bug#37321: 27.0.50; Excessive gc in a use case (el-search)

[Paul Eggert <eggert@cs.ucla.edu>]

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On 9/8/19 8:25 AM, Michael Heerdegen wrote:
> my Emacs seems to have
> become slower independently from gc.  There is still a slow down of
> searching by a factor of at least 3 or so that doesn't seem to be
> related to gc.

Have you tried turning on profiling to see what the problem might be?

I looked into it a bit, and I think the problem is related to this line in 
el-search--flatten-tree:

         (gc-cons-percentage 0.8)) ;Why is binding it here more effective than 
binding it more top-level?

That binding is not effective in general, because it causes Emacs to do most of 
its computation with gc-cons-percentage equal to 0.8, but a small amount of it 
with gc-cons-percentage equal to 0.1 (the default value). This is in an attempt 
to disable most GC. However, the attempt fails in master because when 
gc-cons-percentage drops to 0.1 Emacs does a garbage collection pretty much 
right away. (In Emacs 26, the code lucks out because Emacs happens to not look 
at gc-cons-percentage during the brief time that it is 0.1, so it doesn't GC.)

Proposed ELPA fix attached; it should help performance in Emacs master (where I 
checked in some changes recently to help master work a bit better on this 
example). The abovementioned line shouldn't be needed in Emacs master.

There may be other places in el-search that would benefit from a change similar 
to the attached patch, but I didn't investigate this.

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From 3e85fb302a9cf88ed3668d41c4b2e4cdb7017c54 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Sat, 14 Sep 2019 01:01:20 -0700
Subject: [PATCH] Port GC tuning to Emacs 27 alpha

* packages/el-search/el-search.el (el-search-setup-search):
Set gc-cons-percentage to 0.8 while searching.
---
 packages/el-search/el-search.el | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/packages/el-search/el-search.el b/packages/el-search/el-search.el
index 4e9cd98a4..5e213cf2b 100644
--- a/packages/el-search/el-search.el
+++ b/packages/el-search/el-search.el
@@ -2168,11 +2168,12 @@ files (i.e. file names) to search in.
 With optional FROM-HERE non-nil, the first buffer in this stream
 should be the current buffer, and searching will start at the
 current buffer's point instead of its beginning."
+ (let ((gc-cons-percentage 0.8))
   (el-search-setup-search-1 pattern get-buffer-stream nil setup-function)
   (if (not el-search-occur-flag)
       (el-search-continue-search from-here)
     (setq el-search-occur-flag nil)
-    (el-search-occur)))
+    (el-search-occur))))
 
 (defun el-search-stream-of-directory-files (&optional directory recurse)
   "Return a stream of emacs-lisp files in DIRECTORY.
-- 
2.21.0