diff --git a/dora-runtime/src/gc/swiper/controller.rs b/dora-runtime/src/gc/swiper/controller.rs
index 357acbf25..d75b083af 100644
--- a/dora-runtime/src/gc/swiper/controller.rs
+++ b/dora-runtime/src/gc/swiper/controller.rs
@@ -95,10 +95,12 @@ pub fn stop(
         std::usize::MAX
     };
 
+    let min_semi_size = young.allocated_size();
+
     let rest = config.max_heap_size - config.old_size;
     let target_young_size = rest / 2;
     let target_young_size = min(target_young_size, max_young_size);
-    let target_young_size = max(target_young_size, 2 * PAGE_SIZE);
+    let target_young_size = max(target_young_size, 2 * min_semi_size);
     let young_size = align_page_down(target_young_size / 2) * 2;
 
     if old_size + young_size > config.max_heap_size {
diff --git a/dora-runtime/src/gc/swiper/old.rs b/dora-runtime/src/gc/swiper/old.rs
index e6fe13ab5..391e9aabb 100644
--- a/dora-runtime/src/gc/swiper/old.rs
+++ b/dora-runtime/src/gc/swiper/old.rs
@@ -187,6 +187,7 @@ impl OldGenProtected {
         if let Some(page) = self.allocate_page(vm) {
             self.pages.push(page);
             self.pages.sort();
+            self.size += PAGE_SIZE;
 
             self.top = page.object_area_start();
             self.current_limit = page.object_area_end();
@@ -210,6 +211,7 @@ impl OldGenProtected {
             .position(|&p| p == page)
             .expect("missing page");
         self.pages.swap_remove(idx);
+        self.size -= PAGE_SIZE;
         os::discard(page.start(), page.size());
     }
 
diff --git a/dora-runtime/src/gc/swiper/young.rs b/dora-runtime/src/gc/swiper/young.rs
index cd13af195..99c85337f 100644
--- a/dora-runtime/src/gc/swiper/young.rs
+++ b/dora-runtime/src/gc/swiper/young.rs
@@ -130,6 +130,12 @@ impl YoungGen {
         self.current_size.store(size, Ordering::Relaxed);
     }
 
+    pub fn allocated_size(&self) -> usize {
+        let protected = self.alloc.protected.lock();
+        let to_committed = self.to_committed();
+        protected.current_limit.offset_from(to_committed.start())
+    }
+
     pub fn protect_from(&self) {
         if cfg!(debug_assertions) || self.protect {
             let from_space = self.from_committed();
diff --git a/dora-runtime/src/object.rs b/dora-runtime/src/object.rs
index ef04c1178..469e7070b 100644
--- a/dora-runtime/src/object.rs
+++ b/dora-runtime/src/object.rs
@@ -672,10 +672,11 @@ where
     let cls = vm.class_instances.idx(clsid);
     let vtable = cls.vtable.read();
     let vtable: &VTable = vtable.as_ref().unwrap();
-    let mut handle: Ref<Str> = ptr.into();
+    let handle: Ref<Str> = ptr.into();
     handle
-        .header_mut()
+        .header()
         .set_vtblptr(Address::from_ptr(vtable as *const VTable));
+    handle.header().set_metadata_raw(0);
 
     handle
 }