diff --git a/tidb-server/main.go b/tidb-server/main.go
index f2d19187fe049..a1062d9fa59a8 100644
--- a/tidb-server/main.go
+++ b/tidb-server/main.go
@@ -405,10 +405,13 @@ func setGlobalVars() {
 	if plannercore.PreparedPlanCacheEnabled() {
 		plannercore.PreparedPlanCacheCapacity = cfg.PreparedPlanCache.Capacity
 		plannercore.PreparedPlanCacheMemoryGuardRatio = cfg.PreparedPlanCache.MemoryGuardRatio
+		if plannercore.PreparedPlanCacheMemoryGuardRatio < 0.0 || plannercore.PreparedPlanCacheMemoryGuardRatio > 1.0 {
+			plannercore.PreparedPlanCacheMemoryGuardRatio = 0.1
+		}
 		plannercore.PreparedPlanCacheMaxMemory = cfg.Performance.MaxMemory
-		if plannercore.PreparedPlanCacheMaxMemory == 0 {
-			total, err := memory.MemTotal()
-			terror.MustNil(err)
+		total, err := memory.MemTotal()
+		terror.MustNil(err)
+		if plannercore.PreparedPlanCacheMaxMemory > total || plannercore.PreparedPlanCacheMaxMemory <= 0 {
 			plannercore.PreparedPlanCacheMaxMemory = total
 		}
 	}
diff --git a/util/kvcache/simple_lru.go b/util/kvcache/simple_lru.go
index 364f0d9c3839b..7120e3a5abb7c 100644
--- a/util/kvcache/simple_lru.go
+++ b/util/kvcache/simple_lru.go
@@ -15,7 +15,6 @@ package kvcache
 
 import (
 	"container/list"
-	"math"
 
 	"github.com/pingcap/tidb/util/memory"
 )
@@ -91,7 +90,8 @@ func (l *SimpleLRUCache) Put(key Key, value Value) {
 
 	memUsed, err := memory.MemUsed()
 	if err != nil {
-		memUsed = math.MaxUint64
+		l.DeleteAll()
+		return
 	}
 
 	for memUsed > uint64(float64(l.quota)*(1.0-l.guard)) || l.size > l.capacity {
@@ -105,7 +105,8 @@ func (l *SimpleLRUCache) Put(key Key, value Value) {
 		if memUsed > uint64(float64(l.quota)*(1.0-l.guard)) {
 			memUsed, err = memory.MemUsed()
 			if err != nil {
-				memUsed = math.MaxUint64
+				l.DeleteAll()
+				return
 			}
 		}
 	}
@@ -123,6 +124,15 @@ func (l *SimpleLRUCache) Delete(key Key) {
 	l.size--
 }
 
+// DeleteAll deletes all elements from the LRU Cache.
+func (l *SimpleLRUCache) DeleteAll() {
+	for lru := l.cache.Back(); lru != nil; lru = l.cache.Back() {
+		l.cache.Remove(lru)
+		delete(l.elements, string(lru.Value.(*cacheEntry).key.Hash()))
+		l.size--
+	}
+}
+
 // Size gets the current cache size.
 func (l *SimpleLRUCache) Size() int {
 	return int(l.size)
diff --git a/util/kvcache/simple_lru_test.go b/util/kvcache/simple_lru_test.go
index 55b30f9b48a92..9ddf9ae52b046 100644
--- a/util/kvcache/simple_lru_test.go
+++ b/util/kvcache/simple_lru_test.go
@@ -192,3 +192,26 @@ func (s *testLRUCacheSuite) TestDelete(c *C) {
 	_, exists = lru.Get(keys[2])
 	c.Assert(exists, IsTrue)
 }
+
+func (s *testLRUCacheSuite) TestDeleteAll(c *C) {
+	lru := NewSimpleLRUCache(3, 0.1, 32<<30)
+
+	keys := make([]*mockCacheKey, 3)
+	vals := make([]int64, 3)
+
+	for i := 0; i < 3; i++ {
+		keys[i] = newMockHashKey(int64(i))
+		vals[i] = int64(i)
+		lru.Put(keys[i], vals[i])
+	}
+	c.Assert(int(lru.size), Equals, 3)
+
+	lru.DeleteAll()
+
+	for i := 0; i < 3; i++ {
+		value, exists := lru.Get(keys[i])
+		c.Assert(exists, IsFalse)
+		c.Assert(value, IsNil)
+		c.Assert(int(lru.size), Equals, 0)
+	}
+}