[ruby-core:107471] [Ruby master Feature#18568] Explore lazy RubyGems boot to reduce need for --disable-gems

Issue #18568 has been updated by headius (Charles Nutter).


> I think a faster boot time while still having RubyGems enabled by default would be good for all Rubies.

Thank you for providing those links... I knew I had seen that code but could not remember where it was located.

Making these changes in RubyGems would be great, if that is sufficient to reduce boot times. I was not clear on how much of this could live there versus how much needs to be changed in the Ruby boot process.

One thing that has always bothered me about RubyGems is its constant rescanning of gemspecs and lib directories from gems which I think we all agree should NEVER BE MODIFIED. There's no good reason not to build up a local file index, a la [gel](https://github.com/gel-rb/gel). At worst, a `gem reindex` command that rescans all installed gemspecs and libs would be sufficient to support the very few dev-time cases for adding or removing files from an installed gem.

> I worked on https://github.com/rubygems/rubygems/pull/4199 a while ago

This approach looks similar in spirit to `gel` in that it creates an index of each gem's contents. A single index might be even faster but would require different strategies to maintain it. The advantage of a single index would of course be a single file read, and potentially that file produces exactly the structure we need to quickly find requirable files.

I'm excited to see this moving forward. On @tenderlove's recommendation I am adding a note to the next dev meeting.

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Feature #18568: Explore lazy RubyGems boot to reduce need for --disable-gems
https://bugs.ruby-lang.org/issues/18568#change-96374

* 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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