[ruby-core:118638] [Ruby master Bug#20641] `lib/bundled_gems.rb` makes `Kernel.require` over 100x slower

["byroot (Jean Boussier) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20641 has been updated by byroot (Jean Boussier).


Some extra context. This codepath is only active if using Bundler >= 2.5.0.


----------------------------------------
Bug #20641: `lib/bundled_gems.rb` makes `Kernel.require` over 100x slower
https://bugs.ruby-lang.org/issues/20641#change-109164

* Author: byroot (Jean Boussier)
* Status: Open
* Backport: 3.1: DONTNEED, 3.2: DONTNEED, 3.3: REQUIRED
----------------------------------------
I just discovered this while profiling Active Record's test suite, and I noticed 40% of the runtime was in `$LOAD_PATH.resolve_feature_path`, so much I thought it was a profiler bug.

But it turns out it's real. Various APIs do call `require` late, for instance Psych calls `require 'date'` every single time it parses a date: https://github.com/ruby/psych/blob/be0ba74e5613c20f213403e15914d24944c2652d/lib/psych/scalar_scanner.rb#L64

I've put together a quick benchmark:

```ruby
# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  gem "benchmark-ips"
end

Benchmark.ips do |x|
  x.report(RUBY_VERSION) { require "erb" }
  x.save! "/tmp/require.bench"
  x.compare!
end
```

And the difference is massive:

```
Calculating -------------------------------------
               3.2.2      6.450M (± 1.2%) i/s -     32.822M in   5.089538s

Comparison:
               3.2.2:  6449899.4 i/s
               3.3.3:    46996.8 i/s - 137.24x  slower
```

And that's with a small `$LOAD_PATH`, the bigger the application, the worse it is, if I add:

```ruby
100.times do |i|
  $LOAD_PATH.unshift("/tmp/empty-#{i}")
end
```

```
Calculating -------------------------------------
               3.3.3      6.198k (± 6.1%) i/s -     30.968k in   5.018955s

Comparison:
               3.2.2:  3380939.6 i/s
               3.3.3:     6198.4 i/s - 545.46x  slower
```

I'm looking at a way to speed this up, but this is big enough that I believe we should backport the fix.

cc @hsbt 



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