# 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¶

New in 5.4.0. It is no longer necessary to set the Makefile variable RESTART to True or False. Instead you can set it to None (or omit setting it at all, since this is the default), in which case the setrun.py file will be used to determine if this is a restart run (in which case the previous output directory should be added to, rather than replaced).

## 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.

New in 5.4.0. Gauge output from the original run is no longer overwritten by the second run. Instead gauge output from the restart run will be appended to the end of each gaugeXXXXX.txt file. (If multiple restarts are performed from the same checkpoint file then these will accumulate in an undesirable fashion, but for for most purposes this does the right thing.)