# Checkpoint / restart¶

Enzo-E can perform checkpoint dumps to save the current state, and can read them at initialization to continue a simulation where it left off. Setting up checkpointing involves including methods in the Method parameter group, and provided a schedule for them. Restarting requires adding a small number of parameters to a parameter file to turn on restart, and where the checkpoint files are located.

## Checkpoint¶

Method {
...
list = [ "order_morton", "check", ... ];
order_morton {
schedule { var = "cycle";  start = 5;  step = 5;  }
}
check {
schedule { var = "cycle";  start = 5;  step = 5;  }
dir = [ "Check-%02d", "cycle" ];
num_files = 4;
ordering = "order_morton";
}
...
}


This example writes checkpoint dumps every 5 cycles, starting with the 5th cycle.

See the input/Checkpoint/test_cosmo-checkpoint.in parameter file for a working example of writing checkpoint directories.

Note

Currently, there is a restriction that the domain blocking must be square (2D) or cubical (3D), have a power-of-two blocking along the axes, and the Adapt:min_level parameter must be set such that the coarsest (negative) level is a single block. For example, if Mesh:root_blocks = [ 16, 16, 16], then Adapt:min_level must be initialized to 4 ( $$= log_2 16$$ ).

## Restart¶

To restart from a checkpoint directory, edit a copy of the original parameter file to add two parameters, Initial:restart and Initial:restart_dir. For example, to restart the problem in the previous “Checkpoint” section at cycle 50, one would add the following:

Initial {

restart = true;
restart_dir = "Check-50";
}


Note that it’s also possible to rerun with some parameters modified, for example different linear solver convergence criteria or different mesh refinement criteria. The main limitation is that the simulation data cannot change on restart: you cannot add new fields, new particle types or attributes, or change the number of root-level blocks or block sizes. You can restart with more (or fewer) processors, though the constraint that the number of processors must be at most the number of root-level blocks must still be satisfied.

The parameter file input/Checkpoint/test_cosmo-restart.in is the “restart” counterpoint to the “checkpoint” example mentioned in the previous section.