Add REMEMBERED_WB_UNPROTECTED_OBJECTS_LIMIT_RATIO

[Feature #19571]

This commit adds the environment variable
`RUBY_GC_HEAP_REMEMBERED_WB_UNPROTECTED_OBJECTS_LIMIT_RATIO` which is
used to calculate the `remembered_wb_unprotected_objects_limit` using a
ratio of `old_objects`. This should improve performance by reducing
major GC because, in a major GC, we mark all of the old objects, so we
should have more uncollectible WB unprotected objects before starting a
major GC. The default has been set to 0.01 (1% of old objects).

On one of [Shopify's highest traffic Ruby apps, Storefront Renderer](https://shopify.engineering/how-shopify-reduced-storefront-response-times-rewrite),
we saw significant improvements after deploying this patch in
production. In the graphs below, we have the `tuned` group which uses
`RUBY_GC_HEAP_REMEMBERED_WB_UNPROTECTED_OBJECTS_LIMIT_RATIO=0.01` (the
default value), and an `untuned` group, which turns this feature off
with `RUBY_GC_HEAP_REMEMBERED_WB_UNPROTECTED_OBJECTS_LIMIT_RATIO=0`. We
see that the tuned group spends significantly less time in GC, on
average 0.67x of the time compared to the untuned group and 0.49x for
p99. We see this improvement in GC time translate to improvements in
response times. The average response time is now 0.96x of the time
compared to the untuned group and 0.86x for p99.

https://user-images.githubusercontent.com/15860699/229559078-e23e8ce4-5f1f-4a2f-b5ef-5769f92b8c70.png

gc.c

@@ -291,6 +291,9 @@ rb_gc_guarded_ptr_val(volatile VALUE *ptr, VALUE val)
 #ifndef GC_HEAP_GROWTH_MAX_SLOTS
 #define GC_HEAP_GROWTH_MAX_SLOTS 0 /* 0 is disable */
 #endif
+#ifndef GC_HEAP_REMEMBERED_WB_UNPROTECTED_OBJECTS_LIMIT_RATIO
+# define GC_HEAP_REMEMBERED_WB_UNPROTECTED_OBJECTS_LIMIT_RATIO 0.01
+#endif
 #ifndef GC_HEAP_OLDOBJECT_LIMIT_FACTOR
 #define GC_HEAP_OLDOBJECT_LIMIT_FACTOR 2.0
 #endif
@@ -347,6 +350,7 @@ typedef struct {
     double heap_free_slots_min_ratio;
     double heap_free_slots_goal_ratio;
     double heap_free_slots_max_ratio;
+    double uncollectible_wb_unprotected_objects_limit_ratio;
     double oldobject_limit_factor;
 
     size_t malloc_limit_min;
@@ -369,6 +373,7 @@ static ruby_gc_params_t gc_params = {
     GC_HEAP_FREE_SLOTS_MIN_RATIO,
     GC_HEAP_FREE_SLOTS_GOAL_RATIO,
     GC_HEAP_FREE_SLOTS_MAX_RATIO,
+    GC_HEAP_REMEMBERED_WB_UNPROTECTED_OBJECTS_LIMIT_RATIO,
     GC_HEAP_OLDOBJECT_LIMIT_FACTOR,
 
     GC_MALLOC_LIMIT_MIN,
@@ -8346,7 +8351,10 @@ gc_marks_finish(rb_objspace_t *objspace)
         if (full_marking) {
             /* See the comment about RUBY_GC_HEAP_OLDOBJECT_LIMIT_FACTOR */
             const double r = gc_params.oldobject_limit_factor;
-            objspace->rgengc.uncollectible_wb_unprotected_objects_limit = (size_t)(objspace->rgengc.uncollectible_wb_unprotected_objects * r);
+            objspace->rgengc.uncollectible_wb_unprotected_objects_limit = MAX(
+                (size_t)(objspace->rgengc.uncollectible_wb_unprotected_objects * r),
+                (size_t)(objspace->rgengc.old_objects * gc_params.uncollectible_wb_unprotected_objects_limit_ratio)
+            );
             objspace->rgengc.old_objects_limit = (size_t)(objspace->rgengc.old_objects * r);
         }
 
@@ -11749,6 +11757,7 @@ ruby_gc_set_params(void)
     get_envparam_double("RUBY_GC_HEAP_FREE_SLOTS_GOAL_RATIO", &gc_params.heap_free_slots_goal_ratio,
                         gc_params.heap_free_slots_min_ratio, gc_params.heap_free_slots_max_ratio, TRUE);
     get_envparam_double("RUBY_GC_HEAP_OLDOBJECT_LIMIT_FACTOR", &gc_params.oldobject_limit_factor, 0.0, 0.0, TRUE);
+    get_envparam_double("RUBY_GC_HEAP_REMEMBERED_WB_UNPROTECTED_OBJECTS_LIMIT_RATIO", &gc_params.uncollectible_wb_unprotected_objects_limit_ratio, 0.0, 0.0, TRUE);
 
     if (get_envparam_size("RUBY_GC_MALLOC_LIMIT", &gc_params.malloc_limit_min, 0)) {
         malloc_limit = gc_params.malloc_limit_min;

test/ruby/test_gc.rb

@@ -383,6 +383,15 @@ def test_gc_parameter
       assert_in_out_err([env, "-w", "-e", "exit"], "", [], /RUBY_GC_OLDMALLOC_LIMIT_MAX=16000000/, "")
       assert_in_out_err([env, "-w", "-e", "exit"], "", [], /RUBY_GC_OLDMALLOC_LIMIT_GROWTH_FACTOR=2.0/, "")
     end
+
+    ["0.01", "0.1", "1.0"].each do |i|
+      env = {"RUBY_GC_HEAP_OLDOBJECT_LIMIT_FACTOR" => "0", "RUBY_GC_HEAP_REMEMBERED_WB_UNPROTECTED_OBJECTS_LIMIT_RATIO" => i}
+      assert_separately([env, "-W0"], __FILE__, __LINE__, <<~RUBY)
+        GC.disable
+        GC.start
+        assert_equal((GC.stat[:old_objects] * #{i}).to_i, GC.stat[:remembered_wb_unprotected_objects_limit])
+      RUBY
+    end
   end
 
   def test_profiler_enabled