diff --git a/src/nasal_gc.cpp b/src/nasal_gc.cpp
index b565c7e..12ed05f 100644
--- a/src/nasal_gc.cpp
+++ b/src/nasal_gc.cpp
@@ -25,7 +25,7 @@ void gc::mark() {
     mark_context_root(bfs);
 
     // concurrent mark, experimental
-    if (memory.size()>UINT16_MAX && bfs.size()>32) {
+    if (memory.size() > gc::concurrent_threshold() && bfs.size() > 16) {
         flag_concurrent_mark_triggered = true;
         usize size = bfs.size();
         std::thread t0(&gc::concurrent_mark, this, std::ref(bfs), 0, size/4);
@@ -179,6 +179,29 @@ void gc::mark_map(std::vector<var>& bfs_queue, nas_map& mp) {
 }
 
 void gc::sweep() {
+    if (memory.size() > gc::concurrent_threshold()) {
+        flag_concurrent_mark_triggered = true;
+        usize size = memory.size();
+        std::vector<free_list> collect = {{}, {}, {}, {}};
+        std::thread t0(&gc::concurrent_sweep, this, std::ref(collect[0]), 0, size/4);
+        std::thread t1(&gc::concurrent_sweep, this, std::ref(collect[1]), size/4, size/2);
+        std::thread t2(&gc::concurrent_sweep, this, std::ref(collect[2]), size/2, size/4*3);
+        std::thread t3(&gc::concurrent_sweep, this, std::ref(collect[3]), size/4*3, size);
+        t0.join();
+        t1.join();
+        t2.join();
+        t3.join();
+
+        for (auto& i : collect) {
+            for (int j = 0; j < gc_type_size; ++j) {
+                for (auto ptr : i[j]) {
+                    unused[j].push_back(ptr);
+                }
+            }
+        }
+        return;
+    }
+
     for(auto i : memory) {
         if (i->mark==nas_val::gc_status::uncollected) {
             i->clear();
@@ -190,6 +213,19 @@ void gc::sweep() {
     }
 }
 
+void gc::concurrent_sweep(free_list& collect, usize begin, usize end) {
+    for (usize iter = begin; iter < end; ++ iter) {
+        auto i = memory[iter];
+        if (i->mark==nas_val::gc_status::uncollected) {
+            i->clear();
+            collect[static_cast<u8>(i->type)-static_cast<u8>(vm_type::vm_str)].push_back(i);
+            i->mark = nas_val::gc_status::collected;
+        } else if (i->mark==nas_val::gc_status::found) {
+            i->mark = nas_val::gc_status::uncollected;
+        }
+    }
+}
+
 void gc::extend(const vm_type type) {
     const u8 index = static_cast<u8>(type)-static_cast<u8>(vm_type::vm_str);
     size[index] += incr[index];
@@ -223,7 +259,7 @@ void gc::extend(const vm_type type) {
     }
 
     // if incr[index] = 1, this will always be 1
-    incr[index] = incr[index] + incr[index]/2;
+    incr[index] = incr[index] + incr[index];
 }
 
 void gc::init(const std::vector<std::string>& constant_strings,
diff --git a/src/nasal_gc.h b/src/nasal_gc.h
index 76631aa..4e3691d 100644
--- a/src/nasal_gc.h
+++ b/src/nasal_gc.h
@@ -20,6 +20,14 @@
 
 namespace nasal {
 
+struct free_list {
+    std::vector<nas_val*> elem[gc_type_size];
+
+    auto& operator[](i64 index) {
+        return elem[index];
+    }
+};
+
 struct gc {
     /* main context temporary storage */
     context main_context;
@@ -39,18 +47,18 @@ struct gc {
     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
-    std::vector<nas_val*> unused[gc_type_size]; // gc free list
+    free_list unused; // gc free list
 
     /* heap increase size */
     u64 incr[gc_type_size] = {
-        128, // vm_str
-        128, // vm_vec
-        64,  // vm_hash
+        256, // vm_str
+        512, // vm_vec
+        512, // vm_hash
         256, // vm_func
         256, // vm_upval
-        16,  // vm_obj
-        16,  // vm_co
-        2,   // vm_map
+        4,   // vm_obj
+        4,   // vm_co
+        1,   // vm_map
     };
     // total memory usage, not very accurate
     u64 total_memory_usage = 0;
@@ -86,6 +94,11 @@ private:
     void mark_co(std::vector<var>&, nas_co&);
     void mark_map(std::vector<var>&, nas_map&);
     void sweep();
+    void concurrent_sweep(free_list&, usize, usize);
+
+    static const auto concurrent_threshold() {
+        return UINT16_MAX * 16;
+    }
 
 public:
     void extend(const vm_type);
diff --git a/test/gc_test.nas b/test/gc_test.nas
index c8f526d..6f72d76 100644
--- a/test/gc_test.nas
+++ b/test/gc_test.nas
@@ -17,9 +17,9 @@ var test_func = func(test_processes...) {
         f();
         duration = time_stamp.elapsedMSec();
         info = runtime.gc.info();
-        println(duration, " ms, gc ",
-            (info.total-gc_total)*100/duration, "%, ",
-            1000/duration, " count/sec"
+        println(duration, " ms,\tgc ",
+            (info.total-gc_total)*100/duration, "%,\t",
+            1000/duration, " loop/sec"
         );
         gc_total = info.total;
     }
@@ -28,13 +28,13 @@ var test_func = func(test_processes...) {
         test_process_total.elapsedMSec(), " ms");
     
     info = runtime.gc.info();
-    println("##-gc----------------");
-    println("total    : ", info.total, " ms");
-    println("average  : ", info.average, " ms");
-    println("max gc   : ", info.max_gc, " ms");
-    println("max mark : ", info.max_mark, " ms");
-    println("max sweep: ", info.max_sweep, " ms");
-    println("---------------------");
+    println("+##-gc-----------------");
+    println("| total    : ", info.total, " ms");
+    println("| average  : ", info.average, " ms");
+    println("| max gc   : ", info.max_gc, " ms");
+    println("| max mark : ", info.max_mark, " ms");
+    println("| max sweep: ", info.max_sweep, " ms");
+    println("+----------------------");
 }
 
 var MAX_ITER_NUM = 2e5;
@@ -92,12 +92,12 @@ var append_hash_in_hash = func {
 
 for (var i = 0; i < 10; i += 1) {
     test_func(
-        ["vec", append_vec],
-        ["hash", append_hash],
-        ["func", append_func],
-        ["vec<vec>", append_vec_in_vec],
-        ["vec<hash>", append_hash_in_vec],
-        ["hash<str, vec>", append_vec_in_hash],
+        ["vec            ", append_vec],
+        ["hash           ", append_hash],
+        ["func           ", append_func],
+        ["vec<vec>       ", append_vec_in_vec],
+        ["vec<hash>      ", append_hash_in_vec],
+        ["hash<str, vec> ", append_vec_in_hash],
         ["hash<str, hash>", append_hash_in_hash]
     );
 }
\ No newline at end of file