How to Set CPU Threads in 7-Zip CLI

This article explains how to control CPU thread allocation in the 7-Zip command-line interface (CLI). By using the multithreading switch, you can specify exactly how many processor cores 7-Zip utilizes, allowing you to maximize compression speed or limit resource consumption to keep your system responsive during background tasks.

The CPU Thread Switch: -mmt

The switch used to control CPU thread allocation in the 7-Zip CLI is -mmt.

This parameter tells the 7-Zip compression engine how many simultaneous threads to create. It can be written as -mmt{N}, where {N} is the number of threads, or combined with boolean values to turn multithreading on or off.

Syntax and Parameters

  • -mmt=on or -mmt: Enables multithreading using the default number of threads detected for your CPU.
  • -mmt=off: Disables multithreading entirely, forcing 7-Zip to run on a single thread.
  • -mmt={N}: Sets the thread count to a specific number N (e.g., -mmt=4 limits execution to 4 threads).

Note: The equals sign (=) is optional. For example, -mmt4 and -mmt=4 function identically.

Command Examples

1. Limit Compression to 4 Threads

To compress a directory using four specific threads:

7z a -mmt=4 archive.7z /path/to/files

2. Maximize Performance

To utilize all available CPU threads (for example, on an 8-core, 16-thread processor):

7z a -mmt=16 archive.7z /path/to/files

3. Run in the Background (Single Thread)

To ensure 7-Zip does not impact other running applications, restrict it to a single CPU core:

7z a -mmt=off archive.7z /path/to/files

Important Considerations

  • Compression Algorithm Support: Multithreading behavior depends heavily on the compression algorithm. LZMA2 handles high thread counts exceptionally well, whereas the older LZMA format only supports up to 2 threads for compression. Formats like BZip2 also scale well with multithreading.
  • Memory Usage: Increasing the thread count increases the amount of RAM required for the operation, particularly when using large dictionary sizes.
  • Decompression Limitations: For .7z archives, decompression is typically single-threaded unless using algorithms like BZip2 or chunks compressed independently under specific LZMA2 configurations.