yesterday in the trunk as well). Then you will be sure that when you forget to update, you'll see a NULL pointer. - you commit a heinous crime in that you change the dependencies of a variable with cancel/subscribe during cloning. That is illegal and will for sure break everything. Note that creating a propagator during cloning will commit that crime, too. It is very very unlikely (but not impossible, of course) that the cloning in Gecode has a snatch but, as said, very unlikely given that its used by anything else. Hope that helps. Please do not hesitate to ask some more Christian -- Christian Schulte, www.ict.kth.se/~cschulte/ -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Filip Konvicka Sent: Thursday, February 12, 2009 2:21 PM To: [email protected] Cc: Luboš Moric Subject: [gecode-users] Cloning problems Hi, [Sorry, this is a looong message...] we're hunting a serious bug that occurs during space cloning in 2.2.0. The bug occurs very rarely, but we have a testcase that triggers this behavior. We have many constraints in the problem instance and the solver should post as many propagators as possible. We have a custom branching for this, which posts one propagator at a time in commit(), while the alternative is not to post the propagator (i.e. a no-op). Because we're only looking for the first solution, in the case of a failure we no longer need the path back to the root in the recomputation tree, so we decided to use our own simple search engine for this. The standard DFS search engine exhibits exactly the same behavior (both with recomputation on and off), and we don't see any problems with our search engine. Everything seems to work for the vast majority of the test cases, but there are a few instances that cause problems (probably) during cloning (can be probably also be caused by some earlier bad subscibe or unsubscribe). From our point of view, there is nothing wrong or special about the instances. The crashes occur at the same location both on Linux and Windows, in both release and debug builds. Changing memory management (e.g. never deleting Spaces in the search engine) can cause the crash to occur at slightly different places (e.g. some propagation during status() after clone() finishes). One particular case we're looking at now crashes at core.icc:2270, where f[0] is a bad pointer (0xfeeefeee at Windows). We're not sure how this can happen - we know that in this case n==2 at core.icc:2255, so idx[0] is bad pointer at core.icc:2252. This is also what Valgrind says on Linux (bad read of size 4). When we were trying to debug the other cases, we found out that the subscription list in a variable in the cloned space contained an actor link that was probably copied incorrectly as it seemed as a pure ActorLink like Space::a_actors, having a totally different address than the rest of the actors (probably belonging to the original space object). When we tried to find out when this actor link entered the list, we ended up in VarImp<VIC>::update again. We're (of course:-)) using FloatVars in the model, and we eliminated all other kinds of variables and propagators. In our case, pc_max==1 and free_bits==0. We find it difficult to understand what is happening during cloning. We would appreciate if someone explaned the basic idea. We only have floatvars, propagators and one branching (no advisors or other types of actors/branchings/advisors). We know how VarImp<VIC>::resize works, that's easy. In VarImp<VIC>::enter, we can't see why you do "--idx[0];" as the first iteration of the for cycle overwrites it (as long as pc>0, of course). May be just optimization of course. As for VarImp<VIC>::update, we only guess...we suspect that a) the original x->idx[0] is destroyed somewhere so it needs to get restored from a memcpy backup at idx[0], b) ActorLink::_prev is probably used to map old actors to new ones (thus the "->prev()". We did not dig deep enough to be sure though, so we'd welcome some guidance here. Cheers, Filip _______________________________________________ Gecode users mailing list [email protected] https://www.gecode.org/mailman/listinfo/gecode-users _______________________________________________ Gecode users mailing list [email protected] https://www.gecode.org/mailman/listinfo/gecode-users
