Run unrar on Specific CPU Cores with taskset

This article explains how to restrict the unrar utility to specific CPU cores using the Linux taskset command. It covers the core syntax for setting CPU affinity at execution time, examples for targeting individual or multiple cores, methods for modifying already running processes, and the practical benefits of controlling CPU usage during resource-heavy decompression tasks.

Yes, You Can Use taskset with unrar

The taskset utility sets or retrieves the CPU affinity of a process in Linux. Because unrar runs as a standard userspace process, you can launch it through taskset to restrict its execution to one or more designated processor cores. This is useful for preventing large or multi-part archive extractions from consuming all available computing resources on a server or workstation.

Basic Syntax

To launch unrar bound to specific CPU cores, use the -c (or --cpu-list) flag with taskset, followed by a comma-separated list or range of core identifiers, and then your standard unrar command:

taskset -c <core-list> unrar <command> <archive.rar> [destination]

CPU cores are numbered starting from 0. For example, on a 4-core machine, the cores are 0, 1, 2, and 3.

Practical Examples

Restrict unrar to a Single Core

To run an extraction restricted entirely to CPU Core 0:

taskset -c 0 unrar x archive.rar

This isolates the workload so the remaining cores remain entirely free for other system tasks.

Restrict unrar to a Specific List of Cores

To bind unrar to use only Core 1 and Core 3:

taskset -c 1,3 unrar x archive.rar /path/to/destination/

Restrict unrar to a Range of Cores

To bind unrar across a range of cores, such as Cores 2 through 5:

taskset -c 2-5 unrar e archive.rar

Assigning Cores to an Already Running unrar Process

If an unrar process is already executing, you can change its CPU affinity dynamically using the process ID (PID) and the -p flag:

  1. Locate the PID of the process: 
    pgrep unrar
  2. Apply the desired CPU cores to that PID: 
    taskset -cp 0,1 <PID>

Key Considerations

  • Threading Behavior: While older versions of unrar are largely single-threaded, newer versions and custom builds may spawn multiple threads. taskset will restrict all child threads created by unrar to the specified core mask.
  • Permissions: Standard user privileges are sufficient to assign core affinity to your own unrar commands. Modifying the affinity of processes owned by other users requires sudo or root privileges.
  • I/O Bottlenecks: Restricting CPU cores reduces CPU contention, but archive decompression is frequently disk I/O intensive. If system performance remains slow, consider pairing taskset with ionice to limit disk priority alongside CPU affinity.