gc-maybe (or the Garbage Collector Maybe Trick)

I have used GCMH (the Garbage Collector Magic Hack) for many years. It’s great.

GCMH hooks itself in pre/post-command hooks. It (re)sets its idle timer on every command, which seems to be a good strategy, but also feels a bit too much.

Conversely, all that gc-maybe-mode does is this:

• With an idle timer (default is 8 seconds), call garbage-collect-maybe (from Emacs 28.1 or above) with a factor based on the current gc-cons-percentage, so it garbage collects only if needed;
• Add hooks to minibuffer-setup-hook, to raise the GC cons threshold, and to minibuffer-exit-hook, to restore the GC cons threshold.

That’s it. It is similar to the GCMH hack, but as a cheap trick.

Also, with a “:before” advice, we can raise the GC cons threshold (gc-maybe-raise-threshold or gc-maybe-raise-threshold-briefly) for known costly functions.

When raising the threshold briefly, a “restore” timer is set, so it resets to default values (gc-maybe-restore-threshold) after an idle delay.

See M-x customize-group RET gc-maybe for all user options.

Setup

(use-package gc-maybe
  :vc (:url "https://github.com/bcardoso/gc-maybe/")
  :demand t
  :config
  ;; Default values
  (setopt gc-maybe-cons-threshold (* 16 1024 1024))
  (setopt gc-maybe-cons-percentage 0.2)
  (setopt gc-maybe-idle-delay 8)
  (setopt gc-maybe-idle-restore 5)

  ;; Enable mode to set the idle timer and minibuffer hooks
  (gc-maybe-mode +1)

  ;; Optionally log GC data in a buffer
  ;; (setopt gc-maybe-log-stats-in-buffer t)

  ;; Advice example
  ;; (advice-add 'org-ql-select :before #'gc-maybe-raise-threshold-briefly)
  )

Related

• GCMH: the Garbage Collector Magic Hack
  • [2019-03-11] The original magic Emacs garbage collection hack
• EmacsConf - 2023 - talks - emacs-gc-stats: Does garbage collection actually slow down Emacs?
  • (Results) Please help collecting statistics to optimize Emacs GC defaults : emacs
• [2021-03-07] Opportunistic GC on emacs-devel mailing list