rein in GC during tests by making them run (at most) once per second

this can provide a significant performance boost during testing, by
preventing the GC from running too frequently.

activesupport/lib/active_support/testing/garbage_collection.rb

@@ -3,6 +3,9 @@ module ActiveSupport
     module GarbageCollection
       def self.included(base)
         base.teardown :scrub_leftover_instance_variables
+
+        base.setup :begin_gc_deferment
+        base.teardown :reconsider_gc_deferment
       end
 
       private
@@ -14,6 +17,27 @@ module ActiveSupport
           remove_instance_variable(var)
         end
       end
+
+      # Minimum interval, in seconds, at which to run GC. Might be less
+      # frequently than this, if a single test takes longer than this to
+      # run.
+      DEFERRED_GC_THRESHOLD = (ENV['DEFERRED_GC_THRESHOLD'] || 1.0).to_f
+
+      @@last_gc_run = Time.now
+
+      def begin_gc_deferment
+        GC.disable if DEFERRED_GC_THRESHOLD > 0
+      end
+
+      def reconsider_gc_deferment
+        if DEFERRED_GC_THRESHOLD > 0 && Time.now - @@last_gc_run >= DEFERRED_GC_THRESHOLD
+          GC.enable
+          GC.start
+          GC.disable
+
+          @@last_gc_run = Time.now
+        end
+      end
     end
   end
 end

activesupport/test/test_test.rb

@@ -135,9 +135,9 @@ class SetupAndTeardownTest < ActiveSupport::TestCase
   teardown :foo, :sentinel, :foo
 
   def test_inherited_setup_callbacks
-    assert_equal [:reset_callback_record, :foo], self.class._setup_callbacks.map(&:raw_filter)
+    assert_equal [:begin_gc_deferment, :reset_callback_record, :foo], self.class._setup_callbacks.map(&:raw_filter)
     assert_equal [:foo], @called_back
-    assert_equal [:scrub_leftover_instance_variables, :foo, :sentinel, :foo], self.class._teardown_callbacks.map(&:raw_filter)
+    assert_equal [:scrub_leftover_instance_variables, :reconsider_gc_deferment, :foo, :sentinel, :foo], self.class._teardown_callbacks.map(&:raw_filter)
   end
 
   def setup
@@ -167,9 +167,9 @@ class SubclassSetupAndTeardownTest < SetupAndTeardownTest
   teardown :bar
 
   def test_inherited_setup_callbacks
-    assert_equal [:reset_callback_record, :foo, :bar], self.class._setup_callbacks.map(&:raw_filter)
+    assert_equal [:begin_gc_deferment, :reset_callback_record, :foo, :bar], self.class._setup_callbacks.map(&:raw_filter)
     assert_equal [:foo, :bar], @called_back
-    assert_equal [:scrub_leftover_instance_variables, :foo, :sentinel, :foo, :bar], self.class._teardown_callbacks.map(&:raw_filter)
+    assert_equal [:scrub_leftover_instance_variables, :reconsider_gc_deferment, :foo, :sentinel, :foo, :bar], self.class._teardown_callbacks.map(&:raw_filter)
   end
 
   protected