.. _no_mpi_jobs:


Non-MPI Jobs
=================================

In GRAVITON, non-MPI jobs refer to those that **do not require inter-node communication** or distributed memory execution. These are typically serial jobs (single process), or multi-process jobs that run entirely within a single node or across isolated CPU cores.

Examples include:
- Serial programs that run on a single core
- Shared-memory parallel jobs using OpenMP or multi-threading (e.g., Python multiprocessing, ROOT TThread)

Recommended QOS
---------------

To submit non-MPI jobs, you must use one of the QOS types reserved for this purpose: ``cosmo``, ``hep``, or ``std``. These QOS values determine the job's scheduling behavior and resource limits. The differences between them are described in the architecture section of the documentation.

Example 1: Multiple Independent Tasks 
---------------------------------------

Below is a **GRAVITON-compliant** SLURM batch script for running a non-MPI job:

.. code-block:: bash

   #!/bin/bash
   #SBATCH --job-name=myjob
   #SBATCH --output=slurm_logs/job_%j.out
   #SBATCH --error=slurm_logs/job_%j.err
   #SBATCH --qos=cosmo
   #SBATCH --ntasks=25
   #SBATCH --cpus-per-task=1
   #SBATCH --time=10:00:00

   ./my_program

**Explanation**

- ``--ntasks=25``: Requests 25 independent processes, each running ``./my_program`` once (useful for embarrassingly parallel workloads).
- ``--cpus-per-task=1``: Each task will use a single CPU core.
- ``--qos=cosmo``: Assigns the job to a serial-friendly partition.
- ``--time=``: Defines the job's maximum runtime.

In this example, SLURM will allocate enough resources (e.g., a node or part of a node) to launch 25 tasks, and execute ``./my_program`` 25 times concurrently (one per task).

Submitting the job:

.. code-block:: bash

   sbatch job.sh

SLURM will manage the task distribution internally, and all standard output/error will be written to the files specified with ``--output`` and ``--error``.


Example 2: Single Task Using Multiple CPU Cores (Multithreaded or OpenMP)
---------------------------------------------------------------------------

Below is a **GRAVITON-compliant** SLURM batch script for running a non-MPI job that uses multiple CPU cores within a single task (e.g., via OpenMP, multithreading, or Python multiprocessing):

.. code-block:: bash

   #!/bin/bash
   #SBATCH --job-name=my_multicore_job
   #SBATCH --output=slurm_logs/job_%j.out
   #SBATCH --error=slurm_logs/job_%j.err
   #SBATCH --qos=hep
   #SBATCH --ntasks=1
   #SBATCH --cpus-per-task=25
   #SBATCH --time=10:00:00

   ./my_program

**Explanation**

- ``--ntasks=1``: Only one task is launched.
- ``--cpus-per-task=25``: That single task is allocated 25 CPU cores to run a multi-threaded or parallelized workload.
- ``--qos=hep`` and ``--time=`` as in previous examples.

This pattern is ideal for programs that:

- Use multi-threading (OpenMP, TBB, etc.)
- Spawn multiple processes within one job (e.g., Python `multiprocessing`)
- Internally manage task parallelism (e.g., MadGraph, Pythia with internal multicore support)

Job submission remains the same:

.. code-block:: bash

   sbatch job.sh

Example 3: Double memory
-------------------------

Below is a **GRAVITON-compliant** SLURM batch script for running a non-MPI job that uses multiple CPU cores **and requests double memory per core**:

.. code-block:: bash

   #!/bin/bash
   #SBATCH --job-name=my_highmem_job
   #SBATCH --output=slurm_logs/job_%j.out
   #SBATCH --error=slurm_logs/job_%j.err
   #SBATCH --qos=hep
   #SBATCH --ntasks=25
   #SBATCH --cpus-per-task=1
   #SBATCH --constraint=double_mem
   #SBATCH --time=10:00:00

   ./my_program

**Explanation**

- ``--ntasks=25`` and ``--cpus-per-task=1``: Launches a single task with 25 CPU cores.
- ``--constraint=double_mem``: Requests **double the default memory per core**, which in this case results in **7.6 GB × 25 cores = 190 GB**.
- ``--qos=hep``: Indicates the job will run in the serial partition, since the core count does not exceed 56.

Submit the job as usual:

.. code-block:: bash

   sbatch job.sh