# Restart

In this tutorial we explain how to restart a simulation from a certain time point and to make a simulation restartable from the last timestep of the simulation run.

## How to Make it Possible to Restart the Simulation?

You are preparing your simulation with writing the musubi.lua file and you have not yet run it. To make a restart possible you have to do the following:

1. At first, you have to create the restart folder in your simulation directory with mkdir restart.
2. Add the following code to your musubi.lua file somewhere at the end:
restart = {
write = 'restart/',
time_control = {
min = { iter = rest_start },
max = { iter = tmax_phy },
interval = { iter = interval_phy }
}
}


After that, you are able to test if the restart table is working. For every timestep, Musubi writes each a .lua file and a .lsb file to ./restart/. For example, here is a lua file for a random timestep:

 binary_name = {
'restart/channel_9.021E-03.lsb'
}
solver_configFile = 'musubi.lua'
mesh = './mesh/'
weights = ''
time_point = {
sim =    9.021097956087902E-03,
iter = 5,
clock =  103.734125999999996E-03
}
nElems = 2048
nDofs = 1
solver = 'Musubi_v2.0'
varsys = {
systemname = 'fluid_incompressible',
variable = {
{
name = 'pdf',
ncomponents = 19,
state_varpos = { 1, 2, 3, 4, 5, 6, 7, 8,
9, 10, 11, 12, 13, 14, 15, 16,
17, 18, 19 }
}
},
nScalars = 19,
nStateVars = 1,
nAuxScalars = 4,
nAuxVars = 2
}


It refers itself to the binary file which is the .lsb file.

Now it is possible to restart your simulation from each timestep that is available in the restart folder.

## Restart your Simulation from any Timestep

If you wish to restart from this random timestep, you will have to change the restart table in musubi.lua to:

restart = {
write = 'restart/',
time_control = {
min = { iter = rest_start },
max = { iter = tmax_phy },
interval = { iter = interval_phy }
}
}


## Restart your Simulation right after the Last Timestep

If you wish to restart your simulation from the last timestep when data was written to the restart folder, you will have to change:

read = 'restart/channel_header_9.021E-03.lua',


to

read = 'restart/channel_lastHeader.lua',


Musubi uses a pretty easy syntax in this case for restart files. The filename is "simulation_name"+"lastHeader.lua".

Now you have learned how to restart a simulation from any time point you wish to. This is very useful while you make use of expensive ressources like supercomputers or while it takes a long time to simulate to time x and you wish to go on from time x to time y.

Next chapter: Initial Conditions →