聊聊jvm的code cache

摘要： 序 本文主要研究一下jvm的code cache Code Cache JVM生成的native code存放的記憶體空間稱之為Code Cache；JIT編譯、JNI等都會編譯程式碼到native code，其中JIT生成的native code佔用了Code Cache的絕大部...

序

本文主要研究一下jvm的code cache

Code Cache

JVM生成的native code存放的記憶體空間稱之為Code Cache；JIT編譯、JNI等都會編譯程式碼到native code，其中JIT生成的native code佔用了Code Cache的絕大部分空間

相關引數

Codecache Size Options

• -XX:InitialCodeCacheSize

用於設定初始CodeCache大小

• -XX:ReservedCodeCacheSize

用於設定Reserved code cache的最大大小，通常預設是240M

• -XX:CodeCacheExpansionSize

用於設定code cache的expansion size，通常預設是64K

Codecache Flush Options

• -XX:+UseCodeCacheFlushing

是否在code cache滿的時候先嚐試清理一下，如果還是不夠用再關閉編譯，預設為false

Compilation Policy Options

• -XX:CompileThreshold

方法觸發編譯時的呼叫次數，預設是10000

• -XX:OnStackReplacePercentage

方法中迴圈執行部分程式碼的執行次數觸發OSR編譯時的閾值，預設是140

Compilation Limit Options

• -XX:MaxInlineLevel

針對巢狀呼叫的最大內聯深度，預設為9

• -XX:MaxInlineSize

方法可以被內聯的最大bytecode大小，預設為35

• -XX:MinInliningThreshold

方法可以被內聯的最小呼叫次數，預設為250

• -XX:+InlineSynchronizedMethods

是否允許內聯synchronized methods，預設為true

Diagnostic Options

• -XX:+PrintFlagsFinal(預設沒有啟用 )

用於檢視所有可設定的引數及最終值(JDK 6 update 21開始才可以用 ），預設是不包括diagnostic或experimental系的。如果要在-XX:+PrintFlagsFinal的輸出裡看到這兩種引數的資訊，分別需要顯式指定-XX:+UnlockDiagnosticVMOptions / -XX:+UnlockExperimentalVMOptions(-XX:+PrintCommandLineFlags 這個引數的作用是顯示出VM初始化完畢後所有跟最初的預設值不同的引數及它們的值 )

• -XX:+PrintCodeCache(預設沒有啟用 )

-XX:+PrintCodeCache用於jvm關閉時輸出code cache的使用情況

• -XX:+PrintCodeCacheOnCompilation(預設沒有啟用 )

用於在方法每次被編譯時輸出code cache的使用情況

檢視Code Cache的使用情況

-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啟動引數加上-XX:+PrintCodeCache，可以在jvm關閉時輸出code cache的使用情況
• 這裡分了non-profiled nmethods、profiled nmethods、non-nmethods三部分來展示
• 其中size就是限制的最大大小，used表示實際使用量，max_used就是使用大小的high water mark，free由size-used得來

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也可以檢視code cache的使用情況，輸出跟-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也可以檢視code cache的使用情況(Code部分 )，Compiler部分為Memory tracking used by the compiler when generating code

使用MemoryPoolMXBean檢視

@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屬於NON_HEAP，其輸出如下：

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應用檢視

/ # 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，通過/actuator/metrics介面提供相關指標查詢功能，其中code cache在jvm.memory.used這個metric中

小結

• JVM生成的native code存放的記憶體空間稱之為Code Cache；JIT編譯、JNI等都會編譯程式碼到native code，其中JIT生成的native code佔用了Code Cache的絕大部分空間
• -XX:ReservedCodeCacheSize用於設定Reserved code cache的最大大小，通常預設是240M；對於有些應用來說240M可能太大，code cache可能都填不滿，相當於unconstrained，此時JIT就會繼續編譯任何它認為可以編譯的code

• 檢視Code Cache的記憶體使用情況有好幾種方法：

  Code部分
  

doc

• 15 Codecache Tuning
• JVM的編譯策略
• What are ReservedCodeCacheSize and InitialCodeCacheSize?
• Why does the JVM have a maximum inline depth?
• Code Cache滿導致應用效能降低