In order to make great use of a super computer one needs to be able to tell it what should be done.
And there comes [SLURM](https://slurm.schedmd.com/documentation.html) as a workload manager for HPCs.
There is a quickstart guide [here](https://slurm.schedmd.com/quickstart.html) and in the sections below some information on how to solve the most pressing needs.
```bash
squeue
JOBID PARTITION NAME USER ST TIME NODES NODELIST(REASON)
```
What does that mean? The Queue is empty and is waiting for your jobs.
So let's give the computer something to think about.
Open your favorite Editor (Gedit, Emacs, vim, nano, ...) and put the code below into the file, save it as `test.job`
```bash
#!/bin/bash
# SLURM specific commands
#SBATCH --job-name=test
#SBATCH --output=test.log
#SBATCH --ntasks=1
#SBATCH --time=01:30
# Your Code below here
srun hostname
srun sleep 60
```
Now we submit this job to the queue:
```bash
sbatch test.job
```
and look agin into the queue
```
JOBID PARTITION NAME USER ST TIME NODES NODELIST(REASON)
211 compute test_mpi mblasche R 0:02 1 jet03
```
It is running on jet03 and about a minute later it should be finished and you should see the log file `test.log` with something like that in it:
```
jet03.jet.local
```
The hostname of the node running our test script. Of course that is a silly job. However, there are things to learn for this example:
1. Writing a `job` file and using the `#SBATCH` commands to specify
*`job-name`, this is just for you
*`output`, this is just for you
*`ntasks`, this tells slurm how many threads you should be allowed to have
*`time`, this tells slurm how much time it should give your job
2. Submit the job with `sbatch`
3. Check the Queue and Status of your job with `squeue`
## Writing a job file
There are things to consider when writing a job
## Interactive Session
It is possible to ask for an interactive session on any node. When your job runs you can always `ssh` to that node and maybe check things, although your file system is available everywhere, there is no need for that.
An interactive session can be started like this
How to run an interactive job
```bash
srun --pty bash -i
```
The following options also work with non-interactive jobs (sbatch, salloc, etc.)
How to run on a specific node
```bash
# use the -w / --nodelist option
srun -w jet05 --pty bash -i
```
How to acquire each node exclusively
```bash
# use the --exclusive option
srun -w jet05 --exclusive--pty bash -i
```
## MPI
How to submit an MPI batch job
```bash
#!/bin/bash
# Some example sbatch options.
# See also https://slurm.schedmd.com/sbatch.html
#SBATCH --job-name=testprog
#SBATCH --output=testprog.out
#SBATCH --ntasks=7
# Load the OpenMPI module.
module load openmpi/4.0.5-gcc-8.3.1-773ztsv
# Run the program with mpirun. Additional mpirun options
# are not required. mpirun will get the info (e.g., hosts)
