[ruby-core:107480] [Ruby master Feature#18568] Explore lazy RubyGems boot to reduce need for --disable-gems

Issue #18568 has been updated by Eregon (Benoit Daloze).


byroot (Jean Boussier) wrote in #note-9:
> Another thing that could be explored would be to leverage `RubyVM::InstructionSequence.compile_file(path)`. If we indeed have a mapping of the gems content, we could define `load_iseq`, at least for the duration of rubygems's "boot".

That would only work on CRuby, so as such would be unusable on anything but CRuby.
I think we should rather load less Ruby code while starting RubyGems.

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Feature #18568: Explore lazy RubyGems boot to reduce need for --disable-gems
https://bugs.ruby-lang.org/issues/18568#change-96382

* Author: headius (Charles Nutter)
* Status: Open
* Priority: Normal
----------------------------------------
In https://bugs.ruby-lang.org/issues/17684 there was debate about whether the `--disable-gems` flag should be removed. Several folks were in favor, since Ruby without RubyGems is fairly limited, but others wanted to keep the flag for small, fast command line scripts that do not depend on RubyGems.

Lazily loading RubyGems might be a middle ground, and it has been explored in some depth by TruffleRuby:

https://github.com/oracle/truffleruby/blob/master/src/main/ruby/truffleruby/core/lazy_rubygems.rb

@eregon shows how this improves their startup time in this article from a couple years ago:

https://eregon.me/blog/2019/04/24/how-truffleruby-startup-became-faster-than-mri.html

I believe this approach has merit and could be beneficial to both CRuby and JRuby if we can collaborate on how the lazy loading should happen and figuring out where the edges are. @eregon may know some of those edges if they have run into them in TruffleRuby.

A simple test of `--disable-gems` on CRuby 3.1 shows what an impact it has, which we might be able to duplicate in a lazy boot WITHOUT losing RubyGems functionality and default gem upgrading:

```
$ time ruby -e 1

real    0m0.107s
user    0m0.068s
sys     0m0.030s

$ time ruby --disable-gems -e 1

real    0m0.019s
user    0m0.007s
sys     0m0.008s
```

Over 80% of CRuby's base startup is due to eagerly booting RubyGems. We can do better!



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