缓存伪共享问题以及解决方案缓存行填充

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