perf: re-enable native-comp JIT and hand GC to gcmh

early-init.el disabled JIT native compilation with (setq native-comp-deferred-compilation nil), the obsolete alias of native-comp-jit-compilation. Despite the comment, setting it nil turns JIT off entirely rather than making it synchronous. Most modules then ran interpreted for the daemon's lifetime, and the native-comp-speed/jobs settings in system-defaults.el were dead. The "Selecting deleted buffer" async race that prompted the disable was an Emacs 28/29 issue. This is 30.2. I re-enabled it with native-comp-jit-compilation t and silent async warnings.

GC was pinned at the stock 800KB: early-init restored it post-startup and the minibuffer setup/exit hooks bounced back to it. That's Emacs's bare-editor default, far too low for 184 packages, so GC pauses fired often during completion, agenda, and LSP/AI work. I replaced both hand-rolled mechanisms with gcmh, which keeps the threshold at 1GB during activity and collects on idle.

Verified a clean full launch in a throwaway daemon (JIT on, gcmh active, no backtrace) and gcmh's threshold cycle in batch.

1 files changed, 30 insertions, 0 deletions

diff --git a/modules/gcmh-config.el b/modules/gcmh-config.el
new file mode 100644
index 000000000..beceb1a01
--- /dev/null
+++ b/modules/gcmh-config.el
@@ -0,0 +1,30 @@
+;;; gcmh-config.el --- Garbage collection strategy via gcmh -*- lexical-binding: t -*-
+
+;;; Commentary:
+;; gcmh (the Garbage Collector Magic Hack) owns `gc-cons-threshold' for the
+;; session.  It keeps the threshold very high while you are active so GC never
+;; pauses mid-edit, then drops it and collects on idle, when a pause is
+;; invisible.  This replaces the old hand-rolled scheme -- a stock-800KB restore
+;; in early-init.el plus a minibuffer setup/exit bump -- which pinned GC at
+;; 800000 (Emacs's bare-editor default), far too low for a config this size and
+;; the cause of frequent GC pauses during completion, agenda builds, and LSP/AI
+;; activity.
+;;
+;; Kept in its own module, not system-defaults.el: that module is pre-loaded by
+;; the comp-errors test harness, which has no package system, so an `:ensure'
+;; package there errors at load time.  early-init.el bumps the threshold to
+;; `most-positive-fixnum' for startup and deliberately does not restore it;
+;; `gcmh-mode' takes ownership from here on.
+
+;;; Code:
+
+(use-package gcmh
+  :ensure t
+  :demand t
+  :config
+  (setq gcmh-idle-delay 'auto                          ; scale the idle GC delay to GC cost
+        gcmh-high-cons-threshold (* 1 1024 1024 1024)) ; 1 GB during activity
+  (gcmh-mode 1))
+
+(provide 'gcmh-config)
+;;; gcmh-config.el ends here