https://git.ruby-lang.org/ruby.git/commit/?id=0264424d58

From 0264424d58e0eb3ff6fc42b7b4164b6e3b8ea8ca Mon Sep 17 00:00:00 2001
From: Peter Zhu <peter@p...>
Date: Mon, 15 Aug 2022 09:14:35 -0400
Subject: Add test for GC thrashing of young object creation

This test will prevent performance regressions like [Bug #18929].
---
 test/ruby/test_gc.rb | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/test/ruby/test_gc.rb b/test/ruby/test_gc.rb
index a5d7f4dbaa..a1e782daa9 100644
--- a/test/ruby/test_gc.rb
+++ b/test/ruby/test_gc.rb
@@ -402,6 +402,28 @@ class TestGc < Test::Unit::TestCase https://github.com/ruby/ruby/blob/trunk/test/ruby/test_gc.rb#L402
     eom
   end
 
+  def test_thrashing_for_young_objects
+    # This test prevents bugs like [Bug #18929]
+
+    assert_separately %w[--disable-gem], __FILE__, __LINE__, <<-RUBY
+      # Warmup to make sure heap stabilizes
+      1_000_000.times { Object.new }
+
+      before_stats = GC.stat
+
+      1_000_000.times { Object.new }
+
+      after_stats = GC.stat
+
+      # Should not be thrashing in page creation
+      assert_equal before_stats[:heap_allocated_pages], after_stats[:heap_allocated_pages]
+      assert_equal 0, after_stats[:heap_tomb_pages]
+      assert_equal 0, after_stats[:total_freed_pages]
+      # Only young objects, so should not trigger major GC
+      assert_equal before_stats[:major_gc_count], after_stats[:major_gc_count]
+    RUBY
+  end
+
   def test_gc_internals
     assert_not_nil GC::INTERNAL_CONSTANTS[:HEAP_PAGE_OBJ_LIMIT]
     assert_not_nil GC::INTERNAL_CONSTANTS[:RVALUE_SIZE]
-- 
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