缓存伪共享
共享对象存在同一个缓存中，由于MESI协议，一个对象中一些不需要改变的属性因为其他改变的属性，导致整个对象的缓存进入到M被修改状态。
MESI缓存一致性协议：https://blog.csdn.net/huangyueranbbc/article/details/84554271
![]()
目前的CPU是通常按照32或者64字节的缓存行(Cache Line)进行读取，如果读取的数据在同一个CacheLine，就存在缓存伪共享的问题。
对象被放入一个CacheLine中，根据MSEI协议，其中一个属性改变，其他所有没有改变的属性也变得不可共享。
填充Cache Line缓存块
通过填充对象，将对象中常被改变的属性和不常改变的属性分开到不通缓存Cache Line中。避免缓存的伪共享。
未填充对象：
public class DataPadding{
int value;
long modifyTime;
boolean flag;
long createTime;
char key;
}
对象结构:
# Running 64-bit HotSpot VM.
# Using compressed oop with 3-bit shift.
# Using compressed klass with 3-bit shift.
# Objects are 8 bytes aligned.
# Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
# Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
com.hyr.jol.demo.JOLSample_02_Alignment$DataPadding object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 12 (object header) N/A
12 4 int DataPadding.value N/A
16 8 long DataPadding.modifyTime N/A
24 8 long DataPadding.createTime N/A
32 2 char DataPadding.key N/A
34 1 boolean DataPadding.flag N/A
35 1 (alignment/padding gap)
36 4 com.hyr.jol.demo.JOLSample_02_Alignment DataPadding.this$0 N/A
Instance size: 40 bytes
Space losses: 1 bytes internal + 0 bytes external = 1 bytes total
填充后对象：
public class DataPadding{
long a1,a2,a3,a4,a5,a6,a7,a8;//防止与前一个对象产生伪共享
int value;
long modifyTime;
long b1,b2,b3,b4,b5,b6,b7,b8;//防止不相关变量伪共享;
boolean flag;
long c1,c2,c3,c4,c5,c6,c7,c8;//
long createTime;
char key;
long d1,d2,d3,d4,d5,d6,d7,d8;//防止与下一个对象产生伪共享
}
对象结构:
# Running 64-bit HotSpot VM.
# Using compressed oop with 3-bit shift.
# Using compressed klass with 3-bit shift.
# Objects are 8 bytes aligned.
# Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
# Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
com.hyr.jol.demo.JOLSample_02_Alignment$DataPadding object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 12 (object header) N/A
12 4 int DataPadding.value N/A
16 8 long DataPadding.a1 N/A
24 8 long DataPadding.a2 N/A
32 8 long DataPadding.a3 N/A
40 8 long DataPadding.a4 N/A
48 8 long DataPadding.a5 N/A
56 8 long DataPadding.a6 N/A
64 8 long DataPadding.a7 N/A
72 8 long DataPadding.a8 N/A
80 8 long DataPadding.modifyTime N/A
88 8 long DataPadding.b1 N/A
96 8 long DataPadding.b2 N/A
104 8 long DataPadding.b3 N/A
112 8 long DataPadding.b4 N/A
120 8 long DataPadding.b5 N/A
128 8 long DataPadding.b6 N/A
136 8 long DataPadding.b7 N/A
144 8 long DataPadding.b8 N/A
152 8 long DataPadding.c1 N/A
160 8 long DataPadding.c2 N/A
168 8 long DataPadding.c3 N/A
176 8 long DataPadding.c4 N/A
184 8 long DataPadding.c5 N/A
192 8 long DataPadding.c6 N/A
200 8 long DataPadding.c7 N/A
208 8 long DataPadding.c8 N/A
216 8 long DataPadding.createTime N/A
224 8 long DataPadding.d1 N/A
232 8 long DataPadding.d2 N/A
240 8 long DataPadding.d3 N/A
248 8 long DataPadding.d4 N/A
256 8 long DataPadding.d5 N/A
264 8 long DataPadding.d6 N/A
272 8 long DataPadding.d7 N/A
280 8 long DataPadding.d8 N/A
288 2 char DataPadding.key N/A
290 1 boolean DataPadding.flag N/A
291 1 (alignment/padding gap)
292 4 com.hyr.jol.demo.JOLSample_02_Alignment DataPadding.this$0 N/A
Instance size: 296 bytes
Space losses: 1 bytes internal + 0 bytes external = 1 bytes total
JDK1.8解决缓存伪共享
JDK1.8中增加了Contended注解方式来解决缓存伪共享问题。
在JDK1.8中，新增了一种注解@sun.misc.Contended，来使各个变量在Cache line中分隔开。注意，jvm需要添加参数-XX:-RestrictContended才能开启此功能
未填充对象：
public class DataPadding{
int value;
long modifyTime;
boolean flag;
long createTime;
char key;
}
对象结构：
# Running 64-bit HotSpot VM.
# Using compressed oop with 0-bit shift.
# Using compressed klass with 0-bit shift.
# Objects are 8 bytes aligned.
# Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
# Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
com.hyr.jol.demo.JOLSample_02_Alignment$DataPadding object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 12 (object header) N/A
12 4 int DataPadding.value N/A
16 8 long DataPadding.modifyTime N/A
24 8 long DataPadding.createTime N/A
32 2 char DataPadding.key N/A
34 1 boolean DataPadding.flag N/A
35 1 (alignment/padding gap)
36 4 com.hyr.jol.demo.JOLSample_02_Alignment DataPadding.this$0 N/A
Instance size: 40 bytes
Space losses: 1 bytes internal + 0 bytes external = 1 bytes total
使用Contended注解：
public class DataPadding {
@sun.misc.Contended("group1")
int value;
@sun.misc.Contended("group1")
long modifyTime;
@sun.misc.Contended("group2")
boolean flag;
@sun.misc.Contended("group3")
long createTime;
@sun.misc.Contended("group3")
char key;
}
对象结构：
# Running 64-bit HotSpot VM.
# Using compressed oop with 0-bit shift.
# Using compressed klass with 3-bit shift.
# Objects are 8 bytes aligned.
# Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
# Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
com.hyr.jol.demo.JOLSample_02_Alignment$DataPadding object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 12 (object header) N/A
12 4 com.hyr.jol.demo.JOLSample_02_Alignment DataPadding.this$0 N/A
16 128 (alignment/padding gap)
144 4 int DataPadding.value N/A
148 4 (alignment/padding gap)
152 8 long DataPadding.modifyTime N/A
160 128 (alignment/padding gap)
288 1 boolean DataPadding.flag N/A
289 135 (alignment/padding gap)
424 8 long DataPadding.createTime N/A
432 2 char DataPadding.key N/A
434 6 (loss due to the next object alignment)
Instance size: 440 bytes
Space losses: 395 bytes internal + 6 bytes external = 401 bytes total
如果想深入了解，可以看下关于CPU Cache、Linux Cache相关的知识。
项目输出对象结构用的JOL。相关技术可以参考我的Github项目：
https://github.com/huangyueranbbc/JVM_AGENT_DEMO
![]()
