Building cmake launches too many parallel jobs and makes the computer unresponsive

[denilsonsa]

Output of brew config

HOMEBREW_VERSION: 5.1.11-6-g65eea70
ORIGIN: https://github.com/Homebrew/brew
HEAD: 65eea70c95e0b55bf369a8c3bd5c5706e5de4a77
Last commit: 4 hours ago
Branch: main
Core tap JSON: 11 May 10:32 UTC
Core cask tap JSON: 11 May 10:32 UTC
HOMEBREW_PREFIX: /Users/myusername/.homebrew
HOMEBREW_REPOSITORY: /Users/myusername/.homebrew
HOMEBREW_CELLAR: /Users/myusername/.homebrew/Cellar
HOMEBREW_CASK_OPTS: []
HOMEBREW_DISPLAY: /var/run/com.apple.launchd.vK6Ly9cI8K/org.xquartz:0
HOMEBREW_DOWNLOAD_CONCURRENCY: 24
HOMEBREW_EDITOR: vim
HOMEBREW_FORBID_PACKAGES_FROM_PATHS: set
HOMEBREW_MAKE_JOBS: 12
HOMEBREW_SORBET_RUNTIME: set
Homebrew Ruby: 4.0.3 => /Users/myusername/.homebrew/Library/Homebrew/vendor/portable-ruby/4.0.3/bin/ruby
CPU: dodeca-core 64-bit arm_lobos
Clang: 21.0.0 build 2100
Git: 2.54.0 => /Users/myusername/.homebrew/bin/git
Curl: 8.7.1 => /usr/bin/curl
macOS: 26.4.1-arm64
CLT: 26.4.1.0.1775747724
Xcode: 26.4.1
Metal Toolchain: N/A
Rosetta 2: false

Output of brew doctor

Warning: Your Homebrew's prefix is not /opt/homebrew.

Most of Homebrew's bottles (binary packages) can only be used with the default prefix.
Consider uninstalling Homebrew and reinstalling into the default prefix.

This is a Tier 3 configuration:
  https://docs.brew.sh/Support-Tiers#tier-3
You can report issues with Tier 3 configurations to Homebrew/* repositories!
Read the above document before opening any issues or PRs.


Warning: Homebrew's "sbin" was not found in your PATH but you have installed
formulae that put executables in /Users/myusername/.homebrew/sbin.
Consider setting your PATH for example like so:
  echo 'export PATH="/Users/myusername/.homebrew/sbin:$PATH"' >> ~/.zshrc

Description of issue

I'm using homebrew in a non-standard location because I don't have root access on this machine. I understand this is mostly unsupported, but has been working fine. As a side-effect, some packages can't be unpacked directly from bottles, but instead have to be compiled from source. That is fine.

However, while (re)compiling cmake, I noticed my MacBook M3 Pro was quite slow and unresponsive. Upon looking at htop, I saw it had a load average over 30, which is too much given this machine has 12 CPU cores.

---

Questions:

1. How can I limit the amount of parallel jobs from homebrew?
2. Can homebrew itself figure out sane default values? In other words, can we make homebrew not hog down the machine? (Without the need for any custom configuration.)

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When looking at other packaging systems, I see:

• Gentoo passes --parallel=$(get_makeopts_jobs "$(get_nproc)").
• Arch uses --parallel=$(/usr/bin/getconf _NPROCESSORS_ONLN)
• Debian has some logic
• Nix uses --parallel=''${NIX_BUILD_CORES:-1}

Homebrew uses --parallel=#{ENV.make_jobs}, however I couldn't find any info on where or how to set "make_jobs".

[SMillerDev]

https://docs.brew.sh/Manpage#:~:text=cache/Homebrew/Logs.-,HOMEBREW_MAKE_JOBS,-Use%20this%20value

[denilsonsa]

Thanks!

However, observe how brew config already shows HOMEBREW_MAKE_JOBS: 12. Yet, somehow building cmake was too heavy.

Someone mentioned Ninja while debugging cmake issues on Arch. Which makes me think if the cmake build system is using Ninja and somehow disregarding the explicit number of parallel jobs we've asked.

Can someone please confirm? can someone please try to reproduce this issue? (i.e. try building cmake from source and observing the system load.)

[ZhongRuoyu]

Overriding HOMEBREW_MAKE_JOBS does work for me when building cmake. Try setting it to 1 and you should observe that the build becomes de-parallelized.

Note that having HOMEBREW_MAKE_JOBS set to 12 means that 12 jobs are run concurrently, so all of your 12 cores will be busy building cmake. If you have other ongoing tasks, the slowness is sort of expected. If that's not desired, please adjust it to a lower value.

Can homebrew itself figure out sane default values?

If Homebrew doesn't use all cores, one can also complain like: "Why is Homebrew not using all my CPU cores even when my computer is idle? That makes it slow!" The choice is yours: set HOMEBREW_MAKE_JOBS to whatever value that suits your need.