Manually compact GC before fork

[schneems]

Rub 2.7.0 introduced GC.compact which allows manual compaction of Ruby slots. A good time to do this is before forking so that memory fragmentation can be reduced.

• https://www.ruby-lang.org/en/news/2019/12/17/ruby-2-7-0-rc1-released/
• https://www.youtube.com/watch?v=1F3gXYhQsAY

One issue with memory fragmentation when forking is that while a page in memory might have only one free slot, as that slot is written to after fork, the entire page is written so the benefit of CoW optimizations are greatly reduced. Manually compacting GC reduces the number of pages with empty slots.

This PR manually compacts memory right before Puma forks and after other before_fork hooks are called.

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[MSP-Greg]

JFYI, using ruby master in actions:

https://github.com/ruby/psych/blob/master/.github/workflows/ubuntu-rvm.yml

[liaden]

Would it be worthwhile to add a GC.start to increase the resulting compactness? Edit: Nevermind, it already invokes GC https://github.com/ruby/ruby/blob/master/gc.c#L8438

At work, we often will use eye to monitor child processes for memory bloat and have eye kill them. It would be beneficial to have this only done on the first fork instead of every time we fork.

[schneems]

I'm getting failures on GitHub actions for 2.7.0 and 2.3.x and 2.3.8 with just

Run actions/setup-ruby@v1
  with:
    ruby-version: 2.7.0-preview3
##[error]Version 2.7.0-preview3 not found
##[error]Node run failed with exit code 1

Any idea how to debug or what needs to be changed @MSP-Greg ?

[MSP-Greg]

@schneems

I'm getting failures on GitHub actions for 2.7.0 and 2.3.x and 2.3.8

At present, standard GitHub actions just has Ruby 2.4.x, 2.5.x, & 2.6.x installed. Anything else needs code using a container, installing and using rvm, or a special action step.

@dentarg & PR #2087 adds Ruby 2.3 in a container. The Ruby/Psych code I linked to adds ruby-head using rvm. I can add 2.3 & ruby-head to Windows, but that won't run this code as fork is not available...

[MSP-Greg]

All the Travis 2.7.0 tests passed, and all the Actions tests that are not broken passed.

[schneems]

All the Travis 2.7.0 tests passed, and all the Actions tests that are not broken passed.

Seems good!