Currently, a thorn writer must declare the maximum number of timelevels for a variable in the interface.ccl file, then allocate a specific number of timelevels for which storage is allocated in the schedule.ccl file. There are rules for how many timelevels a variable should have. For example, to evolve using MoL I think you need at least two, whereas to evolve using mesh refinement with time prolongation order 2 you need three.
The problem is that the person writing the thorn shouldn't have to know how it is going to be used. If I write an evolution thorn, I should not care whether it is used with mesh refinement or not. I certainly shouldn't have to care what prolongation order is going to be used.
Would it be possible for Cactus to automatically determine the number of timelevels needed for a given variable? My proposal is that it should not be necessary for the user to specify the number of timelevels in the interface.ccl or schedule.ccl file for variables declared in that thorn. The only time a thorn writer should have to do that is if that thorn specifically uses the other timelevels. For example, a thorn which couples to MoL will probably only ever read or write to the current timelevel. MoL, on the other hand, could tell Cactus that it needs a certain number of timelevels at runtime for those variables. Similarly for mesh refinement.
This goes along with the planned changes to the scheduling system to make it easier to program Cactus thorns and make it harder to make errors.