开启NMT

NMT目前只能在启动的时候开启，不支持动态开关NMT功能。开启NMT功能，会带来5%到10%的额外开销

通过-XX:NativeMemoryTracking=[off | summary | detAIl]来开启NMT功能。

• off 关闭NMT功能，默认关闭
• summary 仅收集子系统聚合的内存使用情况
• detail 收集各个调用点的内存使用情况。

使用jcmd功能获取NMT数据

可以使用jcmd收集数据，或者是与基线进行对比

jcmd <pid> VM.native_memory [summary | detail | baseline | summary.diff | detail.diff | shutdown] [scale= KB | MB | GB]

实战演示

jcmd 27 VM.native_memory summary scale=MB

Native Memory Tracking:

(Omitting categories weighting less than 1MB)

Total: reserved=2830MB, committed=1288MB
-                 JAVA Heap (reserved=1024MB, committed=1024MB)
                            (mmap: reserved=1024MB, committed=1024MB)

-                     Class (reserved=1025MB, committed=6MB)
                            (classes #8980)
                            (  instance classes #8430, array classes #550)
                            (malloc=1MB #20044)
                            (mmap: reserved=1024MB, committed=5MB)
                            (  Metadata:   )
                            (    reserved=40MB, committed=38MB)
                            (    used=38MB)
                            (    waste=0MB =0.34%)
                            (  Class space:)
                            (    reserved=1024MB, committed=5MB)
                            (    used=5MB)
                            (    waste=0MB =2.88%)

-                    Thread (reserved=312MB, committed=16MB)
                            (thread #156)
                            (stack: reserved=312MB, committed=16MB)

-                      Code (reserved=243MB, committed=18MB)
                            (malloc=1MB #6733)
                            (mmap: reserved=242MB, committed=17MB)

-                        GC (reserved=83MB, committed=83MB)
                            (malloc=13MB #7985)
                            (mmap: reserved=70MB, committed=70MB)

-                  Internal (reserved=14MB, committed=14MB)
                            (malloc=14MB #10083)

-                     Other (reserved=61MB, committed=61MB)
                            (malloc=61MB #126)

-                    Symbol (reserved=9MB, committed=9MB)
                            (malloc=8MB #224838)
                            (arena=1MB #1)

-    Native Memory Tracking (reserved=5MB, committed=5MB)
                            (tracking overhead=4MB)

-        Shared class space (reserved=12MB, committed=12MB)
                            (mmap: reserved=12MB, committed=12MB)

-                 Metaspace (reserved=40MB, committed=38MB)
                            (mmap: reserved=40MB, committed=38MB)