用戶態與內核態
JDK早期,synchronized 叫做重量級鎖, 因爲申請鎖資源必須通過kernel, 系統調用
;hello.asm
;write(int fd, const void *buffer, size_t nbytes)
section data
msg db "Hello", 0xA
len equ $ - msg
section .text
global _start
_start:
mov edx, len
mov ecx, msg
mov ebx, 1 ;文件描述符1 std_out
mov eax, 4 ;write函數系統調用號 4
int 0x80
mov ebx, 0
mov eax, 1 ;exit函數系統調用號
int 0x80
CAS
Compare And Swap (Compare And Exchange) / 自旋 / 自旋鎖 / 無鎖 (無重量鎖)
因爲經常配合循環操作,直到完成爲止,所以泛指一類操作
cas(v, a, b) ,變量v,期待值a, 修改值b
ABA問題
什麼意思呢?就是說一個線程把數據A變爲了B,然後又重新變成了A。此時另外一個線程讀取的時候,發現A沒有變化,就誤以爲是原來的那個A。這就是有名的ABA問題。ABA問題會帶來什麼後果呢?我們舉個例子。
一個小偷,把別人家的錢偷了之後又還了回來,還是原來的錢嗎,你老婆出軌之後又回來,還是原來的老婆嗎?ABA問題也一樣,如果不好好解決就會帶來大量的問題。最常見的就是資金問題,也就是別人如果挪用了你的錢,在你發現之前又還了回來。但是別人卻已經觸犯了法律。
解決辦法(版本號 AtomicStampedReference),基礎類型簡單值不需要版本號
Unsafe
AtomicInteger:
public final int incrementAndGet() {
for (;;) {
int current = get();
int next = current + 1;
if (compareAndSet(current, next))
return next;
}
}
public final boolean compareAndSet(int expect, int update) {
return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
}
Unsafe:
public final native boolean compareAndSwapInt(Object var1, long var2, int var4, int var5);
運用:
package com.mashibing.jol;
import sun.misc.Unsafe;
import java.lang.reflect.Field;
public class T02_TestUnsafe {
int i = 0;
private static T02_TestUnsafe t = new T02_TestUnsafe();
public static void main(String[] args) throws Exception {
//Unsafe unsafe = Unsafe.getUnsafe();
Field unsafeField = Unsafe.class.getDeclaredFields()[0];
unsafeField.setAccessible(true);
Unsafe unsafe = (Unsafe) unsafeField.get(null);
Field f = T02_TestUnsafe.class.getDeclaredField("i");
long offset = unsafe.objectFieldOffset(f);
System.out.println(offset);
boolean success = unsafe.compareAndSwapInt(t, offset, 0, 1);
System.out.println(success);
System.out.println(t.i);
//unsafe.compareAndSwapInt()
}
}
jdk8u: unsafe.cpp:
cmpxchg = compare and exchange
UNSAFE_ENTRY(jboolean, Unsafe_CompareAndSwapInt(JNIEnv *env, jobject unsafe, jobject obj, jlong offset, jint e, jint x))
UnsafeWrapper("Unsafe_CompareAndSwapInt");
oop p = JNIHandles::resolve(obj);
jint* addr = (jint *) index_oop_from_field_offset_long(p, offset);
return (jint)(Atomic::cmpxchg(x, addr, e)) == e;
UNSAFE_END
jdk8u: atomic_linux_x86.inline.hpp 93行
is_MP = Multi Processor
inline jint Atomic::cmpxchg (jint exchange_value, volatile jint* dest, jint compare_value) {
int mp = os::is_MP();
__asm__ volatile (LOCK_IF_MP(%4) "cmpxchgl %1,(%3)"
: "=a" (exchange_value)
: "r" (exchange_value), "a" (compare_value), "r" (dest), "r" (mp)
: "cc", "memory");
return exchange_value;
}
jdk8u: os.hpp is_MP()
static inline bool is_MP() {
// During bootstrap if _processor_count is not yet initialized
// we claim to be MP as that is safest. If any platform has a
// stub generator that might be triggered in this phase and for
// which being declared MP when in fact not, is a problem - then
// the bootstrap routine for the stub generator needs to check
// the processor count directly and leave the bootstrap routine
// in place until called after initialization has ocurred.
return (_processor_count != 1) || AssumeMP;
}
jdk8u: atomic_linux_x86.inline.hpp
#define LOCK_IF_MP(mp) "cmp $0, " #mp "; je 1f; lock; 1: "
最終實現:
cmpxchg = cas修改變量值
lock cmpxchg 指令
硬件:
lock指令在執行後面指令的時候鎖定一個北橋信號
(不採用鎖總線的方式)
markword
工具:JOL = Java Object Layout
<dependencies>
<!-- https://mvnrepository.com/artifact/org.openjdk.jol/jol-core -->
<dependency>
<groupId>org.openjdk.jol</groupId>
<artifactId>jol-core</artifactId>
<version>0.9</version>
</dependency>
</dependencies>
jdk8u: markOop.hpp
// Bit-format of an object header (most significant first, big endian layout below):
//
// 32 bits:
// --------
// hash:25 ------------>| age:4 biased_lock:1 lock:2 (normal object)
// JavaThread*:23 epoch:2 age:4 biased_lock:1 lock:2 (biased object)
// size:32 ------------------------------------------>| (CMS free block)
// PromotedObject*:29 ---------->| promo_bits:3 ----->| (CMS promoted object)
//
// 64 bits:
// --------
// unused:25 hash:31 -->| unused:1 age:4 biased_lock:1 lock:2 (normal object)
// JavaThread*:54 epoch:2 unused:1 age:4 biased_lock:1 lock:2 (biased object)
// PromotedObject*:61 --------------------->| promo_bits:3 ----->| (CMS promoted object)
// size:64 ----------------------------------------------------->| (CMS free block)
//
// unused:25 hash:31 -->| cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && normal object)
// JavaThread*:54 epoch:2 cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && biased object)
// narrowOop:32 unused:24 cms_free:1 unused:4 promo_bits:3 ----->| (COOPs && CMS promoted object)
// unused:21 size:35 -->| cms_free:1 unused:7 ------------------>| (COOPs && CMS free block)
synchronized的橫切面詳解
- synchronized原理
- 升級過程
- 彙編實現
- vs reentrantLock的區別
java源碼層級
synchronized(o)
字節碼層級
monitorenter moniterexit
JVM層級(Hotspot)
package com.mashibing.insidesync;
import org.openjdk.jol.info.ClassLayout;
public class T01_Sync1 {
public static void main(String[] args) {
Object o = new Object();
System.out.println(ClassLayout.parseInstance(o).toPrintable());
}
}
com.mashibing.insidesync.T01_Sync1$Lock object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 05 00 00 00 (00000101 00000000 00000000 00000000) (5)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 49 ce 00 20 (01001001 11001110 00000000 00100000) (536923721)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
com.mashibing.insidesync.T02_Sync2$Lock object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 05 90 2e 1e (00000101 10010000 00101110 00011110) (506368005)
4 4 (object header) 1b 02 00 00 (00011011 00000010 00000000 00000000) (539)
8 4 (object header) 49 ce 00 20 (01001001 11001110 00000000 00100000) (536923721)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes tota
InterpreterRuntime:: monitorenter方法
IRT_ENTRY_NO_ASYNC(void, InterpreterRuntime::monitorenter(JavaThread* thread, BasicObjectLock* elem))
#ifdef ASSERT
thread->last_frame().interpreter_frame_verify_monitor(elem);
#endif
if (PrintBiasedLockingStatistics) {
Atomic::inc(BiasedLocking::slow_path_entry_count_addr());
}
Handle h_obj(thread, elem->obj());
assert(Universe::heap()->is_in_reserved_or_null(h_obj()),
"must be NULL or an object");
if (UseBiasedLocking) {
// Retry fast entry if bias is revoked to avoid unnecessary inflation
ObjectSynchronizer::fast_enter(h_obj, elem->lock(), true, CHECK);
} else {
ObjectSynchronizer::slow_enter(h_obj, elem->lock(), CHECK);
}
assert(Universe::heap()->is_in_reserved_or_null(elem->obj()),
"must be NULL or an object");
#ifdef ASSERT
thread->last_frame().interpreter_frame_verify_monitor(elem);
#endif
IRT_END
synchronizer.cpp
revoke_and_rebias
void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock, bool attempt_rebias, TRAPS) {
if (UseBiasedLocking) {
if (!SafepointSynchronize::is_at_safepoint()) {
BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);
if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {
return;
}
} else {
assert(!attempt_rebias, "can not rebias toward VM thread");
BiasedLocking::revoke_at_safepoint(obj);
}
assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
}
slow_enter (obj, lock, THREAD) ;
}
void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {
markOop mark = obj->mark();
assert(!mark->has_bias_pattern(), "should not see bias pattern here");
if (mark->is_neutral()) {
// Anticipate successful CAS -- the ST of the displaced mark must
// be visible <= the ST performed by the CAS.
lock->set_displaced_header(mark);
if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) {
TEVENT (slow_enter: release stacklock) ;
return ;
}
// Fall through to inflate() ...
} else
if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
assert(lock != mark->locker(), "must not re-lock the same lock");
assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock");
lock->set_displaced_header(NULL);
return;
}
#if 0
// The following optimization isn't particularly useful.
if (mark->has_monitor() && mark->monitor()->is_entered(THREAD)) {
lock->set_displaced_header (NULL) ;
return ;
}
#endif
// The object header will never be displaced to this lock,
// so it does not matter what the value is, except that it
// must be non-zero to avoid looking like a re-entrant lock,
// and must not look locked either.
lock->set_displaced_header(markOopDesc::unused_mark());
ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
}
inflate方法:膨脹爲重量級鎖
鎖升級過程
JDK8 markword實現表:
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new - 偏向鎖 - 輕量級鎖 (無鎖, 自旋鎖,自適應自旋)- 重量級鎖
synchronized優化的過程和markword息息相關
用markword中最低的三位代表鎖狀態 其中1位是偏向鎖位 兩位是普通鎖位
-
Object o = new Object()
鎖 = 0 01 無鎖態
注意:如果偏向鎖打開,默認是匿名偏向狀態 -
o.hashCode()
001 + hashcode00000001 10101101 00110100 00110110 01011001 00000000 00000000 00000000
little endian big endian
00000000 00000000 00000000 01011001 00110110 00110100 10101101 00000000
-
默認synchronized(o)
00 -> 輕量級鎖
默認情況 偏向鎖有個時延,默認是4秒
why? 因爲JVM虛擬機自己有一些默認啓動的線程,裏面有好多sync代碼,這些sync代碼啓動時就知道肯定會有競爭,如果使用偏向鎖,就會造成偏向鎖不斷的進行鎖撤銷和鎖升級的操作,效率較低。-XX:BiasedLockingStartupDelay=0
-
如果設定上述參數
new Object () - > 101 偏向鎖 ->線程ID爲0 -> Anonymous BiasedLock
打開偏向鎖,new出來的對象,默認就是一個可偏向匿名對象101 -
如果有線程上鎖
上偏向鎖,指的就是,把markword的線程ID改爲自己線程ID的過程
偏向鎖不可重偏向 批量偏向 批量撤銷 -
如果有線程競爭
撤銷偏向鎖,升級輕量級鎖
線程在自己的線程棧生成LockRecord ,用CAS操作將markword設置爲指向自己這個線程的LR的指針,設置成功者得到鎖 -
如果競爭加劇
競爭加劇:有線程超過10次自旋, -XX:PreBlockSpin, 或者自旋線程數超過CPU核數的一半, 1.6之後,加入自適應自旋 Adapative Self Spinning , JVM自己控制
升級重量級鎖:-> 向操作系統申請資源,linux mutex , CPU從3級-0級系統調用,線程掛起,進入等待隊列,等待操作系統的調度,然後再映射回用戶空間
(以上實驗環境是JDK11,打開就是偏向鎖,而JDK8默認對象頭是無鎖)
偏向鎖默認是打開的,但是有一個時延,如果要觀察到偏向鎖,應該設定參數
如果計算過對象的hashCode,則對象無法進入偏向狀態!
輕量級鎖重量級鎖的hashCode存在與什麼地方?
答案:線程棧中,輕量級鎖的LR中,或是代表重量級鎖的ObjectMonitor的成員中
關於epoch: (不重要)
批量重偏向與批量撤銷淵源:從偏向鎖的加鎖解鎖過程中可看出,當只有一個線程反覆進入同步塊時,偏向鎖帶來的性能開銷基本可以忽略,但是當有其他線程嘗試獲得鎖時,就需要等到safe point時,再將偏向鎖撤銷爲無鎖狀態或升級爲輕量級,會消耗一定的性能,所以在多線程競爭頻繁的情況下,偏向鎖不僅不能提高性能,還會導致性能下降。於是,就有了批量重偏向與批量撤銷的機制。
原理以class爲單位,爲每個class維護解決場景批量重偏向(bulk rebias)機制是爲了解決:一個線程創建了大量對象並執行了初始的同步操作,後來另一個線程也來將這些對象作爲鎖對象進行操作,這樣會導致大量的偏向鎖撤銷操作。批量撤銷(bulk revoke)機制是爲了解決:在明顯多線程競爭劇烈的場景下使用偏向鎖是不合適的。
一個偏向鎖撤銷計數器,每一次該class的對象發生偏向撤銷操作時,該計數器+1,當這個值達到重偏向閾值(默認20)時,JVM就認爲該class的偏向鎖有問題,因此會進行批量重偏向。每個class對象會有一個對應的epoch字段,每個處於偏向鎖狀態對象的Mark Word中也有該字段,其初始值爲創建該對象時class中的epoch的值。每次發生批量重偏向時,就將該值+1,同時遍歷JVM中所有線程的棧,找到該class所有正處於加鎖狀態的偏向鎖,將其epoch字段改爲新值。下次獲得鎖時,發現當前對象的epoch值和class的epoch不相等,那就算當前已經偏向了其他線程,也不會執行撤銷操作,而是直接通過CAS操作將其Mark Word的Thread Id 改成當前線程Id。當達到重偏向閾值後,假設該class計數器繼續增長,當其達到批量撤銷的閾值後(默認40),JVM就認爲該class的使用場景存在多線程競爭,會標記該class爲不可偏向,之後,對於該class的鎖,直接走輕量級鎖的邏輯。
沒錯,我就是廁所所長
加鎖,指的是鎖定對象
鎖升級的過程
JDK較早的版本 OS的資源 互斥量 用戶態 -> 內核態的轉換 重量級 效率比較低
現代版本進行了優化
無鎖 - 偏向鎖 -輕量級鎖(自旋鎖)-重量級鎖
偏向鎖 - markword 上記錄當前線程指針,下次同一個線程加鎖的時候,不需要爭用,只需要判斷線程指針是否同一個,所以,偏向鎖,偏向加鎖的第一個線程 。hashCode備份在線程棧上 線程銷燬,鎖降級爲無鎖
有爭用 - 鎖升級爲輕量級鎖 - 每個線程有自己的LockRecord在自己的線程棧上,用CAS去爭用markword的LR的指針,指針指向哪個線程的LR,哪個線程就擁有鎖
自旋超過10次,升級爲重量級鎖 - 如果太多線程自旋 CPU消耗過大,不如升級爲重量級鎖,進入等待隊列(不消耗CPU)-XX:PreBlockSpin
自旋鎖在 JDK1.4.2 中引入,使用 -XX:+UseSpinning 來開啓。JDK 6 中變爲默認開啓,並且引入了自適應的自旋鎖(適應性自旋鎖)。
自適應自旋鎖意味着自旋的時間(次數)不再固定,而是由前一次在同一個鎖上的自旋時間及鎖的擁有者的狀態來決定。如果在同一個鎖對象上,自旋等待剛剛成功獲得過鎖,並且持有鎖的線程正在運行中,那麼虛擬機就會認爲這次自旋也是很有可能再次成功,進而它將允許自旋等待持續相對更長的時間。如果對於某個鎖,自旋很少成功獲得過,那在以後嘗試獲取這個鎖時將可能省略掉自旋過程,直接阻塞線程,避免浪費處理器資源。
偏向鎖由於有鎖撤銷的過程revoke,會消耗系統資源,所以,在鎖爭用特別激烈的時候,用偏向鎖未必效率高。還不如直接使用輕量級鎖。
synchronized最底層實現
public class T {
static volatile int i = 0;
public static void n() { i++; }
public static synchronized void m() {}
publics static void main(String[] args) {
for(int j=0; j<1000_000; j++) {
m();
n();
}
}
}
java -XX:+UnlockDiagnosticVMOptions -XX:+PrintAssembly T
C1 Compile Level 1 (一級優化)
C2 Compile Level 2 (二級優化)
找到m() n()方法的彙編碼,會看到 lock comxchg …指令
synchronized vs Lock (CAS)
在高爭用 高耗時的環境下synchronized效率更高
在低爭用 低耗時的環境下CAS效率更高
synchronized到重量級之後是等待隊列(不消耗CPU)
CAS(等待期間消耗CPU)
一切以實測爲準
鎖消除 lock eliminate
public void add(String str1,String str2){
StringBuffer sb = new StringBuffer();
sb.append(str1).append(str2);
}
我們都知道 StringBuffer 是線程安全的,因爲它的關鍵方法都是被 synchronized 修飾過的,但我們看上面這段代碼,我們會發現,sb 這個引用只會在 add 方法中使用,不可能被其它線程引用(因爲是局部變量,棧私有),因此 sb 是不可能共享的資源,JVM 會自動消除 StringBuffer 對象內部的鎖。
鎖粗化 lock coarsening
public String test(String str){
int i = 0;
StringBuffer sb = new StringBuffer():
while(i < 100){
sb.append(str);
i++;
}
return sb.toString():
}
JVM 會檢測到這樣一連串的操作都對同一個對象加鎖(while 循環內 100 次執行 append,沒有鎖粗化的就要進行 100 次加鎖/解鎖),此時 JVM 就會將加鎖的範圍粗化到這一連串的操作的外部(比如 while 虛幻體外),使得這一連串操作只需要加一次鎖即可。
鎖降級(不重要)
https://www.zhihu.com/question/63859501
其實,只被VMThread訪問,降級也就沒啥意義了。所以可以簡單認爲鎖降級不存在!
超線程
一個ALU + 兩組Registers + PC
參考資料
http://openjdk.java.net/groups/hotspot/docs/HotSpotGlossary.html
volatile的用途
1.線程可見性
package com.mashibing.testvolatile;
public class T01_ThreadVisibility {
private static volatile boolean flag = true;
public static void main(String[] args) throws InterruptedException {
new Thread(()-> {
while (flag) {
//do sth
}
System.out.println("end");
}, "server").start();
Thread.sleep(1000);
flag = false;
}
}
2.防止指令重排序
問題:DCL單例需不需要加volatile?
CPU的基礎知識
-
緩存行對齊
緩存行64個字節是CPU同步的基本單位,緩存行隔離會比僞共享效率要高
Disruptorpackage com.mashibing.juc.c_028_FalseSharing; public class T02_CacheLinePadding { private static class Padding { public volatile long p1, p2, p3, p4, p5, p6, p7; // } private static class T extends Padding { public volatile long x = 0L; } public static T[] arr = new T[2]; static { arr[0] = new T(); arr[1] = new T(); } public static void main(String[] args) throws Exception { Thread t1 = new Thread(()->{ for (long i = 0; i < 1000_0000L; i++) { arr[0].x = i; } }); Thread t2 = new Thread(()->{ for (long i = 0; i < 1000_0000L; i++) { arr[1].x = i; } }); final long start = System.nanoTime(); t1.start(); t2.start(); t1.join(); t2.join(); System.out.println((System.nanoTime() - start)/100_0000); } }
MESI
-
僞共享
-
合併寫
CPU內部的4個字節的Bufferpackage com.mashibing.juc.c_029_WriteCombining; public final class WriteCombining { private static final int ITERATIONS = Integer.MAX_VALUE; private static final int ITEMS = 1 << 24; private static final int MASK = ITEMS - 1; private static final byte[] arrayA = new byte[ITEMS]; private static final byte[] arrayB = new byte[ITEMS]; private static final byte[] arrayC = new byte[ITEMS]; private static final byte[] arrayD = new byte[ITEMS]; private static final byte[] arrayE = new byte[ITEMS]; private static final byte[] arrayF = new byte[ITEMS]; public static void main(final String[] args) { for (int i = 1; i <= 3; i++) { System.out.println(i + " SingleLoop duration (ns) = " + runCaseOne()); System.out.println(i + " SplitLoop duration (ns) = " + runCaseTwo()); } } public static long runCaseOne() { long start = System.nanoTime(); int i = ITERATIONS; while (--i != 0) { int slot = i & MASK; byte b = (byte) i; arrayA[slot] = b; arrayB[slot] = b; arrayC[slot] = b; arrayD[slot] = b; arrayE[slot] = b; arrayF[slot] = b; } return System.nanoTime() - start; } public static long runCaseTwo() { long start = System.nanoTime(); int i = ITERATIONS; while (--i != 0) { int slot = i & MASK; byte b = (byte) i; arrayA[slot] = b; arrayB[slot] = b; arrayC[slot] = b; } i = ITERATIONS; while (--i != 0) { int slot = i & MASK; byte b = (byte) i; arrayD[slot] = b; arrayE[slot] = b; arrayF[slot] = b; } return System.nanoTime() - start; } }
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指令重排序
package com.mashibing.jvm.c3_jmm; public class T04_Disorder { private static int x = 0, y = 0; private static int a = 0, b =0; public static void main(String[] args) throws InterruptedException { int i = 0; for(;;) { i++; x = 0; y = 0; a = 0; b = 0; Thread one = new Thread(new Runnable() { public void run() { //由於線程one先啓動,下面這句話讓它等一等線程two. 讀着可根據自己電腦的實際性能適當調整等待時間. //shortWait(100000); a = 1; x = b; } }); Thread other = new Thread(new Runnable() { public void run() { b = 1; y = a; } }); one.start();other.start(); one.join();other.join(); String result = "第" + i + "次 (" + x + "," + y + ")"; if(x == 0 && y == 0) { System.err.println(result); break; } else { //System.out.println(result); } } } public static void shortWait(long interval){ long start = System.nanoTime(); long end; do{ end = System.nanoTime(); }while(start + interval >= end); } }
系統底層如何實現數據一致性
- MESI如果能解決,就使用MESI
- 如果不能,就鎖總線
系統底層如何保證有序性
- 內存屏障sfence mfence lfence等系統原語
- 鎖總線
volatile如何解決指令重排序
1: volatile i
2: ACC_VOLATILE
3: JVM的內存屏障
屏障兩邊的指令不可以重排!保障有序!
4:hotspot實現
bytecodeinterpreter.cpp
int field_offset = cache->f2_as_index();
if (cache->is_volatile()) {
if (support_IRIW_for_not_multiple_copy_atomic_cpu) {
OrderAccess::fence();
}
orderaccess_linux_x86.inline.hpp
inline void OrderAccess::fence() {
if (os::is_MP()) {
// always use locked addl since mfence is sometimes expensive
#ifdef AMD64
__asm__ volatile ("lock; addl $0,0(%%rsp)" : : : "cc", "memory");
#else
__asm__ volatile ("lock; addl $0,0(%%esp)" : : : "cc", "memory");
#endif
}
}