Add string malloc pressure benchmark

benchmarks.yml

@@ -136,6 +136,11 @@ sudoku:
 splay:
   desc: Splay tree operations (insert, find, remove) to stress GC.
   single_file: true
+string_malloc_pressure:
+  desc: Allocates and retains many empty Strings with large reserved capacity to exercise GC under malloc-backed String buffer pressure.
+  category: gc
+  single_file: true
+  default_harness: harness-gc
 tinygql:
   desc: TinyGQL gem parsing a large file in pure Ruby
 

benchmarks/string_malloc_pressure.rb

@@ -0,0 +1,12 @@
+require_relative '../harness/loader'
+
+def build_string_malloc_pressure
+  live = []
+  500.times do
+    live << Array.new(200) { String.new(capacity: 64 * 1024) }
+  end
+end
+
+run_benchmark(10) do
+  build_string_malloc_pressure
+end