# Checkpointing and restarting¶

Warning

These instructions currently only apply to amrclaw and geoclaw codes.

## Checkpointing a computation¶

In this section clawdata refers to the rundata.clawdata attribute of an object of class ClawRunData, as is generally set at the top of a setrun.py file.

The rundata.clawdata.checkpt_style parameter specified in setrun.py (see Specifying classic run-time parameters in setrun.py) determines how often checkpointing is done, if at all.

See the comments in Sample setrun.py module for classic Clawpack for examples.

The checkpoint files are saved in the same output directory as the solution output, with file names of the form fort.tchkNNNNN (a small ASCII file) and fort.chkNNNNN (a large binary file) where NNNNN is the step number on the coarsest level. These files containt all the information needed to restart the computation at this point.

## Restarting a computation¶

To restart a computation from any point where checkpoint files have been saved, modify setrun.py to set:

clawdata.restart = True
clawdata.restart_file = 'fort.chkNNNNN'


where NNNNN is the time step number from which the restart should commence.

You should also modify the output time parameters to specify that the computation should go to a later time than the time of the restart file (which can be found in the file fort.tchkNNNNN).

Note the following in setting the new output times:

• The value clawdata.t0 should generally be left to the original starting time of the computation.

• If clawdata.output_style==1, then clawdata.t0 and clawdata.tfinal along with clawdata.num_output_times are used to determine equally spaced output times. Only those times greater than the restart time will be used as output times.

If clawdata.output_t0==True then a time frame will be output at the restart time (not t0 in general). This may duplicate the final frame that was output from the original computation. Set clawdata.output_t0 = False to avoid this.

• If clawdata.output_style==2, then clawdata.output_times is a list of output times and only those times greater than or equal to the restart time will be used as output times.

## Modifying the Makefile for a restart¶

To restart a computation, it is necessary to modify the Makefile as well as setrun.py. In the Makefile, set:

RESTART = True


This will ensure that the original set of output files (for times up to the restart time) and the checkpoint file will not be deleted. They will still be available in the output directory along with the newly created output files.

## Output files after a restart¶

After running the restarted computation, the original set of output files should still be in the output directory along with a new set from the second run. Note that one output time may be repeated in two frames if clawdata.output_t0 == True in the restarted run.

Note that any gauge output from the first run will be overwritten by the second run. If you wish to preserve the gauge output from the first run, currently you must copy the output file fort.gauge to another location, say fort.gauge1, before doing the restart run. Then you could catenate the two to get gauge output for the entire run, e.g.:

cd _output
mv fort.gauge fort.gauge2
cat fort.gauge1 fort.gauge2 > fort.gauge


This creates a file fort.gauge that contains the entire gauge history.

TODO: This should be simplified.