Free all empty heap pages in Process.warmup

This commit adds `free_empty_pages` which frees all empty heap pages and
moves the number of pages freed to the allocatable pages counter. This
is used in Process.warmup to improve performance because page
invalidation from copy-on-write is slower than allocating a new page.

1 files changed, 22 insertions, 0 deletions

diff --git a/test/ruby/test_process.rb b/test/ruby/test_process.rb
index 38dcb8054f..095ab27f5d 100644
--- a/test/ruby/test_process.rb
+++ b/test/ruby/test_process.rb
@@ -2725,4 +2725,26 @@ EOS
       assert_include(ObjectSpace.dump(obj), '"coderange":"7bit"')
     end;
   end
+
+  def test_warmup_frees_pages
+    assert_separately([{"RUBY_GC_HEAP_FREE_SLOTS_MAX_RATIO" => "1.0"}, "-W0"], "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      TIMES = 10_000
+      ary = Array.new(TIMES)
+      TIMES.times do |i|
+        ary[i] = Object.new
+      end
+      ary.clear
+      ary = nil
+
+      total_pages_before = GC.stat(:heap_eden_pages) + GC.stat(:heap_allocatable_pages)
+
+      Process.warmup
+
+      # Number of pages freed should cause equal increase in number of allocatable pages.
+      assert_equal(total_pages_before, GC.stat(:heap_eden_pages) + GC.stat(:heap_allocatable_pages))
+      assert_equal(0, GC.stat(:heap_tomb_pages))
+      assert_operator(GC.stat(:total_freed_pages), :>, 0)
+    end;
+  end
 end