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engine: note that --cpus does not change the reported CPU count #26348

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engine: note that --cpus does not change the reported CPU count
manunicholasjacob Oct 14, 2026
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engine: name GNU coreutils 9.8 as the nproc version that reads the quota
manunicholasjacob Oct 15, 2026
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28 changes: 28 additions & 0 deletions content/manuals/engine/containers/resource_constraints.md
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Expand Up @@ -174,6 +174,34 @@ Which is the equivalent to manually specifying `--cpu-period` and `--cpu-quota`;
$ docker run -it --cpu-period=100000 --cpu-quota=50000 ubuntu /bin/bash
```

The `--cpus`, `--cpu-period`, and `--cpu-quota` flags limit how much CPU time
the container gets. They don't change the number of CPUs that the operating
system reports inside the container. On a host with 20 CPUs, a container
started with `--cpus=2` still reports 20 online CPUs:

```console
$ docker run --rm --cpus=2 ubuntu getconf _NPROCESSORS_ONLN
20
```

Programs that size a thread pool from this count, for example with Python's
`os.cpu_count()`, start more busy threads than the quota can run at the same
time, and the kernel throttles the container for most of each period. Some
tools also read the quota, so their output depends on the image. GNU coreutils
9.8 and later make `nproc` follow a cgroup v2 CPU quota, and the `nproc` in
the `ubuntu:26.04` image reports 2 in the same container. The `nproc` in
`ubuntu:24.04` (GNU coreutils 9.4) reports 20.

To keep the thread count in line with the limit, set the program's thread
count explicitly, for example with `OMP_NUM_THREADS` or a command-line option.
For programs that read their CPU affinity, such as `nproc`, you can instead
restrict the container to specific CPUs with `--cpuset-cpus`:

```console
$ docker run --rm --cpuset-cpus=0,1 ubuntu nproc
2
```

### Configure the real-time scheduler

You can configure your container to use the real-time scheduler, for tasks which
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