聊聊jvm的CompressedClassSpace

序

本文主要研究一下jvm的CompressedClassSpace

CompressedClassSpace

• java8移除了permanent generation，然后class metadata存储在native memory中，其大小默认是不受限的，可以通过-XX:MaxMetaspaceSize来限制
• 如果开启了-XX:+UseCompressedOops及-XX:+UseCompressedClassesPointers(默认是开启)，则UseCompressedOops会使用32-bit的offset来代表java object的引用，而UseCompressedClassPointers则使用32-bit的offset来代表64-bit进程中的class pointer；可以使用CompressedClassSpaceSize来设置这块的空间大小
• 如果开启了指针压缩，则CompressedClassSpace分配在MaxMetaspaceSize里头，即MaxMetaspaceSize=Compressed Class Space Size + Metaspace area (excluding the Compressed Class Space) Size

查看CompressedClassSpace大小

jcmd pid GC.heap_info

/ # jcmd 1 GC.heap_info
1:
Shenandoah Heap
 524288K total, 144896K committed, 77232K used
 2048 x 256K regions
Status: not cancelled
Reserved region:
 - [0x00000000e0000000, 0x0000000100000000)

 Metaspace       used 45675K, capacity 46867K, committed 47104K, reserved 1091584K
  class space    used 5406K, capacity 5838K, committed 5888K, reserved 1048576K

可以看到整个metaspace使用了45675K，其中class space使用了5406K，而Metaspace area (excluding the Compressed Class Space)使用了45675K-5406K=40269K；整个metaspace的reserved大小为1091584K，其中class space的reserved大小为1048576K

jcmd pid VM.native_memory

/ # jcmd 1 VM.native_memory
1:

Native Memory Tracking:

Total: reserved=2224403KB, committed=238187KB
-                 Java Heap (reserved=524288KB, committed=144896KB)
                            (mmap: reserved=524288KB, committed=144896KB)

-                     Class (reserved=1092940KB, committed=48460KB)
                            (classes #8563)
                            (  instance classes #7988, array classes #575)
                            (malloc=1356KB #20589)
                            (mmap: reserved=1091584KB, committed=47104KB)
                            (  Metadata:   )
                            (    reserved=43008KB, committed=41216KB)
                            (    used=40286KB)
                            (    free=930KB)
                            (    waste=0KB =0.00%)
                            (  Class space:)
                            (    reserved=1048576KB, committed=5888KB)
                            (    used=5407KB)
                            (    free=481KB)
                            (    waste=0KB =0.00%)

-                    Thread (reserved=37130KB, committed=2846KB)
                            (thread #36)
                            (stack: reserved=36961KB, committed=2676KB)
                            (malloc=127KB #189)
                            (arena=42KB #70)

-                      Code (reserved=529360KB, committed=15420KB)
                            (malloc=968KB #4745)
                            (mmap: reserved=528392KB, committed=14452KB)

-                        GC (reserved=21844KB, committed=7724KB)
                            (malloc=5460KB #9644)
                            (mmap: reserved=16384KB, committed=2264KB)

-                  Compiler (reserved=165KB, committed=165KB)
                            (malloc=34KB #455)
                            (arena=131KB #5)

-                  Internal (reserved=3758KB, committed=3758KB)
                            (malloc=1710KB #6582)
                            (mmap: reserved=2048KB, committed=2048KB)

-                     Other (reserved=32KB, committed=32KB)
                            (malloc=32KB #3)

-                    Symbol (reserved=10277KB, committed=10277KB)
                            (malloc=7456KB #225421)
                            (arena=2821KB #1)

-    Native Memory Tracking (reserved=4235KB, committed=4235KB)
                            (malloc=10KB #126)
                            (tracking overhead=4225KB)

-               Arena Chunk (reserved=176KB, committed=176KB)
                            (malloc=176KB)

-                   Logging (reserved=7KB, committed=7KB)
                            (malloc=7KB #264)

-                 Arguments (reserved=18KB, committed=18KB)
                            (malloc=18KB #500)

-                    Module (reserved=165KB, committed=165KB)
                            (malloc=165KB #1708)

-                 Safepoint (reserved=4KB, committed=4KB)
                            (mmap: reserved=4KB, committed=4KB)

-                   Unknown (reserved=4KB, committed=4KB)
                            (mmap: reserved=4KB, committed=4KB)

可以看到class部分，reserved大小为1092940KB，其中Metadata的reserved大小为43008KB，Class space的reserved大小为1048576KB；其中Metadata使用了40286KB，而Class space使用了5407KB

jmx查看

    @GetMapping("/meta")
    public Object getMetaspaceSize(){
        return ManagementFactory.getPlatformMXBeans(MemoryPoolMXBean.class)
                .stream()
                .filter(e -> MemoryType.NON_HEAP == e.getType())
                .filter(e -> e.getName().equals("Metaspace") || e.getName().equals("Compressed Class Space"))
                .map(e -> "name:"+e.getName()+",info:"+e.getUsage())
                .collect(Collectors.toList());
    }

输出如下：

/ # curl -i localhost:8080/memory/meta
HTTP/1.1 200
Content-Type: application/json;charset=UTF-8
Transfer-Encoding: chunked
Date: Sun, 31 Mar 2019 03:06:55 GMT

["name:Metaspace,info:init = 0(0K) used = 46236784(45153K) committed = 47710208(46592K) max = -1(-1K)","name:Compressed Class Space,info:init = 0(0K) used = 5482736(5354K) committed = 6029312(5888K) max = 1073741824(1048576K)"]

这里可以看到Metaspace总共使用了45153K，其中Compressed Class Space部分使用了5354K，而Metaspace area (excluding the Compressed Class Space)使用了45153K-5354K=39799K；而这里显示的Metaspace的max为-1，其中Compressed Class Space部分max值为1048576K即1G

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: Sun, 31 Mar 2019 02:52:51 GMT

{"name":"jvm.memory.used","description":"The amount of used memory","baseUnit":"bytes","measurements":[{"statistic":"VALUE","value":6.4449464E7}],"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:Metaspace"
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: Sun, 31 Mar 2019 02:54:56 GMT

{"name":"jvm.memory.used","description":"The amount of used memory","baseUnit":"bytes","measurements":[{"statistic":"VALUE","value":4.7468312E7}],"availableTags":[]}

/ # curl -i "http://localhost:8080/actuator/metrics/jvm.memory.used?tag=area:nonheap&tag=id:Compressed%
20Class%20Space"
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: Sun, 31 Mar 2019 02:55:18 GMT

{"name":"jvm.memory.used","description":"The amount of used memory","baseUnit":"bytes","measurements":[{"statistic":"VALUE","value":5609952.0}],"availableTags":[]}

springboot使用micrometer，通过/actuator/metrics接口提供相关指标查询功能，其中Metaspace及Compressed Class Space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

• java8移除了permanent generation，然后class metadata存储在native memory中，其大小默认是不受限的，可以通过-XX:MaxMetaspaceSize来限制；如果开启了-XX:+UseCompressedOops及-XX:+UseCompressedClassesPointers(默认是开启)，则UseCompressedOops会使用32-bit的offset来代表java object的引用，而UseCompressedClassPointers则使用32-bit的offset来代表64-bit进程中的class pointer；可以使用CompressedClassSpaceSize来设置这块的空间大小
• 开启了指针压缩，则CompressedClassSpace分配在MaxMetaspaceSize里头，即MaxMetaspaceSize=Compressed Class Space Size + Metaspace area (excluding the Compressed Class Space) Size

• 查看CompressedClassSpace的内存使用情况有好几种方法：

  • jcmd pid GC.heap_info(Metaspace为总的部分，包含了class space，而Metaspace area (excluding the Compressed Class Space)需要自己计算即total-class space)
  • jcmd pid VM.native_memory(class为总的部分，包含了Metaspace area (excluding the Compressed Class Space)及Class Space)
  • 使用JMX来获取NON_HEAP类型中的name为Metaspace及Compressed Class Space的MemoryPoolMXBean可以得到Metaspace及Compressed Class Space的使用情况(JMX得到的Metaspace为总的部分，而Metaspace area (excluding the Compressed Class Space)需要自己计算即total-class space)
  • 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Metaspace及Compressed Class Space在jvm.memory.used这个metric中

doc

• Class Metadata
• 7.19.3 -XX:CompressedClassSpaceSize
• JDK 8: UseCompressedClassPointers vs. UseCompressedOops
• JVM源码分析之Metaspace解密
• 聊聊jvm的PermGen与Metaspace