Restarting simulation

Tristan v2 allows simulations to be paused and continued later. For that the code can store all the field and particle data into separate files. When restarted, the simulation can read all the data from that file and initialize all the relevant quantities as if it never stopped running in the first place.

Saving restart files #

Directory where the code stores all the restart finales in binary format is specified at runtime with the flag -r:

./tristan-mp2d ... -r [CHECKPOINT_DIRECTORY]

If not specified, the CHECKPOINT_DIRECTORY will default to just restart.

Rest of the configurations for the restart are specified in the input file:

<restart>
  do_restart    = 0              # perform simulation restart [0]
  cpu_group     = 50             # simultaneously written restart files [50]
  enable        = 0              # enable/disable restart [0]
  start         = 1              # first restart step [0]
  interval      = 1000           # step interval between restarts [10000]
  rewrite       = 1              # 1 = rewrite each restart; 0 = separate restart each time [0]

There are two options for the rewrite variable: you can either create a single restart folder and overwrite it once every interval timesteps (rewrite = 1), or you can create separate folders for each of the restart steps (rewrite = 0). All the directories will be saved in the path specified above – [restart_dir_name]. If rewrite = 1 – there will be just one directory called step_00000/ where all the restart files will be saved. If rewrite = 0 – you’ll see several directories of the form step_*****, where ***** will be the number of the restart.

Restarting simulation #

To restart the simulation with the data stored in the restart directory you need to include the following flag when running:

./tristan-mp2d ... -R [CONTINUE_FROM_DIRECTORY]

do_restart vs enable

To write the restart files (i.e., dump the checkpoint), one needs to put enable = 1 in the <restart> section of the configuration file. To actually perform the restart (i.e., continue the simulation), make sure to change the flag do_restart to 1, otherwise, the code will simply start from step 0.

[CONTINUE_FROM_DIRECTORY] is the directory where all the restart files are stored, i.e., typically it will be of the form: restart/step_*****. Notice that here you’ll also need to specify the subdirectory step_ where all the files are actually stored. That way you can specify which restart point you actually want to use.

Every cpu will be writing its own restart file in the specified directory. Because of that when you actually restart the simulation, you cannot change the meshblock or tile distribution. Allocated sizes of particle tiles can be altered, but not their spatial dimensions.

Also the fundamental scaling parameters, such as the speed of light, the skin depth etc will be read from the restart file, and the values specified in the new input will be ignored. The user-specified parameters, however, will still be determined read from the input, as the userReadInput() function will still be called during the restart. However, user-specific particle and field initializations will be ignored.