From: "Dan0042 (Daniel DeLorme) via ruby-core" <ruby-core@...>
Date: 2024-07-29T17:39:16+00:00
Subject: [ruby-core:118720] [Ruby master Misc#20652] Memory allocation for gsub has increased from Ruby 2.7 to 3.3

Issue #20652 has been updated by Dan0042 (Daniel DeLorme).


byroot (Jean Boussier) wrote in #note-4:
> Maybe we could add a new Regexp flag to turn off this behavior?

My first reaction was "Yes! This is exactly was we need!" but after thinking more it feels un-rubyish. We shouldn't have to write code to micro-tweak the performance like that. Ideally the interpreter/jit should handle micro-optimizations. I'd be happier to see something like:
```ruby
def foo(str)
  str if str =~ /rx/
  #in this method we don't use $~ and friends, so the interpreter doesn't have to allocate MatchData
  #yes in theory there's an incompatibility with eval, but in practice I believe that's a non-issue (I'm open to be shown otherwise)
end
```


shyouhei (Shyouhei Urabe) wrote in #note-5:
> Nobody thinks `gsub` modifies `$~` behind the scene.

I'm not sure this is what you meant, but I definitely expect ANY regexp operation to modify `$~` (except where documented otherwise, like `Regexp#match?`)
BTW this includes String#sub; I sometimes write code like: `name, prefix = str.sub(/^(the) /i), $1`


----------------------------------------
Misc #20652: Memory allocation for gsub has increased from Ruby 2.7 to 3.3
https://bugs.ruby-lang.org/issues/20652#change-109255

* 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?



-- 
https://bugs.ruby-lang.org/
 ______________________________________________
 ruby-core mailing list -- ruby-core@ml.ruby-lang.org
 To unsubscribe send an email to ruby-core-leave@ml.ruby-lang.org
 ruby-core info -- https://ml.ruby-lang.org/mailman3/lists/ruby-core.ml.ruby-lang.org/