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, 44 insertions, 0 deletions

diff --git a/gc.c b/gc.c
index 6c4bffa95d..34334f3278 100644
--- a/gc.c
+++ b/gc.c
@@ -9647,11 +9647,55 @@ gc_start_internal(rb_execution_context_t *ec, VALUE self, VALUE full_mark, VALUE
     return Qnil;
 }
 
+static void
+free_empty_pages(void)
+{
+    rb_objspace_t *objspace = &rb_objspace;
+
+    for (int i = 0; i < SIZE_POOL_COUNT; i++) {
+        /* Move all empty pages to the tomb heap for freeing. */
+        rb_size_pool_t *size_pool = &size_pools[i];
+        rb_heap_t *heap = SIZE_POOL_EDEN_HEAP(size_pool);
+        rb_heap_t *tomb_heap = SIZE_POOL_TOMB_HEAP(size_pool);
+
+        size_t freed_pages = 0;
+
+        struct heap_page **next_page_ptr = &heap->free_pages;
+        struct heap_page *page = heap->free_pages;
+        while (page) {
+            /* All finalizers should have been ran in gc_start_internal, so there
+            * should be no objects that require finalization. */
+            GC_ASSERT(page->final_slots == 0);
+
+            struct heap_page *next_page = page->free_next;
+
+            if (page->free_slots == page->total_slots) {
+                heap_unlink_page(objspace, heap, page);
+                heap_add_page(objspace, size_pool, tomb_heap, page);
+                freed_pages++;
+            }
+            else {
+                *next_page_ptr = page;
+                next_page_ptr = &page->free_next;
+            }
+
+            page = next_page;
+        }
+
+        *next_page_ptr = NULL;
+
+        size_pool_allocatable_pages_set(objspace, size_pool, size_pool->allocatable_pages + freed_pages);
+    }
+
+    heap_pages_free_unused_pages(objspace);
+}
+
 void
 rb_gc_prepare_heap(void)
 {
     rb_objspace_each_objects(gc_set_candidate_object_i, NULL);
     gc_start_internal(NULL, Qtrue, Qtrue, Qtrue, Qtrue, Qtrue);
+    free_empty_pages();
 }
 
 static int