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

[Paul Eggert]

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.

0001-Port-GC-tuning-to-Emacs-27-alpha.patch
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