緩存僞共享問題以及解決方案緩存行填充
緩存僞共享
共享對象存在同一個緩存中，因爲MESI協議，一個對象中一些不須要改變的屬性由於其餘改變的屬性，致使整個對象的緩存進入到M被修改狀態。java
MESI緩存一致性協議：https://blog.csdn.net/huangyueranbbc/article/details/84554271git
目前的CPU是一般按照32或者64字節的緩存行(Cache Line)進行讀取，若是讀取的數據在同一個CacheLine，就存在緩存僞共享的問題。github
對象被放入一個CacheLine中，根據MSEI協議，其中一個屬性改變，其餘全部沒有改變的屬性也變得不可共享。緩存
填充Cache Line緩存塊
經過填充對象，將對象中常被改變的屬性和不常改變的屬性分開到不通緩存Cache Line中。避免緩存的僞共享。ruby
未填充對象：
public class DataPadding{
int value;
long modifyTime;
boolean flag;
long createTime;
char key;
}
對象結構:
# 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$0 N/A
Instance size: 40 bytes
Space losses: 1 bytes internal + 0 bytes external = 1 bytes total
填充後對象：
public class DataPadding{
long a1,a2,a3,a4,a5,a6,a7,a8;//防止與前一個對象產生僞共享
int value;
long modifyTime;
long b1,b2,b3,b4,b5,b6,b7,b8;//防止不相關變量僞共享;
boolean flag;
long c1,c2,c3,c4,c5,c6,c7,c8;//
long createTime;
char key;
long d1,d2,d3,d4,d5,d6,d7,d8;//防止與下一個對象產生僞共享
}
對象結構:
# Running 64-bit HotSpot VM.
# Using compressed oop with 3-bit shift.
# Using compressed klass with 3-bit shift.
# Objects are 8 bytes aligned.
# Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
# Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
com.hyr.jol.demo.JOLSample_02_Alignment$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$0 N/A
Instance size: 296 bytes
Space losses: 1 bytes internal + 0 bytes external = 1 bytes total
JDK1.8解決緩存僞共享
JDK1.8中增長了Contended註解方式來解決緩存僞共享問題。jvm
在JDK1.8中，新增了一種註解@sun.misc.Contended，來使各個變量在Cache line中分隔開。注意，jvm須要添加參數-XX:-RestrictContended才能開啓此功能 oop
未填充對象：
public class DataPadding{
int value;
long modifyTime;
boolean flag;
long createTime;
char key;
}
對象結構：
# Running 64-bit HotSpot VM.
# Using compressed oop with 0-bit shift.
# Using compressed klass with 0-bit shift.
# Objects are 8 bytes aligned.
# Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
# Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
com.hyr.jol.demo.JOLSample_02_Alignment$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$0 N/A
Instance size: 40 bytes
Space losses: 1 bytes internal + 0 bytes external = 1 bytes total
使用Contended註解：
public class DataPadding {
@sun.misc.Contended("group1")
int value;
@sun.misc.Contended("group1")
long modifyTime;
@sun.misc.Contended("group2")
boolean flag;
@sun.misc.Contended("group3")
long createTime;
@sun.misc.Contended("group3")
char key;
}
對象結構：
# Running 64-bit HotSpot VM.
# Using compressed oop with 0-bit shift.
# Using compressed klass with 3-bit shift.
# Objects are 8 bytes aligned.
# Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
# Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
com.hyr.jol.demo.JOLSample_02_Alignment$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
若是想深刻了解，能夠看下關於CPU Cache、Linux Cache相關的知識。this
項目輸出對象結構用的JOL。相關技術能夠參考個人Github項目：https://github.com/huangyueranbbc/JVM_AGENT_DEMOspa
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