# 2.13. Restart example¶

Micromagnetic simulations can last for many hours or even many days. It is then important to be able to save periodically the state of the simulation, in such a way that, if a hardware failure or a power cut occurs, the simulation can be restarted exactly at the point where its state was last saved. In this example we show how an nmag script can be modified to be “restartable”. The only thing the user needs to do is to periodically save the state of the simulation in what we call a “restart file”. The simulation can then be restarted using the appropriate command line option.

The restart feature applies only to the hysteresis method.

## 2.13.1. Saving the state of the simulation¶

We re-consider the cubic anisotropy example (Cubic anisotropy simulation script) and replace the last line:

sim.hysteresis(Hs)


with the following lines:

from nmag import every, at
sim.hysteresis(Hs, save=[('averages', 'fields', at('stage_end')),
('restart', at('stage_end') | every('step', 1000))])


The first two lines reproduce the default behaviour: the fields and their averages are saved at the end of each stage. The third line specifies that the restart file should be saved at the end of each stage and also every 1000 steps.

For convenience the modified script cube_restartable.py is shown below:

import nmag
from nmag import SI, si

# Create the simulation object
sim = nmag.Simulation()

# Define the magnetic material (data from OOMMF materials file)
Fe = nmag.MagMaterial(name="Fe",
Ms=SI(1700e3, "A/m"),
exchange_coupling=SI(21e-12, "J/m"),
anisotropy=nmag.cubic_anisotropy(axis1=[1, 0, 0],
axis2=[0, 1, 0],
K1=SI(48e3, "J/m^3")))

# Set the initial magnetisation
sim.set_m([0, 0, 1])

# Launch the hysteresis loop
Hs = nmag.vector_set(direction=[1.0, 0, 0.0001],
norm_list=[0, 1, [], 19, 19.1, [], 21, 22, [], 50],
units=0.001*si.Tesla/si.mu0)
from nmag import every, at
sim.hysteresis(Hs, save=[('averages', 'fields', at('stage_end')),
('restart', at('stage_end') | every('step', 1000))])


## 2.13.2. Starting and restarting the simulation¶

We will now demonstrate how the discussed nmag script can be restarted. To do that, we will have to interrupt it artificially. We start the simulation in the usual way:

$nsim cube_restartable.py  We interrupt the execution after the hysteresis loop has started and several stages have been computed. Do this by pressing simultaneously the keys CTRL and C (in the same terminal window where nsim was started), thus simulating what could have been the result of a power cut. We then use the command: $ ncol cube_restartable stage step time


to see at what point of the hysteresis loop the simulation was interrupted. We obtain (for this particular interruption):

 1            330  3.320127110062e-11
2            480  5.042492488627e-10
3            640  9.926580643272e-10
4            805  1.464971830453e-09
5            980  1.927649646634e-09
6           1150  2.406521613682e-09
7           1340  2.882400372552e-09
8           1515  3.371522550051e-09
9           1705  3.863380029345e-09
10           1920  4.365560120394e-09
11           2095  4.893234441813e-09
12           2295  5.436617525896e-09
13           2480  5.997866344586e-09
14           2680  6.570733097131e-09
15           2890  7.172534305054e-09
16           3100  7.803577637245e-09
17           3315  8.462827284047e-09


The simulation was interrupted at the seventeenth stage. We now try to run the simulation again with the command:

$nsim cube_restartable.py  obtaining the following output: <snip> NmagUserError: Error: Found old file ./cube_restartable_dat.ndt -- cannot proceed. To start a simulation script with old data files present you either need to use '--clean' (and then the old files will be deleted), or use '--restart' in which case the run will be continued.  nsim suggests the possible alternatives. We can start the simulation from scratch with the command (but this will override any data from the previous run): $ nsim cube_restartable.py --clean


or we can continue from the configuration which was last saved:

\$ nsim cube_restartable.py --restart


Here we choose the second possibility. After the simulation has finished we issue again the command ncol cube_restartable stage step time, obtaining the following output:

    1            330  3.320127110062e-11
2            480  5.042492488627e-10
3            640  9.926580643272e-10
4            805  1.464971830453e-09
5            980  1.927649646634e-09
6           1150  2.406521613682e-09
7           1340  2.882400372552e-09
8           1515  3.371522550051e-09
9           1705  3.863380029345e-09
10           1920  4.365560120394e-09
11           2095  4.893234441813e-09
12           2295  5.436617525896e-09
13           2480  5.997866344586e-09
14           2680  6.570733097131e-09
15           2890  7.172534305054e-09
16           3100  7.803577637245e-09
17           3315  8.462827284047e-09
stage           step          #time
<>             <>           #<s>
18           3715  8.519843629989e-09
19           3975  9.300878866142e-09
...


The two lines between stage 17 and 18 stand as a reminder that the simulation was restarted at that point. (They need to be removed manually from the cube_restartable_dat.ndt file, before ncol can work in the usual way on the ndt file.)