Adding requirements to the Cactus scheduler

Problem Outline

One of the currently most complex aspects of programming with Cactus is writing schedule.ccl files for new routines, in particular if mesh refinement is used. The basic problem is that it is very difficult to ensure that routines are executed in the correct order, i.e. that all grid variables which are required for a routine are actually calculated beforehand. It is also difficult to ensure that boundary conditions (and synchronisation and symmetry boundaries) are applied when needed, in particular after regridding.

The Cactus schedule consists of several independent "parts": There are schedule bins defined by the flesh, there are schedule groups defined by infrastructure thorns (e.g. MoL or HydroBase), and there is the recursive Berger-Oliger algorithm traversing the bins implemented in Carpet. It is for the end-user difficult to see which groups are executed when and on what refinement level, and in which order this occurs.

The Cactus schedule offers "before" and "after" clauses to ensure a partial ordering between routines. Unfortunately, this ordering applies only to routines within the same schedule group and the same schedule bin and refinement level. It is not possible to ensure a particular order between routines in different schedule groups or schedule bins, and it is very complex to ensure that a routine is executed e.g. after another routine has been executed on all refinement levels.

There is one example setup that illustrates this problem. When setting up initial conditions for a hydrodynamics evolution, one may e.g. want to first set up a neutron star, then calculate its maximum density, and then set up the atmosphere to a value depending on this maximum density. Making this possible in Cactus required introducing a new schedule bin "postpostinitial" to the flesh, and requires careful arrangement of schedule groups defined by ADMBase and HydroBase. Even now that this is possible, it is probably not possible to ensure at run time that these actions occur in a correct order.

Suggested Solution

To resolve this issue, and to generally simplify the way in which schedule.ccl files are designed and written, I suggest the following:

• Each scheduled routine declares which grid variables it reads and which grid variables it writes
• Since most routine write only parts of grid variables, the routine would also specify which part it reads/writes, e.g. the interior, outer boundary, symmetry boundary, etc.
• This allows the Cactus scheduler in a first step to validate the schedule and detect cases where a required variable has not been defined, or where a variable is calculated multiple times or synchronized multiple times
• In a second step this will also allow the Cactus scheduler to completely derive the schedule from these declarations. This may even make it possible to execute routines in parallel if they are independent. Even SYNC statements can be automatically derived, and schedule groups would not be necessary any more.

One particular issue arises with routines which modify a variable, e.g. imposing the constraint that <math>\tilde A^i_i=0</math>. These routines read and write the same variable, and it is thus not immediately clear why they should be executed or in which order they should be executed. To resolve this, I suggest to add a tag to variables, declaring that this routine "reads Aij:original" and writes "Aij:constraints-enforced". Each other routine accessing this variables would then also need to declare whether it reads or writes the original Aij or the Aij with constraints enforced.

Another issue arises with loops in the schedule. This is currently mostly used by MoL for the sub-timesteps. I have currently no good idea for handling this, and I suggest to punt and implement a special case for this.

Current State

There is a patch to the flesh that allows adding REQUIRES and PROVIDES clauses to the schedule block for every routine. These clauses can be arbitrary strings (there is no syntax checking done), and they are stored in the schedule database of the flesh and are ignored by default. There is a suggestion to rename these clauses to READS and WRITES; this has not yet been done.

Carpet has a file Requirements.cc that detects the presence of these clauses and performs rudimentary checks. These checks are probably useless in their current form.

Next Steps

To bring this project further, we need to define how the "reads" and "writes" clauses should look like. As mentioned above, it is insufficient to list only grid variables there, since most routines access only parts of grid variables. Ian Hinder volunteered to come up with an initial plan for what kinds of "parts" there should be (e.g. interior, outer boundary, symmetry boundary, ghost zone, etc.).

Since current schedules, even for WaveToy, are already very complex, it would be beneficial to have an example code with a very simple schedule, i.e. a schedule that does not use coordinates, boundary conditions, symmetries, or MoL. (Since these thorns are probably required by Carpet, they would still be active, but the example code itself would be independent of these.) We are looking for a volunteer for this.