# Running Interactive Jobs

### The Merlin7 'interactive' partition

On the **`merlin7`** cluster, it is recommended to always run interactive jobs on the **`interactive`** partition.
This partition allows CPU oversubscription (up to four users may share the same CPU) and **has the highest scheduling priority**. Access to this partition is typically quick, making it a convenient extension of the login nodes for interactive workloads.

On the **`gmerlin7`** cluster, additional interactive partitions are available, but these are primarily intended for CPU-only workloads (such like compiling GPU-based software, or creating an allocation for submitting jobs to Grace-Hopper nodes).

!!! warning
    Because **GPU resources are scarce and expensive**, interactive allocations on GPU nodes that use GPUs should only be submitted when strictly necessary and well justified.

## Running interactive jobs

There are two different ways for running interactive jobs in Slurm. This is possible by using
the ``salloc`` and ``srun`` commands:

* **``salloc``**: to obtain a Slurm job allocation (a set of nodes), execute command(s), and then release the allocation when the command is finished.
* **``srun``**: is used for running parallel tasks.

### srun

Is run is used to run parallel jobs in the batch system. It can be used within a batch script
(which can be run with ``sbatch``), or within a job allocation (which can be run with ``salloc``).
Also, it can be used as a direct command (in example, from the login nodes).

When used inside a batch script or during a job allocation, ``srun`` is constricted to the 
amount of resources allocated by the ``sbatch``/``salloc`` commands. In ``sbatch``, usually 
these resources are defined inside the batch script with the format ``#SBATCH <option>=<value>``. 
In other words, if you define in your batch script or allocation 88 tasks (and 1 thread / core) 
and 2 nodes, ``srun`` is constricted to these amount of resources (you can use less, but never 
exceed those limits).

When used from the login node, usually is used to run a specific command or software in an 
interactive way. ``srun`` is a blocking process (it will block bash prompt until the ``srun`` 
command finishes, unless you run it in background with ``&``). This can be very useful to run 
interactive software which pops up a Window and then submits jobs or run sub-tasks in the 
background (in example, **Relion**, **cisTEM**, etc.)

Refer to ``man srun`` for exploring all possible options for that command.

<details>
<summary>[Show 'srun' example]: Running 'hostname' command on 3 nodes, using 2 cores (1 task/core) per node</summary>
<pre class="terminal code highlight js-syntax-highlight plaintext" lang="plaintext" markdown="false">
caubet_m@login001:~> srun --clusters=merlin7 --partition=interactive --ntasks=6 --ntasks-per-node=2 --nodes=3 hostname
cn001.merlin7.psi.ch
cn001.merlin7.psi.ch
cn002.merlin7.psi.ch
cn002.merlin7.psi.ch
cn003.merlin7.psi.ch
cn003.merlin7.psi.ch
</pre>
</details>

### salloc

**``salloc``** is used to obtain a Slurm job allocation (a set of nodes). Once job is allocated,
users are able to execute interactive command(s). Once finished (``exit`` or ``Ctrl+D``), 
the allocation is released. **``salloc``** is a blocking command, it is, command will be blocked 
until the requested resources are allocated. 

When running **``salloc``**, once the resources are allocated, *by default* the user will get 
a ***new shell on one of the allocated resources*** (if a user has requested few nodes, it will 
prompt a new shell on the first allocated node). However, this behaviour can be changed by adding
a shell (`$SHELL`) at the end of the `salloc` command. In example:

```bash
# Typical 'salloc' call 
salloc --clusters=merlin7 --partition=interactive -N 2 -n 2

# Custom 'salloc' call
#   - $SHELL will open a local shell on the login node from where ``salloc`` is running
salloc --clusters=merlin7 --partition=interactive -N 2 -n 2 $SHELL
```

<details>
<summary>[Show 'salloc' example]: Allocating 2 cores (1 task/core) in 2 nodes (1 core/node) - <i>Default</i></summary>
<pre class="terminal code highlight js-syntax-highlight plaintext" lang="plaintext" markdown="false">
caubet_m@login001:~> salloc --clusters=merlin7 --partition=interactive -N 2 -n 2
salloc: Granted job allocation 161
salloc: Nodes cn[001-002] are ready for job

caubet_m@login001:~> srun hostname
cn002.merlin7.psi.ch
cn001.merlin7.psi.ch

caubet_m@login001:~> exit
exit
salloc: Relinquishing job allocation 161
</pre>
</details>

<details>
<summary>[Show 'salloc' example]: Allocating 2 cores (1 task/core) in 2 nodes (1 core/node) - <i>$SHELL</i></summary>
<pre class="terminal code highlight js-syntax-highlight plaintext" lang="plaintext" markdown="false">
caubet_m@login001:~> salloc --clusters=merlin7 --partition=interactive --ntasks=2 --nodes=2 $SHELL
salloc: Granted job allocation 165
salloc: Nodes cn[001-002] are ready for job
caubet_m@login001:~> srun hostname
cn001.merlin7.psi.ch
cn002.merlin7.psi.ch
caubet_m@login001:~> exit
exit
salloc: Relinquishing job allocation 165
</pre>
</details>

## Running interactive jobs with X11 support

### Requirements

#### Graphical access

[NoMachine](../02-How-To-Use-Merlin/nomachine.md) is the official supported service for graphical 
access in the Merlin cluster. This service is running on the login nodes. Check the 
document [{Accessing Merlin -> NoMachine}](../02-How-To-Use-Merlin/nomachine.md) for details about 
how to connect to the **NoMachine** service in the Merlin cluster.

For other non officially supported graphical access (X11 forwarding):

* For Linux clients, please follow [{How To Use Merlin -> Accessing from Linux Clients}](../02-How-To-Use-Merlin/connect-from-linux.md)
* For Windows clients, please follow [{How To Use Merlin -> Accessing from Windows Clients}](../02-How-To-Use-Merlin/connect-from-windows.md)
* For MacOS clients, please follow [{How To Use Merlin -> Accessing from MacOS Clients}](../02-How-To-Use-Merlin/connect-from-macos.md)

### 'srun' with x11 support

Merlin6 and merlin7 clusters allow running any windows based applications. For that, you need to 
add the option ``--x11`` to the ``srun`` command. In example:

```bash
srun --clusters=merlin7 --partition=interactive --x11 sview
```

will popup a X11 based slurm view of the cluster.

In the same manner, you can create a bash shell with x11 support. For doing that, you need
to add the option ``--pty`` to the ``srun --x11`` command. Once resource is allocated, from
there you can interactively run X11 and non-X11 based commands.

```bash
srun --clusters=merlin7 --partition=interactive --x11 --pty bash
```

<details>
<summary>[Show 'srun' with X11 support examples]</summary>
<pre class="terminal code highlight js-syntax-highlight plaintext" lang="plaintext" markdown="false">
caubet_m@login001:~> srun --clusters=merlin7 --partition=interactive --x11 sview

caubet_m@login001:~> 

caubet_m@login001:~> srun --clusters=merlin7 --partition=interactive --x11 --pty bash

caubet_m@cn003:~> sview

caubet_m@cn003:~> echo "This was an example"
This was an example

caubet_m@cn003:~> exit
exit
</pre>
</details>

### 'salloc' with x11 support

**Merlin6** and **merlin7** clusters allow running any windows based applications. For that, you need to 
add the option ``--x11`` to the ``salloc`` command. In example:

```bash
salloc --clusters=merlin7 --partition=interactive --x11 sview
```

will popup a X11 based slurm view of the cluster.

In the same manner, you can create a bash shell with x11 support. For doing that, you need
to add to run just ``salloc --clusters=merlin7 --partition=interactive --x11``. Once resource is allocated, from 
there you can interactively run X11 and non-X11 based commands.

```bash
salloc --clusters=merlin7 --partition=interactive --x11
```

<details>
<summary>[Show 'salloc' with X11 support examples]</summary>
<pre class="terminal code highlight js-syntax-highlight plaintext" lang="plaintext" markdown="false">
caubet_m@login001:~> salloc --clusters=merlin7 --partition=interactive --x11 sview
salloc: Granted job allocation 174
salloc: Nodes cn001 are ready for job
salloc: Relinquishing job allocation 174

caubet_m@login001:~> salloc --clusters=merlin7 --partition=interactive --x11 
salloc: Granted job allocation 175
salloc: Nodes cn001 are ready for job
caubet_m@cn001:~> 

caubet_m@cn001:~> sview

caubet_m@cn001:~> echo "This was an example"
This was an example

caubet_m@cn001:~> exit
exit
salloc: Relinquishing job allocation 175
</pre>
</details>