Add bench of class with many singletons

bench/bench_subclasses.rb

@@ -1,5 +1,9 @@
 require 'benchmark/ips'
 
+Custom = Class.new(Object)
+# create these once and hold references
+many_singletons = 1000.times.map { c = Custom.new; class << c; end; c }
+
 Benchmark.ips do |bm|
   [1, 5, 10, 50].each do |count|
     bm.report("#{count} thread Numeric.subclasses") do
@@ -24,5 +28,16 @@
         }
       }.each(&:join)
     end
+    bm.report("#{count} thread Custom.subclasses with many singletons") do
+      count.times.map {
+        Thread.new {
+          i = 10_000 / count
+          while i > 0
+            Custom.subclasses
+            i-=1
+          end
+        }
+      }.each(&:join)
+    end
   end
 end