diff --git a/cpp/conf/server_config.template b/cpp/conf/server_config.template
index 215ca45027..2942ffa179 100644
--- a/cpp/conf/server_config.template
+++ b/cpp/conf/server_config.template
@@ -34,7 +34,7 @@ license_config:         # license configure
 
 cache_config:           # cache configure
   cpu_cache_capacity: 16   # how many memory are used as cache, unit: GB, range: 0 ~ less than total memory
-  cache_free_percent: 0.85 # how much memory should be free when cache is full, range: greater than zero ~ 1.0
+  cache_free_percent: 0.85 # old data will be erased from cache when cache is full, this value specify how much memory should be kept, range: greater than zero ~ 1.0
   insert_cache_immediately: false  # insert data will be load into cache immediately for hot query
 
 engine_config:
diff --git a/cpp/src/cache/Cache.cpp b/cpp/src/cache/Cache.cpp
index 0cc804ac5f..a1f2520302 100644
--- a/cpp/src/cache/Cache.cpp
+++ b/cpp/src/cache/Cache.cpp
@@ -163,6 +163,9 @@ void Cache::free_memory() {
 
     int64_t threshhold = capacity_ * freemem_percent_;
     int64_t delta_size = usage_ - threshhold;
+    if(delta_size <= 0) {
+        delta_size = 1;//ensure at least one item erased
+    }
 
     std::set<std::string> key_array;
     int64_t released_size = 0;