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：
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$0 N/AInstance size: 40 bytesSpace losses: 1 bytes internal + 0 bytes external = 1 bytes total
填充后对象：
public class DataPadding{ long a1,a2,a3,a4,a5,a6,a7,a8;//防止与前一个对象产生伪共享 int value; long modifyTime; long b1,b2,b3,b4,b5,b6,b7,b8;//防止不相关变量伪共享; boolean flag; long c1,c2,c3,c4,c5,c6,c7,c8;// long createTime; char key; long d1,d2,d3,d4,d5,d6,d7,d8;//防止与下一个对象产生伪共享} 对象结构:
# Running 64-bit HotSpot VM.# Using compressed oop with 3-bit shift.# Using compressed klass with 3-bit shift.# Objects are 8 bytes aligned.# Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]# Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]com.hyr.jol.demo.JOLSample_02_Alignment$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$0 N/AInstance size: 296 bytesSpace losses: 1 bytes internal + 0 bytes external = 1 bytes total
JDK1.8解决缓存伪共享
JDK1.8中增加了Contended注解方式来解决缓存伪共享问题。
在JDK1.8中，新增了一种注解@sun.misc.Contended，来使各个变量在Cache line中分隔开。注意，jvm需要添加参数-XX:-RestrictContended才能开启此功能
未填充对象：
public class DataPadding{ int value; long modifyTime; boolean flag; long createTime; char key; } 对象结构：
# Running 64-bit HotSpot VM.# Using compressed oop with 0-bit shift.# Using compressed klass with 0-bit shift.# Objects are 8 bytes aligned.# Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]# Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]com.hyr.jol.demo.JOLSample_02_Alignment$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$0 N/AInstance size: 40 bytesSpace losses: 1 bytes internal + 0 bytes external = 1 bytes total
使用Contended注解：
public class DataPadding { @sun.misc.Contended("group1") int value; @sun.misc.Contended("group1") long modifyTime; @sun.misc.Contended("group2") boolean flag; @sun.misc.Contended("group3") long createTime; @sun.misc.Contended("group3") char key;} 对象结构：
# Running 64-bit HotSpot VM.# Using compressed oop with 0-bit shift.# Using compressed klass with 3-bit shift.# Objects are 8 bytes aligned.# Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]# Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]com.hyr.jol.demo.JOLSample_02_Alignment$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 bytesSpace 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：