How to use warm starts in Minizinc?

I'm trying to use the warm start anotation in Minizinc to give a known suboptimal solution to a model.

I started by trying to execute this warm start example from the Minizinc documentation (the only one they provide):

array[1..3] of var 0..10: x;
array[1..3] of var 0.0..10.5: xf;
var bool: b;
array[1..3] of var set of 5..9: xs;
constraint b+sum(x)==1;
constraint b+sum(xf)==2.4;
constraint 5==sum( [ card(xs[i]) | i in index_set(xs) ] );
solve
  :: warm_start_array( [                     %%% Can be on the upper level
    warm_start( x, [<>,8,4] ),               %%% Use <> for missing values
    warm_start( xf, array1d(-5..-3, [5.6,<>,4.7] ) ),
    warm_start( xs, array1d( -3..-2, [ 6..8, 5..7 ] ) )
  ] )
  :: seq_search( [
    warm_start_array( [                      %%% Now included in seq_search to keep order
      warm_start( x, [<>,5,2] ),             %%% Repeated warm_starts allowed but not specified
      warm_start( xf, array1d(-5..-3, [5.6,<>,4.7] ) ),
      warm_start( xs, array1d( -3..-2, [ 6..8, 5..7 ] ) )
    ] ),
    warm_start( [b], [true] ),
    int_search(x, first_fail, indomain_min)
  ] )
  minimize x[1] + b + xf[2] + card( xs[1] intersect xs[3] );

The example runs, and it gets the optimal solution. However, the output displays warnings stating all the warm start anotations were ignored.

Warning, ignored search annotation: warm_start_array([warm_start([[xi(1), xi(2)], [i(5), i(2)]]), warm_start([[xf(0), xf(2)], [f(5.6), f(4.7)]]), warm_start([[xs(0), xs(1), xs(2)], [s(), s()]])])
Warning, ignored search annotation: warm_start([[xb(0)], [b(true)]])
Warning, ignored search annotation: warm_start_array([warm_start([[xi(1), xi(2)], [i(8), i(4)]]), warm_start([[xf(0), xf(2)], [f(5.6), f(4.7)]]), warm_start([[xs(0), xs(1), xs(2)], [s(), s()]])])

I didnt modified anything in the example, just copy-pasted it and ran it in the Minizinc IDE with the Geocode default solver. In case it is relevant, I'm using Windows. I have ran other models and used other search anotations without problems.

In the example there is two blocks of warm stars (one after solve and one inside seq_search). I'm not sure if both are necessary. I tried removing one, then the other, but the warnings still happen for all the remaining warm start anotations. Also I dont get why 'b' isnt refered in the fisrt block.

There is a similar example in git https://github.com/google/or-tools/issues/539 but it also produces the warnings.

If someone could point me out to a working example of warm_start it would be great.

Solution

Your usage of the warm_start annotations are correct, but warm start annotations are currently not supported in most solvers. At the time of writing I believe the warm start annotations are only supported by the Mixed Integer Programming interfaces (CoinBC, Gurobi, CPlex, XPress, and SCIP). Although we've been working on adding support for the annotation in Gecode and Chuffed, support for this annotation has not been included in any of the released versions.