diff --git a/HISTORY b/HISTORY
index e9a3877..b74e877 100644
--- a/HISTORY
+++ b/HISTORY
@@ -1,7 +1,7 @@
 
 Version 1.24  2015-12-25
   * php extension compiled on PHP 7
-  * add skiplist
+  * add skiplist which support stable sort
   * make.sh: use sed to replace perl
 
 Version 1.23  2015-11-16
diff --git a/src/skiplist.c b/src/skiplist.c
index debc650..f77ff71 100644
--- a/src/skiplist.c
+++ b/src/skiplist.c
@@ -18,11 +18,12 @@
 #include "skiplist.h"
 
 int skiplist_init_ex(Skiplist *sl, const int level_count,
-        skiplist_compare_func compare_func, const int alloc_elements_once)
+        skiplist_compare_func compare_func, const int min_alloc_elements_once)
 {
     int bytes;
     int element_size;
     int i;
+    int alloc_elements_once;
     int result;
     struct fast_mblock_man *top_mblock;
 
@@ -33,7 +34,7 @@ int skiplist_init_ex(Skiplist *sl, const int level_count,
         return EINVAL;
     }
 
-    if (level_count > 32) {
+    if (level_count > 20) {
         logError("file: "__FILE__", line: %d, "
                 "level count: %d is too large",
                 __LINE__, level_count);
@@ -50,13 +51,24 @@ int skiplist_init_ex(Skiplist *sl, const int level_count,
     }
     memset(sl->mblocks, 0, bytes);
 
-    for (i=0; i<level_count; i++) {
+    alloc_elements_once = min_alloc_elements_once;
+    if (alloc_elements_once <= 0) {
+        alloc_elements_once = SKIPLIST_DEFAULT_MIN_ALLOC_ELEMENTS_ONCE;
+    }
+    else if (alloc_elements_once > 1024) {
+        alloc_elements_once = 1024;
+    }
+
+    for (i=level_count-1; i>=0; i--) {
         element_size = sizeof(SkiplistNode) + sizeof(SkiplistNode *) * (i + 1);
         if ((result=fast_mblock_init_ex(sl->mblocks + i,
             element_size, alloc_elements_once, NULL, false)) != 0)
         {
             return result;
         }
+        if (alloc_elements_once < 1024 * 1024) {
+            alloc_elements_once *= 2;
+        }
     }
 
     sl->top_level_index = level_count - 1;
@@ -67,6 +79,17 @@ int skiplist_init_ex(Skiplist *sl, const int level_count,
     }
     memset(sl->top, 0, top_mblock->info.element_size);
 
+    sl->tail = (SkiplistNode *)fast_mblock_alloc_object(sl->mblocks + 0);
+    if (sl->tail == NULL) {
+        return ENOMEM;
+    }
+    memset(sl->tail, 0, sl->mblocks[0].info.element_size);
+
+    sl->tail->prev = sl->top;
+    for (i=0; i<level_count; i++) {
+        sl->top->links[i] = sl->tail;
+    }
+
     sl->level_count = level_count;
     sl->compare_func = compare_func;
 
@@ -109,7 +132,7 @@ int skiplist_insert(Skiplist *sl, void *data)
     int level_index;
     SkiplistNode *node;
     SkiplistNode *previous;
-    SkiplistNode *current;
+    SkiplistNode *current = NULL;
 
     level_index = skiplist_get_level_index(sl);
     node = (SkiplistNode *)fast_mblock_alloc_object(sl->mblocks + level_index);
@@ -119,16 +142,16 @@ int skiplist_insert(Skiplist *sl, void *data)
 
     previous = sl->top;
     for (i=sl->top_level_index; i>level_index; i--) {
-        while (previous->links[i] != NULL && sl->compare_func(data,
-                    previous->links[i]->data) > 0)
+        while (previous->links[i] != sl->tail && sl->compare_func(data,
+                    previous->links[i]->data) < 0)
         {
             previous = previous->links[i];
         }
     }
 
     while (i >= 0) {
-        while (previous->links[i] != NULL && sl->compare_func(data,
-                    previous->links[i]->data) > 0)
+        while (previous->links[i] != sl->tail && sl->compare_func(data,
+                    previous->links[i]->data) < 0)
         {
             previous = previous->links[i];
         }
@@ -140,6 +163,8 @@ int skiplist_insert(Skiplist *sl, void *data)
         i--;
     }
 
+    node->prev = previous;
+    current->prev = node;
     node->data = data;
     return 0;
 }
@@ -155,9 +180,9 @@ static SkiplistNode *skiplist_get_previous(Skiplist *sl, void *data,
     found = NULL;
     previous = sl->top;
     for (i=sl->top_level_index; i>=0; i--) {
-        while (previous->links[i] != NULL) {
+        while (previous->links[i] != sl->tail) {
             cmp = sl->compare_func(data, previous->links[i]->data);
-            if (cmp < 0) {
+            if (cmp > 0) {
                 break;
             }
             else if (cmp == 0) {
@@ -188,8 +213,8 @@ int skiplist_delete(Skiplist *sl, void *data)
 
     deleted = previous->links[level_index];
     for (i=level_index; i>=0; i--) {
-        while (previous->links[i] != NULL && sl->compare_func(data,
-                    previous->links[i]->data) > 0)
+        while (previous->links[i] != sl->tail && sl->compare_func(data,
+                    previous->links[i]->data) < 0)
         {
             previous = previous->links[i];
         }
@@ -198,6 +223,8 @@ int skiplist_delete(Skiplist *sl, void *data)
         previous->links[i] = previous->links[i]->links[i];
     }
 
+    deleted->links[0]->prev = previous;
+
     fast_mblock_free_object(sl->mblocks + level_index, deleted);
     return 0;
 }
diff --git a/src/skiplist.h b/src/skiplist.h
index 5566818..95849ee 100644
--- a/src/skiplist.h
+++ b/src/skiplist.h
@@ -6,7 +6,7 @@
 * Please visit the FastDFS Home Page http://www.csource.org/ for more detail.
 **/
 
-//skiplist.h
+//skiplist.h, support stable sort  :)
 #ifndef _SKIPLIST_H
 #define _SKIPLIST_H
 
@@ -21,7 +21,7 @@ typedef int (*skiplist_compare_func)(const void *p1, const void *p2);
 typedef struct skiplist_node
 {
     void *data;
-//    struct skiplist_node *prev;   //for stable sort
+    struct skiplist_node *prev;   //for stable sort
     struct skiplist_node *links[0];
 } SkiplistNode;
 
@@ -32,9 +32,11 @@ typedef struct skiplist
     skiplist_compare_func compare_func;
     struct fast_mblock_man *mblocks;  //node allocators
     SkiplistNode *top;   //the top node
+    SkiplistNode *tail;  //the tail node for interator
 } Skiplist;
 
 typedef struct skiplist_iterator {
+    Skiplist *sl;
     SkiplistNode *current;
 } SkiplistIterator;
 
@@ -42,11 +44,14 @@ typedef struct skiplist_iterator {
 extern "C" {
 #endif
 
+#define SKIPLIST_DEFAULT_MIN_ALLOC_ELEMENTS_ONCE 128
+
 #define skiplist_init(sl, level_count, compare_func) \
-    skiplist_init_ex(sl, level_count, compare_func, 1024)
+    skiplist_init_ex(sl, level_count, compare_func,  \
+    SKIPLIST_DEFAULT_MIN_ALLOC_ELEMENTS_ONCE)
 
 int skiplist_init_ex(Skiplist *sl, const int level_count,
-        skiplist_compare_func compare_func, const int alloc_elements_once);
+        skiplist_compare_func compare_func, const int min_alloc_elements_once);
 
 void skiplist_destroy(Skiplist *sl);
 
@@ -56,19 +61,20 @@ void *skiplist_find(Skiplist *sl, void *data);
 
 static inline void skiplist_iterator(Skiplist *sl, SkiplistIterator *iterator)
 {
-    iterator->current = sl->top->links[0];
+    iterator->sl = sl;
+    iterator->current = sl->tail->prev;
 }
 
 static inline void *skiplist_next(SkiplistIterator *iterator)
 {
     void *data;
 
-    if (iterator->current == NULL) {
+    if (iterator->current == iterator->sl->top) {
         return NULL;
     }
 
     data = iterator->current->data;
-    iterator->current = iterator->current->links[0];
+    iterator->current = iterator->current->prev;
     return data;
 }
 
diff --git a/src/tests/test_skiplist.c b/src/tests/test_skiplist.c
index 4a9c59d..f4fb9dd 100644
--- a/src/tests/test_skiplist.c
+++ b/src/tests/test_skiplist.c
@@ -91,6 +91,72 @@ static void test_delete()
     assert(i==0);
 }
 
+typedef struct record
+{
+    int line;
+    int key;
+} Record;
+
+static int compare_record(const void *p1, const void *p2)
+{
+    return ((Record *)p1)->key - ((Record *)p2)->key;
+}
+
+static int test_stable_sort()
+{
+#define RECORDS 20 
+    int i;
+    int result;
+    int index1;
+    int index2;
+    Skiplist sl;
+    SkiplistIterator iterator;
+    Record records[RECORDS];
+    Record *record;
+    void *value;
+
+    result = skiplist_init_ex(&sl, 16, compare_record, 128);
+    if (result != 0) {
+        return result;
+    }
+
+    for (i=0; i<RECORDS; i++) {
+        records[i].line = i + 1;
+        records[i].key = i + 1;
+    }
+
+    for (i=0; i<RECORDS/4; i++) {
+        index1 = (RECORDS - 1) * (int64_t)rand() / (int64_t)RAND_MAX;
+        index2 = RECORDS - 1 - index1;
+        if (index1 != index2) {
+            records[index1].key = records[index2].key;
+        }
+    }
+
+    for (i=0; i<RECORDS; i++) {
+        if ((result=skiplist_insert(&sl, records + i)) != 0) {
+            return result;
+        }
+    }
+
+    for (i=0; i<RECORDS; i++) {
+        value = skiplist_find(&sl, records + i);
+        assert(value != NULL && ((Record *)value)->key == records[i].key);
+    }
+
+    i = 0;
+    skiplist_iterator(&sl, &iterator);
+    while ((value=skiplist_next(&iterator)) != NULL) {
+        i++;
+        record = (Record *)value;
+        printf("%d => #%d\n", record->key, record->line);
+    }
+    assert(i==RECORDS);
+
+    skiplist_destroy(&sl);
+    return 0;
+}
+
 int main(int argc, char *argv[])
 {
     int i;
@@ -117,6 +183,7 @@ int main(int argc, char *argv[])
         numbers[index2] = tmp;
     }
 
+    fast_mblock_manager_init();
     result = skiplist_init_ex(&sl, 12, compare_func, 128);
     if (result != 0) {
         return result;
@@ -125,6 +192,8 @@ int main(int argc, char *argv[])
     test_insert();
     printf("\n");
 
+    fast_mblock_manager_stat_print(false);
+
     test_delete();
     printf("\n");
 
@@ -135,6 +204,8 @@ int main(int argc, char *argv[])
     printf("\n");
     skiplist_destroy(&sl);
 
+    test_stable_sort();
+
     printf("pass OK\n");
     return 0;
 }