From: "jeremyevans0 (Jeremy Evans) via ruby-core" <ruby-core@...>
Date: 2024-07-29T19:40:16+00:00
Subject: [ruby-core:118723] [Ruby master Misc#20652] Memory allocation for gsub has increased from Ruby 2.7 to 3.3

Issue #20652 has been updated by jeremyevans0 (Jeremy Evans).


Dan0042 (Daniel DeLorme) wrote in #note-19:
> After reading over https://github.com/ruby/ruby/pull/4734/files it seems there's two parts to it.
> 1. use a `set_match` pointer to return the match (this fixes the race condition)
> 2. always allocate a MatchData, never using `rb_backref_get()`
> 
> But it seems to me that #2 is only necessary if `set_match` is used. So what about using `rb_backref_get()` when possible? Like
> ```ruby
> match = set_match ? Qnil : rb_backref_get();
> ```
> 
> @jeremyevans0 wdyt?

I'm not sure it is thread-safe.  This would modify a shared backref in code paths where `set_match` is `NULL`.  I haven't audited the related code, so I'm not sure what code calls `rb_reg_search0` and `rb_reg_match`.  Feel free to give it a try and see if passes the test and reduces allocations.

----------------------------------------
Misc #20652: Memory allocation for gsub has increased from Ruby 2.7 to 3.3
https://bugs.ruby-lang.org/issues/20652#change-109258

* 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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