metaspace观察

jvm metaspace泄露的问题往往不好排查。一个原因是我们不知道jvm加载1个类需要占用多少空间，以至于最后看到1个classloader加载了1000个类，他占用了10m空间是否是合理的。jvm提供了一种解决方式。利用jcmd查看metaspace。

查看空间占用

jcmd pid VM.metaspace

我们可以看到如下的信息。总共36个loaders。metaspace占用情况。

Total Usage ( 36 loaders):

  Non-Class:  113 chunks,      7.09 MB capacity,     6.96 MB ( 98%) used,   129.19 KB (  2%) free,   128 bytes ( <1%) waste,     7.06 KB ( <1%) overhead, deallocated: 61 blocks with 37.80 KB
      Class:   51 chunks,    721.00 KB capacity,   695.72 KB ( 96%) used,    22.09 KB (  3%) free,     8 bytes ( <1%) waste,     3.19 KB ( <1%) overhead, deallocated: 14 blocks with 5.09 KB
       Both:  164 chunks,      7.80 MB capacity,     7.64 MB ( 98%) used,   151.27 KB (  2%) free,   136 bytes ( <1%) waste,    10.25 KB ( <1%) overhead, deallocated: 75 blocks with 42.89 KB

Virtual space:
  Non-class space:        8.00 MB reserved,       7.25 MB ( 91%) committed
      Class space:        1.00 GB reserved,     768.00 KB ( <1%) committed
             Both:        1.01 GB reserved,       8.00 MB ( <1%) committed



Chunk freelists:
   Non-Class:

 specialized chunks:    2, capacity 2.00 KB
       small chunks:   14, capacity 56.00 KB
      medium chunks: (none)
   humongous chunks: (none)
              Total:   16, capacity=58.00 KB
       Class:

 specialized chunks:    1, capacity 1.00 KB
       small chunks:    7, capacity 14.00 KB
      medium chunks: (none)
   humongous chunks: (none)
              Total:    8, capacity=15.00 KB

Waste (percentages refer to total committed size 8.00 MB):
              Committed unused:    136.00 KB (  2%)
        Waste in chunks in use:    136 bytes ( <1%)
         Free in chunks in use:    151.27 KB (  2%)
     Overhead in chunks in use:     10.25 KB ( <1%)
                In free chunks:     73.00 KB ( <1%)
Deallocated from chunks in use:     42.89 KB ( <1%) (75 blocks)
                       -total-:    413.55 KB (  5%)


MaxMetaspaceSize: 17179869184.00 GB
InitialBootClassLoaderMetaspaceSize: 4.00 MB
UseCompressedClassPointers: true
CompressedClassSpaceSize: 1.00 GB

查看classloader状态

因为是classloader相关，所以需要再进一步查看。例如我们可以查看loader的占用情况。

jcmd pid VM.classloader_stats

这里我们需要理解这36个classloader的情况，这里是根据loader分类的。其中有一个unsafe anonymous classes。这个是例如反射等动态产生的classloader，这种loader普遍的情况就是一个classloader加载一个class。这里加载了33个class，就寓意这里有33个classloader，加上剩下的3个，正好36个。

ClassLoader         Parent              CLD*               Classes   ChunkSz   BlockSz  Type
0x0000000000000000  0x0000000000000000  0x0000600002420000    1079   7921664   7906024  <boot class loader>
                                                                33     89088     58048   + unsafe anonymous classes
0x0000000800010a38  0x0000000800010eb0  0x00006000024240a0       5     86016     22048  jdk.internal.loader.ClassLoaders$AppClassLoader
0x0000000800010eb0  0x0000000000000000  0x0000600002424000       4     77824     22936  jdk.internal.loader.ClassLoaders$PlatformClassLoader
Total = 3                                                     1121   8174592   8009056
ChunkSz: Total size of all allocated metaspace chunks
BlockSz: Total size of all allocated metaspace blocks (each chunk has several blocks)

在上面的信息中，我们可以看到classloader加载的Classes，ChunkSz，BlockSz我们从这里的信息中就可以知道哪些classloader占用比较大。

查看classloader委托情况

jcmd pid VM.classloaders

我们可以查看classloader的双亲委托情况，这样就可以拿到更多的运行时的信息，帮助解决问题

+-- <bootstrap>
      |
      +-- "platform", jdk.internal.loader.ClassLoaders$PlatformClassLoader
            |
            +-- "app", jdk.internal.loader.ClassLoaders$AppClassLoader

查看类的依赖情况

jcmd pid VM.class_hierarchy

最后一个是查看类的继承结构，这个是提供一种额外的信息，帮助我们找出问题点。