Add RUBY_GC_HEAP_INIT_SIZE_%d_SLOTS to pre-init pools granularly

The old RUBY_GC_HEAP_INIT_SLOTS isn't really usable anymore as
it initalize all the pools by the same factor, but it's unlikely
that pools will need similar sizes.

In production our 40B pool is 5 to 6 times bigger than our 80B pool.

gc.c

@@ -11749,15 +11749,28 @@ get_envparam_double(const char *name, double *default_value, double lower_bound,
 }
 
 static void
-gc_set_initial_pages(rb_objspace_t *objspace)
+gc_set_initial_pages(rb_objspace_t *objspace, int global_heap_init_slots)
 {
     gc_rest(objspace);
 
     for (int i = 0; i < SIZE_POOL_COUNT; i++) {
         rb_size_pool_t *size_pool = &size_pools[i];
+        char env_key[sizeof("RUBY_GC_HEAP_INIT_SIZE_" "_SLOTS") + DECIMAL_SIZE_OF_BITS(sizeof(size_pool->slot_size) * CHAR_BIT)];
+        snprintf(env_key, sizeof(env_key), "RUBY_GC_HEAP_INIT_SIZE_%d_SLOTS", size_pool->slot_size);
 
-        if (gc_params.heap_init_slots > size_pool->eden_heap.total_slots) {
-            size_t slots = gc_params.heap_init_slots - size_pool->eden_heap.total_slots;
+        size_t pool_init_slots = 0;
+        if (!get_envparam_size(env_key, &pool_init_slots, 0)) {
+            if (global_heap_init_slots) {
+                // If we use the global init slot we ponderate it by slot size
+                pool_init_slots = gc_params.heap_init_slots / (size_pool->slot_size / BASE_SLOT_SIZE);
+            }
+            else {
+                continue;
+            }
+        }
+
+        if (pool_init_slots > size_pool->eden_heap.total_slots) {
+            size_t slots = pool_init_slots - size_pool->eden_heap.total_slots;
             int multiple = size_pool->slot_size / BASE_SLOT_SIZE;
             size_pool->allocatable_pages = slots * multiple / HEAP_PAGE_OBJ_LIMIT;
         }
@@ -11823,7 +11836,10 @@ ruby_gc_set_params(void)
 
     /* RUBY_GC_HEAP_INIT_SLOTS */
     if (get_envparam_size("RUBY_GC_HEAP_INIT_SLOTS", &gc_params.heap_init_slots, 0)) {
-        gc_set_initial_pages(objspace);
+        gc_set_initial_pages(objspace, TRUE);
+    }
+    else {
+        gc_set_initial_pages(objspace, FALSE);
     }
 
     get_envparam_double("RUBY_GC_HEAP_GROWTH_FACTOR", &gc_params.growth_factor, 1.0, 0.0, FALSE);

man/ruby.1

@@ -550,7 +550,12 @@ There are currently 4 possible areas where the GC may be tuned by
 the following 11 environment variables:
 .Bl -hang -compact -width "RUBY_GC_OLDMALLOC_LIMIT_GROWTH_FACTOR"
 .It Ev RUBY_GC_HEAP_INIT_SLOTS
-Initial allocation slots.  Introduced in Ruby 2.1, default: 10000.
+Initial allocation slots. Applies to all slot sizes.  Introduced in Ruby 2.1, default: 10000.
+.Pp
+.It Ev RUBY_GC_HEAP_INIT_SIZE_%d_SLOTS
+Initial allocation of slots in a specific size pool.
+The available size pools can be found in `GC.stat_heap`.
+Introduced in Ruby 3.3.
 .Pp
 .It Ev RUBY_GC_HEAP_FREE_SLOTS
 Prepare at least this amount of slots after GC.

test/ruby/test_gc.rb

@@ -315,6 +315,21 @@ class TestGc < Test::Unit::TestCase
     }
     assert_normal_exit("exit", "[ruby-core:39777]", :child_env => env)
 
+    env = {}
+    GC.stat_heap.each do |_, s|
+      env["RUBY_GC_HEAP_INIT_SIZE_#{s[:slot_size]}_SLOTS"] = "200000"
+    end
+    assert_normal_exit("exit", "", :child_env => env)
+
+    env["RUBY_GC_HEAP_INIT_SLOTS"] = "100000"
+    assert_normal_exit("exit", "", :child_env => env)
+
+    env = {}
+    GC.stat_heap.each do |_, s|
+      env["RUBY_GC_HEAP_INIT_SIZE_#{s[:slot_size]}_SLOTS"] = "0"
+    end
+    assert_normal_exit("exit", "", :child_env => env)
+
     env = {
       "RUBYOPT" => "",
       "RUBY_GC_HEAP_INIT_SLOTS" => "100000"