# Run many jobs on one GPU

## Goal

Pack several PlayMolecule jobs onto a single GPU on a SLURM cluster using NVIDIA MPS, so a small workload doesn't waste a whole device.

## Minimal example

```python
from playmolecule import slurm_mps
from playmolecule.apps import deepsite

eds = [
    deepsite(outdir=f"./run-{i}", pdbfile=f"protein_{i}.pdb")
    for i in range(8)
]

slurm_mps(eds, partition="normalGPU", ncpu=1, ngpu=1)
```

Every {py:class}`~playmolecule.ExecutableDirectory` in `eds` is submitted as a single SLURM job — via {py:func}`~playmolecule.slurm_mps` — that holds one GPU and shares it across the jobs through NVIDIA's Multi-Process Service.

## When to use it

- Small GPU jobs (a few seconds to a few minutes) where individual SLURM submissions would burn more time queueing than running.
- Workloads where one GPU has plenty of memory for several processes — e.g., parameter sweeps over the same model.

Don't use MPS for jobs that are individually GPU-saturating: they'll just serialise without benefit.

## Parameters

`slurm_mps(exec_dirs, **kwargs)` accepts the same SLURM kwargs as `ed.run(queue="slurm", ...)` — `partition`, `ncpu`, `ngpu`, `memory`, `walltime`, `nodelist`, `exclude`, `envvars`, `prerun`, the mail options, and the stream options. See [Run an app on SLURM](run-an-app-on-slurm.md) for the table.

The resource defaults come from the **first** {py:class}`~playmolecule.ExecutableDirectory` in the list (its `execution_resources`, which were copied from the app manifest at setup time). If you mix apps with different defaults, pass `ncpu` / `ngpu` explicitly to be safe.

## Gotchas

- All `ExecutableDirectory`s must live on a shared filesystem (same rule as plain SLURM).
- MPS needs to be enabled on the chosen partition's nodes. Talk to your cluster admin if `slurm_mps` jobs fail with "Failed to start MPS" in their logs.
- The single SLURM job runs to completion when *all* batched jobs finish; one slow job holds the GPU for the others. Group by expected runtime.

## See also

- {py:func}`~playmolecule.slurm_mps`
- [Run an app on SLURM](run-an-app-on-slurm.md)