diff --git a/include/pocketpy/codeobject.h b/include/pocketpy/codeobject.h
index db3eec05..4da74cdc 100644
--- a/include/pocketpy/codeobject.h
+++ b/include/pocketpy/codeobject.h
@@ -71,7 +71,8 @@ struct CodeObject {
     std::vector<int> iblocks;       // block index for each bytecode
     std::vector<LineInfo> lines;
     
-    List consts;
+    small_vector<PyObject*, 8> consts;
+
     pod_vector<StrName> varnames;      // local variables
     NameDictInt varnames_inv;
     std::vector<CodeBlock> blocks;
diff --git a/include/pocketpy/frame.h b/include/pocketpy/frame.h
index b5d84902..06df37cd 100644
--- a/include/pocketpy/frame.h
+++ b/include/pocketpy/frame.h
@@ -126,7 +126,7 @@ struct Frame {
     }
 };
 
-using CallstackContainer = std::vector<Frame>;
+using CallstackContainer = small_vector<Frame, 16>;
 
 struct FrameId{
     CallstackContainer* data;
diff --git a/include/pocketpy/vector.h b/include/pocketpy/vector.h
index 62470969..73c02038 100644
--- a/include/pocketpy/vector.h
+++ b/include/pocketpy/vector.h
@@ -162,7 +162,8 @@ public:
 	const T& top() const { return vec.back(); }
 	T popx(){ T t = std::move(vec.back()); vec.pop_back(); return t; }
     void reserve(int n){ vec.reserve(n); }
-	Container& data() { return vec; }
+	Container& container() { return vec; }
+    const Container& container() const { return vec; }
 };
 
 template <typename T, typename Container=std::vector<T>>
@@ -229,8 +230,8 @@ namespace pkpy
 
         pointer data() { return m_begin; }
         const_pointer data() const { return m_begin; }
-        reference operator[](size_type index) { return data()[index]; }
-        const_reference operator[](size_type index) const { return data()[index]; }
+        reference operator[](size_type index) { return m_begin[index]; }
+        const_reference operator[](size_type index) const { return m_begin[index]; }
         iterator begin() { return m_begin; }
         const_iterator begin() const { return m_begin; }
         iterator end() { return m_end; }
diff --git a/src/compiler.cpp b/src/compiler.cpp
index b9554708..dd4fb3cd 100644
--- a/src/compiler.cpp
+++ b/src/compiler.cpp
@@ -1011,7 +1011,7 @@ __EAT_DOTS_END:
         }
         ctx()->emit_(OP_BEGIN_CLASS, namei, BC_KEEPLINE);
 
-        for(auto& c: this->contexts.data()){
+        for(auto& c: this->contexts.container()){
             if(c.is_compiling_class){
                 SyntaxError("nested class is not allowed");
             }
diff --git a/src/vm.cpp b/src/vm.cpp
index 5ea4804b..b63e9541 100644
--- a/src/vm.cpp
+++ b/src/vm.cpp
@@ -69,7 +69,6 @@ namespace pkpy{
     VM::VM(bool enable_os) : heap(this), enable_os(enable_os) {
         this->vm = this;
         this->_c.error = nullptr;
-        this->callstack.reserve(8);
         _stdout = [](const char* buf, int size) { std::cout.write(buf, size); };
         _stderr = [](const char* buf, int size) { std::cerr.write(buf, size); };
         _main = nullptr;
@@ -124,7 +123,7 @@ namespace pkpy{
 #if PK_DEBUG_EXTRA_CHECK
         if(callstack.empty()) PK_FATAL_ERROR();
 #endif
-        return FrameId(&callstack.data(), callstack.size()-1);
+        return FrameId(&callstack.container(), callstack.size()-1);
     }
 
     void VM::_pop_frame(){
@@ -1262,7 +1261,7 @@ void VM::_raise(bool re_raise){
 
 void ManagedHeap::mark() {
     for(PyObject* obj: _no_gc) PK_OBJ_MARK(obj);
-    for(auto& frame : vm->callstack.data()) frame._gc_mark();
+    for(auto& frame : vm->callstack.container()) frame._gc_mark();
     for(PyObject* obj: vm->s_data) PK_OBJ_MARK(obj);
     for(auto [_, co]: vm->_cached_codes) co->_gc_mark();
     if(vm->_last_exception) PK_OBJ_MARK(vm->_last_exception);