序
本文主要研究一下jvm的Code Cache
Code Cache
JVM生成的native code存放的内存空间称之为Code Cache；JIT编译、JNI等都会编译代码到native code，其中JIT生成的native code占用了Code Cache的绝大部分空间
相关参数
Codecache Size Options
- -XX:InitialCodeCacheSize
用于设置初始CodeCache大小
- -XX:ReservedCodeCacheSize
用于设置Reserved code cache的最大大小，通常默认是240M
- -XX:CodeCacheExpansionSize
用于设置code cache的expansion size，通常默认是64K
Codecache Flush Options
- -XX:+UseCodeCacheFlushing
是否在code cache满的时候先尝试清理一下，如果还是不够用再关闭编译，默认开启
Compilation Policy Options
- -XX:CompileThreshold
方法触发编译时的调用次数，默认是10000
- -XX:OnStackReplacePercentage
方法中循环执行部分代码的执行次数触发OSR编译时的阈值，默认是140
Compilation Limit Options
- -XX:MaxInlineLevel
针对嵌套调用的最大内联深度，默认为9
- -XX:MaxInlineSize
方法可以被内联的最大bytecode大小，默认为35
- -XX:MinInliningThreshold
方法可以被内联的最小调用次数，默认为250
- -XX:+InlineSynchronizedMethods
是否允许内联synchronized methods，默认为true
Diagnostic Options
- -XX:+PrintFlagsFinal(
默认没有启用)
用于查看所有可设置的参数及最终值(
JDK 6 update 21开始才可以用），默认是不包括diagnostic或experimental系的。如果要在-XX:+PrintFlagsFinal的输出里看到这两种参数的信息，分别需要显式指定-XX:+UnlockDiagnosticVMOptions / -XX:+UnlockExperimentalVMOptions(-XX:+PrintCommandLineFlags 这个参数的作用是显示出VM初始化完毕后所有跟最初的默认值不同的参数及它们的值)
- -XX:+PrintCodeCache(
默认没有启用)
-XX:+PrintCodeCache用于jvm关闭时输出code cache的使用情况
- -XX:+PrintCodeCacheOnCompilation(
默认没有启用)
用于在方法每次被编译时输出code cache的使用情况
查看Code Cache的使用情况
-XX:+PrintCodeCache
CodeHeap 'non-profiled nmethods': size=120032Kb used=2154Kb max_used=2160Kb free=117877Kb
bounds [0x00000001178ea000, 0x0000000117b5a000, 0x000000011ee22000]
CodeHeap 'profiled nmethods': size=120028Kb used=10849Kb max_used=11005Kb free=109178Kb
bounds [0x00000001103b3000, 0x0000000110e73000, 0x00000001178ea000]
CodeHeap 'non-nmethods': size=5700Kb used=1177Kb max_used=1239Kb free=4522Kb
bounds [0x000000010fe22000, 0x0000000110092000, 0x00000001103b3000]
total_blobs=5638 nmethods=4183 adapters=435
compilation: enabled
stopped_count=0, restarted_count=0
full_count=0
- jvm启动参数加上-XX:+PrintCodeCache，可以在jvm关闭时输出code cache的使用情况
- 这里分了non-profiled nmethods、profiled nmethods、non-nmethods三部分来展示
- 其中size就是限制的最大大小，used表示实际使用量，max_used就是使用大小的high water mark，free由size-used得来
jcmd pid Compiler.codecache
/ # jcmd 1 Compiler.codecache
1:
CodeHeap 'non-profiled nmethods': size=120036Kb used=1582Kb max_used=1582Kb free=118453Kb
bounds [0x00007f1e42226000, 0x00007f1e42496000, 0x00007f1e4975f000]
CodeHeap 'profiled nmethods': size=120032Kb used=9621Kb max_used=9621Kb free=110410Kb
bounds [0x00007f1e3acee000, 0x00007f1e3b65e000, 0x00007f1e42226000]
CodeHeap 'non-nmethods': size=5692Kb used=1150Kb max_used=1198Kb free=4541Kb
bounds [0x00007f1e3a75f000, 0x00007f1e3a9cf000, 0x00007f1e3acee000]
total_blobs=5610 nmethods=4369 adapters=412
compilation: enabled
stopped_count=0, restarted_count=0
full_count=0
使用jcmd的Compiler.codecache也可以查看code cache的使用情况，输出跟-XX:+PrintCodeCache相同
jcmd pid VM.native_memory
/ # jcmd 1 VM.native_memory
1:
Native Memory Tracking:
Total: reserved=1928023KB, committed=231182KB
- Java Heap (reserved=511488KB, committed=140288KB)
(mmap: reserved=511488KB, committed=140288KB)
- Class (reserved=1090832KB, committed=46608KB)
(classes #8218)
( instance classes #7678, array classes #540)
(malloc=1296KB #19778)
(mmap: reserved=1089536KB, committed=45312KB)
( Metadata: )
( reserved=40960KB, committed=39680KB)
( used=38821KB)
( free=859KB)
( waste=0KB =0.00%)
( Class space:)
( reserved=1048576KB, committed=5632KB)
( used=5190KB)
( free=442KB)
( waste=0KB =0.00%)
- Thread (reserved=37130KB, committed=2806KB)
(thread #36)
(stack: reserved=36961KB, committed=2636KB)
(malloc=127KB #189)
(arena=42KB #70)
- Code (reserved=248651KB, committed=15351KB)
(malloc=963KB #4600)
(mmap: reserved=247688KB, committed=14388KB)
- GC (reserved=21403KB, committed=7611KB)
(malloc=5419KB #9458)
(mmap: reserved=15984KB, committed=2192KB)
- Compiler (reserved=150KB, committed=150KB)
(malloc=20KB #447)
(arena=131KB #5)
- Internal (reserved=3744KB, committed=3744KB)
(malloc=1696KB #6416)
(mmap: reserved=2048KB, committed=2048KB)
- Other (reserved=24KB, committed=24KB)
(malloc=24KB #2)
- Symbol (reserved=10094KB, committed=10094KB)
(malloc=7305KB #219914)
(arena=2789KB #1)
- Native Memory Tracking (reserved=4130KB, committed=4130KB)
(malloc=12KB #158)
(tracking overhead=4119KB)
- Arena Chunk (reserved=177KB, committed=177KB)
(malloc=177KB)
- Logging (reserved=7KB, committed=7KB)
(malloc=7KB #264)
- Arguments (reserved=18KB, committed=18KB)
(malloc=18KB #500)
- Module (reserved=165KB, committed=165KB)
(malloc=165KB #1699)
- Safepoint (reserved=4KB, committed=4KB)
(mmap: reserved=4KB, committed=4KB)
- Unknown (reserved=4KB, committed=4KB)
(mmap: reserved=4KB, committed=4KB)
使用jcmd的VM.native_memory也可以查看code cache的使用情况(
Code部分)，Compiler部分为Memory tracking used by the compiler when generating code
使用MemoryPoolMXBean查看
@Test
public void testGetCodeCacheUsage(){
ManagementFactory.getPlatformMXBeans(MemoryPoolMXBean.class)
.stream()
.filter(e -> MemoryType.NON_HEAP == e.getType())
.filter(e -> e.getName().startsWith("CodeHeap"))
.forEach(e -> {
LOGGER.info("name:{},info:{}",e.getName(),e.getUsage());
});
}
MemoryPoolMXBean包含了HEAP及NON_HEAP，其中code cache属于NON_HEAP，其输出如下：
12:21:10.728 [main] INFO com.example.CodeCacheTest - name:CodeHeap 'non-nmethods',info:init = 2555904(2496K) used = 1117696(1091K) committed = 2555904(2496K) max = 5836800(5700K)
12:21:10.743 [main] INFO com.example.CodeCacheTest - name:CodeHeap 'profiled nmethods',info:init = 2555904(2496K) used = 1543808(1507K) committed = 2555904(2496K) max = 122908672(120028K)
12:21:10.743 [main] INFO com.example.CodeCacheTest - name:CodeHeap 'non-profiled nmethods',info:init = 2555904(2496K) used = 319616(312K) committed = 2555904(2496K) max = 122912768(120032K)
spring boot应用查看
/ # curl -i "http://localhost:8080/actuator/metrics/jvm.memory.used?tag=area:nonheap"
HTTP/1.1 200
Content-Disposition: inline;filename=f.txt
Content-Type: application/vnd.spring-boot.actuator.v2+json;charset=UTF-8
Transfer-Encoding: chunked
Date: Sat, 30 Mar 2019 04:26:39 GMT
{"name":"jvm.memory.used","description":"The amount of used memory","baseUnit":"bytes","measurements":[{"statistic":"VALUE","value":6.5295408E7}],"availableTags":[{"tag":"id","values":["CodeHeap 'non-profiled nmethods'","CodeHeap 'profiled nmethods'","Compressed Class Space","Metaspace","CodeHeap 'non-nmethods'"]}]}
/ # curl -i "http://localhost:8080/actuator/metrics/jvm.memory.used?tag=area:nonheap&tag=id:CodeHeap%20%27non-profiled
%20nmethods%27"
HTTP/1.1 200
Content-Disposition: inline;filename=f.txt
Content-Type: application/vnd.spring-boot.actuator.v2+json;charset=UTF-8
Transfer-Encoding: chunked
Date: Sat, 30 Mar 2019 04:24:58 GMT
{"name":"jvm.memory.used","description":"The amount of used memory","baseUnit":"bytes","measurements":[{"statistic":"VALUE","value":1592448.0}],"availableTags":[]}
springboot使用micrometer，通过/actuator/metrics接口提供相关指标查询功能，其中code cache在jvm.memory.used这个metric中
它是基于MemoryPoolMXBean来实现的，具体详见micrometer-core-1.1.3-sources.jar!/io/micrometer/core/instrument/binder/jvm/JvmMemoryMetrics.java
小结
- JVM生成的native code存放的内存空间称之为Code Cache；JIT编译、JNI等都会编译代码到native code，其中JIT生成的native code占用了Code Cache的绝大部分空间
- -XX:ReservedCodeCacheSize用于设置Reserved code cache的最大大小，通常默认是240M；对于有些应用来说240M可能太大，code cache可能都填不满，相当于unconstrained，此时JIT就会继续编译任何它认为可以编译的code
- 查看Code Cache的内存使用情况有好几种方法：
- jvm启动参数加上-XX:+PrintCodeCache，可以在jvm关闭时输出code cache的使用情况
- 使用jcmd的Compiler.codecache，其输出跟-XX:+PrintCodeCache相同；
- 使用jcmd的VM.native_memory也可以查看code cache的使用情况(
Code部分) - 使用JMX来获取NON_HEAP类型中的name为CodeHeap开头的MemoryPoolMXBean可以得到code cache的使用情况
- 如果是springboot应用，它使用micrometer，通过/actuator/metrics接口提供相关指标查询功能，其中code cache在jvm.memory.used这个metric中
doc
- 15 Codecache Tuning
- JVM的编译策略
- What are ReservedCodeCacheSize and InitialCodeCacheSize?
- Why does the JVM have a maximum inline depth?
- Code Cache满导致应用性能降低