[PATCH 3/3] Start opportunistic GC timer at startup

[Spencer Baugh <sbaugh@catern.com>]

---
 lisp/startup.el | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/lisp/startup.el b/lisp/startup.el
index 9f67dfde12..412ed0ca0e 100644
--- a/lisp/startup.el
+++ b/lisp/startup.el
@@ -1008,6 +1008,32 @@ the `--debug-init' option to view a complete error backtrace."
     (when debug-on-error-should-be-set
       (setq debug-on-error debug-on-error-from-init-file))))
 
+(defcustom gc-opportunistic-eager-factor 2
+  "How eager we should be when GCing while Emacs is idle.
+Emacs will call garbage-collect-maybe with this argument to
+eagerly GC after gc-estimated-time idle seconds have passed.
+If this is nil, we won't perform GC while Emacs is idle.
+"
+  :type 'integer)
+
+(defvar gc-next-opportunistic-timer nil
+  "The timer for the next opportunistic GC
+This is set by `gc-start-opportunistic'.")
+
+(defun gc-start-opportunistic ()
+  "Start a timer to GC after `gc-estimated-time' idle seconds.
+The GC is performed with `garbage-collect-maybe', and is passed
+`gc-opportunistic-eager-factor' as its single argument.
+If `gc-opportunistic-eager-factor' is nil, we won't do anything.
+This function is itself run by an idle timer at Emacs startup."
+  (when gc-opportunistic-eager-factor
+    (when gc-next-opportunistic-timer
+      (cancel-timer gc-next-opportunistic-timer))
+    (setq gc-next-opportunistic-timer
+          (run-with-idle-timer
+           gc-estimated-time nil
+           #'garbage-collect-maybe gc-opportunistic-eager-factor))))
+
 (defun command-line ()
   "A subroutine of `normal-top-level'.
 Amongst another things, it parses the command-line arguments."
@@ -1420,6 +1446,15 @@ please check its value")
 		 (eq face-ignored-fonts old-face-ignored-fonts))
       (clear-face-cache)))
 
+  ;; Start opportunistic GC (after loading the init file, so we obey
+  ;; its settings).  This is desirable for two reason:
+  ;; - It reduces the number of times we have to GC in the middle of
+  ;;   an operation.
+  ;; - It means we GC when the C stack is short, reducing the risk of false
+  ;;   positives from the conservative stack scanning.
+  (when gc-opportunistic-eager-factor
+    (run-with-idle-timer 1 t #'gc-start-opportunistic))
+
   (setq after-init-time (current-time))
   ;; Display any accumulated warnings after all functions in
   ;; `after-init-hook' like `desktop-read' have finalized possible
-- 
2.28.0