聊聊jvm的Code Cache

序

本文主要研究一下jvm的Code Cache

Code Cache

JVM生成的native code存放的內存空間稱之爲Code Cache；JIT編譯、JNI等都會編譯代碼到native code，其中JIT生成的native code佔用了Code Cache的絕大部分空間

相關參數

Codecache Size Options

• -XX:InitialCodeCacheSize

用於設置初始CodeCache大小

• -XX:ReservedCodeCacheSize

用於設置Reserved code cache的最大大小，一般默認是240M

• -XX:CodeCacheExpansionSize

用於設置code cache的expansion size，一般默認是64K

Codecache Flush Options

• -XX:+UseCodeCacheFlushing

是否在code cache滿的時候先嚐試清理一下，若是仍是不夠用再關閉編譯，默認爲false

Compilation Policy Options

• -XX:CompileThreshold

方法觸發編譯時的調用次數，默認是10000

• -XX:OnStackReplacePercentage

方法中循環執行部分代碼的執行次數觸發OSR編譯時的閾值，默認是140

Compilation Limit Options

• -XX:MaxInlineLevel

針對嵌套調用的最大內聯深度，默認爲9

• -XX:MaxInlineSize

方法能夠被內聯的最大bytecode大小，默認爲35

• -XX:MinInliningThreshold

方法能夠被內聯的最小調用次數，默認爲250

• -XX:+InlineSynchronizedMethods

是否容許內聯synchronized methods，默認爲true

Diagnostic Options

• -XX:+PrintFlagsFinal(默認沒有啓用)

用於查看全部可設置的參數及最終值(JDK 6 update 21開始才能夠用），默認是不包括diagnostic或experimental系的。若是要在-XX:+PrintFlagsFinal的輸出裏看到這兩種參數的信息，分別須要顯式指定-XX:+UnlockDiagnosticVMOptions / -XX:+UnlockExperimentalVMOptions(-XX:+PrintCommandLineFlags 這個參數的做用是顯示出VM初始化完畢後全部跟最初的默認值不一樣的參數及它們的值)

• -XX:+PrintCodeCache(默認沒有啓用)

-XX:+PrintCodeCache用於jvm關閉時輸出code cache的使用狀況

• -XX:+PrintCodeCacheOnCompilation(默認沒有啓用)

用於在方法每次被編譯時輸出code cache的使用狀況

查看Code Cache的使用狀況

-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啓動參數加上-XX:+PrintCodeCache，能夠在jvm關閉時輸出code cache的使用狀況
• 這裏分了non-profiled nmethods、profiled nmethods、non-nmethods三部分來展現
• 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也能夠查看code cache的使用狀況，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也能夠查看code cache的使用狀況(Code部分)，Compiler部分爲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屬於NON_HEAP，其輸出以下：

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，經過/actuator/metrics接口提供相關指標查詢功能，其中code cache在jvm.memory.used這個metric中 它是基於MemoryPoolMXBean來實現的，具體詳見micrometer-core-1.1.3-sources.jar!/io/micrometer/core/instrument/binder/jvm/JvmMemoryMetrics.java

小結

• JVM生成的native code存放的內存空間稱之爲Code Cache；JIT編譯、JNI等都會編譯代碼到native code，其中JIT生成的native code佔用了Code Cache的絕大部分空間
• -XX:ReservedCodeCacheSize用於設置Reserved code cache的最大大小，一般默認是240M；對於有些應用來講240M可能太大，code cache可能都填不滿，至關於unconstrained，此時JIT就會繼續編譯任何它認爲能夠編譯的code
• 查看Code Cache的內存使用狀況有好幾種方法：
  • jvm啓動參數加上-XX:+PrintCodeCache，能夠在jvm關閉時輸出code cache的使用狀況
  • 使用jcmd的Compiler.codecache，其輸出跟-XX:+PrintCodeCache相同；
  • 使用jcmd的VM.native_memory也能夠查看code cache的使用狀況(Code部分)
  • 使用JMX來獲取NON_HEAP類型中的name爲CodeHeap開頭的MemoryPoolMXBean能夠獲得code cache的使用狀況
  • 若是是springboot應用，它使用micrometer，經過/actuator/metrics接口提供相關指標查詢功能，其中code cache在jvm.memory.used這個metric中

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滿致使應用性能下降