diff --git a/common/collect/sized_queue.go b/common/collect/sized_queue.go
deleted file mode 100644
index 6386ecef5..000000000
--- a/common/collect/sized_queue.go
+++ /dev/null
@@ -1,24 +0,0 @@
-package collect
-
-type SizedQueue struct {
-	elements []interface{}
-	nextPos  int
-}
-
-func NewSizedQueue(size int) *SizedQueue {
-	return &SizedQueue{
-		elements: make([]interface{}, size),
-		nextPos:  0,
-	}
-}
-
-// Put puts a new element into the queue and pop out the first element if queue is full.
-func (this *SizedQueue) Put(element interface{}) interface{} {
-	res := this.elements[this.nextPos]
-	this.elements[this.nextPos] = element
-	this.nextPos++
-	if this.nextPos == len(this.elements) {
-		this.nextPos = 0
-	}
-	return res
-}
diff --git a/common/collect/sized_queue_test.go b/common/collect/sized_queue_test.go
deleted file mode 100644
index 2456095ff..000000000
--- a/common/collect/sized_queue_test.go
+++ /dev/null
@@ -1,18 +0,0 @@
-package collect_test
-
-import (
-	"testing"
-
-	"github.com/v2ray/v2ray-core/common/collect"
-	v2testing "github.com/v2ray/v2ray-core/testing"
-	"github.com/v2ray/v2ray-core/testing/assert"
-)
-
-func TestSizedQueue(t *testing.T) {
-	v2testing.Current(t)
-
-	queue := collect.NewSizedQueue(2)
-	assert.Pointer(queue.Put(1)).IsNil()
-	assert.Pointer(queue.Put(2)).IsNil()
-	assert.Int(queue.Put(3).(int)).Equals(1)
-}
diff --git a/common/dice/dice.go b/common/dice/dice.go
index 01d2ca804..742d563fb 100644
--- a/common/dice/dice.go
+++ b/common/dice/dice.go
@@ -2,6 +2,7 @@ package dice
 
 import (
 	"math/rand"
+	"time"
 )
 
 func Roll(n int) int {
@@ -10,3 +11,7 @@ func Roll(n int) int {
 	}
 	return rand.Intn(n)
 }
+
+func init() {
+	rand.Seed(time.Now().Unix())
+}
diff --git a/common/net/testing/port.go b/common/net/testing/port.go
index b49d60a79..465f86dbd 100644
--- a/common/net/testing/port.go
+++ b/common/net/testing/port.go
@@ -6,5 +6,5 @@ import (
 )
 
 func PickPort() v2net.Port {
-	return v2net.Port(30000 + dice.Roll(10000))
+	return v2net.Port(30000 + dice.Roll(5000))
 }
diff --git a/proxy/vmess/protocol/userset.go b/proxy/vmess/protocol/userset.go
index b36df0a5a..2f12289c8 100644
--- a/proxy/vmess/protocol/userset.go
+++ b/proxy/vmess/protocol/userset.go
@@ -4,7 +4,6 @@ import (
 	"sync"
 	"time"
 
-	"github.com/v2ray/v2ray-core/common/collect"
 	"github.com/v2ray/v2ray-core/common/serial"
 	"github.com/v2ray/v2ray-core/proxy/vmess"
 )
@@ -31,10 +30,10 @@ func (this Timestamp) HashBytes() []byte {
 }
 
 type idEntry struct {
-	id      *vmess.ID
-	userIdx int
-	lastSec Timestamp
-	hashes  *collect.SizedQueue
+	id             *vmess.ID
+	userIdx        int
+	lastSec        Timestamp
+	lastSecRemoval Timestamp
 }
 
 type UserSet interface {
@@ -67,21 +66,24 @@ func NewTimedUserSet() UserSet {
 
 func (us *TimedUserSet) generateNewHashes(nowSec Timestamp, idx int, entry *idEntry) {
 	var hashValue [16]byte
+	var hashValueRemoval [16]byte
 	idHash := IDHash(entry.id.Bytes())
 	for entry.lastSec <= nowSec {
 		idHash.Write(entry.lastSec.Bytes())
 		idHash.Sum(hashValue[:0])
 		idHash.Reset()
 
-		hash2Remove := entry.hashes.Put(hashValue)
+		idHash.Write(entry.lastSecRemoval.Bytes())
+		idHash.Sum(hashValueRemoval[:0])
+		idHash.Reset()
 
 		us.access.Lock()
 		us.userHash[hashValue] = indexTimePair{idx, entry.lastSec}
-		if hash2Remove != nil {
-			delete(us.userHash, hash2Remove.([16]byte))
-		}
+		delete(us.userHash, hashValueRemoval)
 		us.access.Unlock()
+
 		entry.lastSec++
+		entry.lastSecRemoval++
 	}
 }
 
@@ -101,19 +103,19 @@ func (us *TimedUserSet) AddUser(user *vmess.User) error {
 	nowSec := time.Now().Unix()
 
 	entry := &idEntry{
-		id:      user.ID,
-		userIdx: idx,
-		lastSec: Timestamp(nowSec - cacheDurationSec),
-		hashes:  collect.NewSizedQueue(2*cacheDurationSec + 1),
+		id:             user.ID,
+		userIdx:        idx,
+		lastSec:        Timestamp(nowSec - cacheDurationSec),
+		lastSecRemoval: Timestamp(nowSec - cacheDurationSec*3),
 	}
 	us.generateNewHashes(Timestamp(nowSec+cacheDurationSec), idx, entry)
 	us.ids = append(us.ids, entry)
 	for _, alterid := range user.AlterIDs {
 		entry := &idEntry{
-			id:      alterid,
-			userIdx: idx,
-			lastSec: Timestamp(nowSec - cacheDurationSec),
-			hashes:  collect.NewSizedQueue(2*cacheDurationSec + 1),
+			id:             alterid,
+			userIdx:        idx,
+			lastSec:        Timestamp(nowSec - cacheDurationSec),
+			lastSecRemoval: Timestamp(nowSec - cacheDurationSec*3),
 		}
 		us.generateNewHashes(Timestamp(nowSec+cacheDurationSec), idx, entry)
 		us.ids = append(us.ids, entry)