diff --git a/src/nasal_gc.cpp b/src/nasal_gc.cpp
index 12ed05f..00975b9 100644
--- a/src/nasal_gc.cpp
+++ b/src/nasal_gc.cpp
@@ -194,9 +194,11 @@ void gc::sweep() {
 
         for (auto& i : collect) {
             for (int j = 0; j < gc_type_size; ++j) {
-                for (auto ptr : i[j]) {
-                    unused[j].push_back(ptr);
-                }
+                unused[j].insert(
+                    unused[j].end(),
+                    std::make_move_iterator(i[j].begin()),
+                    std::make_move_iterator(i[j].end())
+                );
             }
         }
         return;
@@ -240,21 +242,21 @@ void gc::extend(const vm_type type) {
     }
     switch(type) {
         case vm_type::vm_str:
-            total_memory_usage += incr[index] * sizeof(std::string); break;
+            total_object_count += incr[index] * sizeof(std::string); break;
         case vm_type::vm_vec:
-            total_memory_usage += incr[index] * sizeof(nas_vec); break;
+            total_object_count += incr[index] * sizeof(nas_vec); break;
         case vm_type::vm_hash:
-            total_memory_usage += incr[index] * sizeof(nas_hash); break;
+            total_object_count += incr[index] * sizeof(nas_hash); break;
         case vm_type::vm_func:
-            total_memory_usage += incr[index] * sizeof(nas_func); break;
+            total_object_count += incr[index] * sizeof(nas_func); break;
         case vm_type::vm_upval:
-            total_memory_usage += incr[index] * sizeof(nas_upval); break;
+            total_object_count += incr[index] * sizeof(nas_upval); break;
         case vm_type::vm_ghost:
-            total_memory_usage += incr[index] * sizeof(nas_ghost); break;
+            total_object_count += incr[index] * sizeof(nas_ghost); break;
         case vm_type::vm_co:
-            total_memory_usage += incr[index] * sizeof(nas_co); break;
+            total_object_count += incr[index] * sizeof(nas_co); break;
         case vm_type::vm_map:
-            total_memory_usage += incr[index] * sizeof(nas_map); break;
+            total_object_count += incr[index] * sizeof(nas_map); break;
         default: break;
     }
 
@@ -283,22 +285,22 @@ void gc::init(const std::vector<std::string>& constant_strings,
         strs[i] = var::gcobj(new nas_val(vm_type::vm_str));
         strs[i].val.gcobj->immutable = 1;
         strs[i].str() = constant_strings[i];
-        total_memory_usage += strs[i].str().size();
-        total_memory_usage += sizeof(std::string);
+        total_object_count += strs[i].str().size();
+        total_object_count += sizeof(std::string);
     }
 
     // record arguments
     env_argv.resize(argv.size());
     for (u64 i = 0; i < argv.size(); ++i) {
         // incremental initialization, avoid memory leak in repl mode
-        if (env_argv[i].is_str() && env_argv[i].str()==argv[i]) {
+        if (env_argv[i].is_str() && env_argv[i].str() == argv[i]) {
             continue;
         }
         env_argv[i] = var::gcobj(new nas_val(vm_type::vm_str));
         env_argv[i].val.gcobj->immutable = 1;
         env_argv[i].str() = argv[i];
-        total_memory_usage += env_argv[i].str().size();
-        total_memory_usage += sizeof(std::string);
+        total_object_count += env_argv[i].str().size();
+        total_object_count += sizeof(std::string);
     }
 }
 
diff --git a/src/nasal_gc.h b/src/nasal_gc.h
index 4e3691d..36adead 100644
--- a/src/nasal_gc.h
+++ b/src/nasal_gc.h
@@ -38,16 +38,16 @@ struct gc {
 
     /* runtime context */
     context* running_context = nullptr;
-    nas_co* cort = nullptr; // running coroutine
+    nas_co* cort = nullptr;         // running coroutine
 
-    /*  temporary space used in native/module functions */
+    /*  temporary space used in native / module functions */
     var temp = nil;
 
     /* constants and memory pool */
-    std::vector<var> strs = {};        // reserved address for const vm_str
-    std::vector<var> env_argv = {};    // command line arguments
-    std::vector<nas_val*> memory;      // gc memory
-    free_list unused; // gc free list
+    std::vector<var> strs = {};     // reserved address for const vm_str
+    std::vector<var> env_argv = {}; // command line arguments
+    std::vector<nas_val*> memory;   // gc memory
+    free_list unused;               // gc free list
 
     /* heap increase size */
     u64 incr[gc_type_size] = {
@@ -60,8 +60,9 @@ struct gc {
         4,   // vm_co
         1,   // vm_map
     };
-    // total memory usage, not very accurate
-    u64 total_memory_usage = 0;
+
+    // total object count
+    u64 total_object_count = 0;
 
     /* values for analysis */
     u64 size[gc_type_size];
@@ -115,8 +116,9 @@ public:
         return worktime * 1.0 / den * 1000.0;
     }
 
+    // not very accurate
     double get_total_memory() const {
-        return total_memory_usage * 1.0 / 1024.0 / 1024.0;
+        return total_object_count * 3.5 / 1024.0 / 1024.0;
     }
 
 public:
diff --git a/src/nasal_type.h b/src/nasal_type.h
index fb283ed..7cda862 100644
--- a/src/nasal_type.h
+++ b/src/nasal_type.h
@@ -51,7 +51,7 @@ struct nas_map;   // mapper
 // nas_val includes gc-managed types
 struct nas_val {
     enum class gc_status: u8 {
-        uncollected = 0,   
+        uncollected = 0,
         collected,
         found
     };