From: "byroot (Jean Boussier) via ruby-core" <ruby-core@...>
Date: 2024-07-26T06:15:14+00:00
Subject: [ruby-core:118689] [Ruby master Misc#20652] Memory allocation for gsub has increased from Ruby 2.7 to 3.3

Issue #20652 has been updated by byroot (Jean Boussier).


>  If anyone can come up with an approach that fixes #17507 without causing an increase in memory usage

I guess I'm a bit surprised by that because I wouldn't have thought the backref is only accessible by the same fiber, but maybe I'm missing something.

But on a more general note, I very very often wish I'd have a way to not have a `MatchData` created by methods that take a Regexp, it's very rare you need the `MatchData` created by `gsub` or `Regexp===`. Most of the time it's fine, but when you are trying to optimize a hotspot, it's really something you'd want to be able to skip. It's in big part why `Regexp#match?` was added and popularized, but is only usable in a subset of cases.

Maybe we could add a new Regexp flag to turn off this behavior?

e.g.

```ruby
case str
when /^a/
  p $~ # => #<MatchData "a">
when /^b/c
  p $~ # => nil
end
```

----------------------------------------
Misc #20652: Memory allocation for gsub has increased from Ruby 2.7 to 3.3
https://bugs.ruby-lang.org/issues/20652#change-109225

* Author: orisano (Nao Yonashiro)
* Status: Open
* Assignee: jeremyevans0 (Jeremy Evans)
----------------------------------------
I recently upgraded from ruby 2.7.7 to 3.3.1 and noticed that the GC load increased.
When I used the allocation profiler to investigate, I found that memory allocation from gsub had increased.

The problem was code like this:
```ruby
s = "foo              "
s.gsub(/ (\s+)/) { " #{'&nbsp;' * Regexp.last_match(1).length}" }
```

When I compared the results of heap-profiler between 2.7.7 and 3.3.1, I found that MatchData was increasing.

https://gist.github.com/orisano/98792dee260106e9b6fcb45bbabeb1e6

https://github.com/ruby/ruby/commit/abc0304cb28cb9dcc3476993bc487884c139fd11

I discovered that the cause is this commit, which stopped reusing backref to avoid race conditions.
Is there a way to reuse backref while still avoiding race conditions?



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