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How to Limit CPU and Memory Use for Linux Multiprocessing Jobs

A job-wide cgroup boundary is the practical way to limit CPU and memory across a Linux multiprocessing tree. Learn how systemd and Docker controls work, what memory limits do, and how to size Python workers.
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To limit a Linux multiprocessing job as a whole, run it inside a cgroup boundary—typically a systemd scope on a systemd-managed host or a container with Docker resource limits. Set an aggregate CPU quota and a hard memory maximum, then choose a worker count that fits both budgets. A per-process limit or CPU pinning alone does not provide the same job-wide control.

Choose a boundary that covers the whole job

A multiprocessing program can create workers and other descendant processes. Put the launcher and its descendants in one resource-control boundary so the limits apply to the job as a group, rather than trying to configure every worker individually. On a host-managed job, use a systemd scope or service; if the workload already runs in Docker, use the container’s resource controls. Both configure Linux resource-control mechanisms, but the effective limits can also be narrowed by parent cgroups. Check the host’s cgroup version and the settings that actually apply to the job.

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For a job launched directly on a systemd host

This illustrative command starts a transient scope with a CPU quota and memory maximum:

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systemd-run --scope -p CPUQuota=200% -p MemoryMax=4G python job.py

CPUQuota=200% represents a maximum CPU bandwidth equivalent to two CPUs’ worth of runtime. MemoryMax=4G requests a 4 GiB hard memory setting. Actual behavior depends on the systemd version, cgroup configuration, parent limits, and how the unit properties are parsed on the host; verify the resulting unit settings rather than assuming the requested values are the effective ones. See the systemd resource-control documentation.

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For a job already running in Docker

Use container limits at launch, for example:

docker run --cpus=2 --memory=4g IMAGE COMMAND

--cpus sets a CPU access cap, while --memory sets a memory constraint. Do not substitute --cpu-shares for a hard CPU cap: shares are a relative weight that matters when CPU is contested. Check the Docker version and host/runtime configuration. The Docker resource constraints documentation describes these controls.

Does a CPU quota limit cores or CPU time?

A quota limits CPU-time bandwidth over the scheduler’s quota period; it does not pin a process to a particular set of cores. In systemd, CPUQuota=200% means a maximum equivalent to two CPUs’ worth of runtime. By contrast, AllowedCPUs= restricts where tasks may run. The parent cgroup can narrow that CPU list, and EffectiveCPUs= shows the resulting configuration. Affinity can help with locality or keep a job off selected CPUs, but affinity by itself is not an aggregate CPU-time cap. See the systemd resource-control documentation.

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What happens when a cgroup memory limit is reached?

With cgroup v2, memory.high is a pressure and throttling boundary: crossing it causes heavy reclaim pressure, but does not itself invoke the OOM killer. memory.max is the principal hard memory limit. The Linux Kernel Documentation describes it as: “Memory usage hard limit. This is the main mechanism to limit memory usage of a cgroup.” If usage reaches the limit and cannot be reduced, the kernel invokes the OOM killer within the cgroup; usage can temporarily exceed the limit. A hard limit can therefore cause allocations to fail or processes to be terminated. See the Linux kernel cgroup v2 documentation.

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Leave headroom in the memory budget for the parent process, workers, shared-memory objects, libraries, and other processes in the job. A memory limit that matches only the expected worker allocations can still be too tight once the full job is counted.

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How do I choose a Python multiprocessing worker count?

Set the pool size explicitly when you need a predictable starting point, for example Pool(processes=n). For CPU-bound work, a reasonable initial rule is to keep the number of workers within the usable CPU budget. That is only a CPU-oriented starting point, not a memory-sizing formula.

In Python 3.13 and later, Pool(processes=None) uses os.process_cpu_count() by default instead of os.cpu_count(). The former reports the logical CPUs usable by the calling thread and can be lower than the machine-wide count. It can account for CPU affinity, but it does not universally convert a cgroup CPU quota into an ideal worker count. See the Python multiprocessing documentation.

Size for memory as well as CPU

Estimate the parent’s and each worker’s memory footprint under representative workload conditions. Include both per-process and shared allocations, then choose a worker count that stays below the job’s memory budget with headroom. CPU count alone cannot tell you how many workers fit in RAM, and there is no universal workers-per-GiB ratio that applies across workloads.

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Clean up pools and account for worker resources

Use a Pool as a context manager or explicitly close or terminate it so workers do not outlive the work that needs them. The maxtasksperchild option replaces a worker after a selected number of tasks, which can help release resources accumulated by long-lived workers. On POSIX systems, the spawn and forkserver start methods also launch a resource tracker for named resources such as semaphores and SharedMemory; include those resources when troubleshooting unexpected usage. See the Python multiprocessing documentation.

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Why per-process limits are not enough

Python’s Unix resource module exposes limits such as RLIMIT_CPU, which limits processor time for an individual process and sends SIGXCPU when crossed, and RLIMIT_AS, which limits an individual process’s address space. These are not a simple aggregate CPU-and-memory ceiling for a multiprocessing tree. Use a cgroup boundary for the whole job; per-process limits can be supplementary controls where appropriate. See the Python resource module documentation.

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