[ruby-core:30323] [Feature #3322] Simple Patch to make ruby copy-on-write-friendly

Feature #3322: Simple Patch to make ruby copy-on-write-friendly
http://redmine.ruby-lang.org/issues/show/3322

Author: Daniel DeLorme
Status: Open, Priority: Normal
Category: core

A lot has been said about ruby's lack of COW-friendliness; a full solution like REE has a performance impact, but I think I've come up with a simple trick that solves the problem of pre-forking servers. Basically, this patch implements GC.leak which marks all existing objects as ineligible for GC, and existing heaps as ineligible for allocation of new objects. It may sound hacky, but it's exactly the point of pre-forking in most cases: load a bunch of code & data and then keep it in memory forever.

Compare the memory usage of these:
  ./ruby -e 'x=(1..40_000_000).map{""};          Process.fork; x=[]; GC.start; sleep 10'
  ./ruby -e 'x=(1..40_000_000).map{""}; GC.leak; Process.fork; x=[]; GC.start; sleep 10'

This patch has the advantage of being 
  1) simple
  2) free of negative performance impact
  3) implemented and available now

PS: As far as I can tell the patch works fine, but I'm not terribly well-versed about the innards of ruby so I'd appreciate some code review. Thanks.


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