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。

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。