Don't immediately promote children of old objects

[Feature #19678]

References from an old object to a write barrier protected young object
will not immediately promote the young object. Instead, the young object
will age just like any other object, meaning that it has to survive
three collections before being promoted to the old generation.
References from an old object to a write barrier unprotected object will
place the parent object in the remember set for marking during minor
collections. This allows the child object to be reclaimed in minor
collections at the cost of increased time for minor collections.

On one of [Shopify's highest traffic Ruby apps, Storefront
Renderer](https://shopify.engineering/how-shopify-reduced-storefront-response-times-rewrite),
we saw significant improvements after deploying this feature in
production. We compare the GC time and response time of web workers that
have the original behaviour (non-experimental group) and this new
behaviour (experimental group). We see that with this feature we spend
significantly less time in the GC, 0.81x on average, 0.88x on p99, and
0.45x on p99.9.

This translates to improvements in average response time (0.96x) and p99
response time (0.92x).

1 files changed, 26 insertions, 0 deletions

diff --git a/test/ruby/test_gc.rb b/test/ruby/test_gc.rb
index 3ff2d8bff2..7e160817c8 100644
--- a/test/ruby/test_gc.rb
+++ b/test/ruby/test_gc.rb
@@ -652,4 +652,30 @@ class TestGc < Test::Unit::TestCase
     Module.new.class_eval( (["# shareable_constant_value: literal"] +
                             (0..100000).map {|i| "M#{ i } = {}" }).join("\n"))
   end
+
+  def test_old_to_young_reference
+    original_gc_disabled = GC.disable
+
+    require "objspace"
+
+    old_obj = Object.new
+    4.times { GC.start }
+
+    assert_include ObjectSpace.dump(old_obj), '"old":true'
+
+    young_obj = Object.new
+    old_obj.instance_variable_set(:@test, young_obj)
+
+    # Not immediately promoted to old generation
+    3.times do
+      assert_not_include ObjectSpace.dump(young_obj), '"old":true'
+      GC.start
+    end
+
+    # Takes 4 GC to promote to old generation
+    GC.start
+    assert_include ObjectSpace.dump(young_obj), '"old":true'
+  ensure
+    GC.enable if !original_gc_disabled
+  end
 end