为什么 JVM 报告的已提交内存比 linux 进程驻留集大小更多

当运行一个启用了本机内存跟踪(-XX:NativeMemoryTracking=detail参见https://docs.oracle.com/javase/8/docs/technotes/guides/vm/nmt-8.html和https://docs.oracle.com/javase/8/docs/technotes/guides/troubleshoot/tooldescr007.html)的Java应用程序(in-Thread)时，我可以看到JVM在不同类别中使用了多少内存.

我在jdk 1.8.045上的应用程序显示:

Native Memory Tracking:

Total: reserved=4023326KB, committed=2762382KB
-                 Java Heap (reserved=1331200KB, committed=1331200KB)
                            (mmap: reserved=1331200KB, committed=1331200KB) 

-                     Class (reserved=1108143KB, committed=64559KB)
                            (classes #8621)
                            (malloc=6319KB #17371) 
                            (mmap: reserved=1101824KB, committed=58240KB) 

-                    Thread (reserved=1190668KB, committed=1190668KB)
                            (thread #1154)
                            (stack: reserved=1185284KB, committed=1185284KB)
                            (malloc=3809KB #5771) 
                            (arena=1575KB #2306)

-                      Code (reserved=255744KB, committed=38384KB)
                            (malloc=6144KB #8858) 
                            (mmap: reserved=249600KB, committed=32240KB) 

-                        GC (reserved=54995KB, committed=54995KB)
                            (malloc=5775KB #217) 
                            (mmap: reserved=49220KB, committed=49220KB) 

-                  Compiler (reserved=267KB, committed=267KB)
                            (malloc=137KB #333) 
                            (arena=131KB #3)

-                  Internal (reserved=65106KB, committed=65106KB)
                            (malloc=65074KB #29652) 
                            (mmap: reserved=32KB, committed=32KB) 

-                    Symbol (reserved=13622KB, committed=13622KB)
                            (malloc=12016KB #128199) 
                            (arena=1606KB #1)

-    Native Memory Tracking (reserved=3361KB, committed=3361KB)
                            (malloc=287KB #3994) 
                            (tracking overhead=3075KB)

-               Arena Chunk (reserved=220KB, committed=220KB)
                            (malloc=220KB) 

这显示了2.7GB的提交内存，包括1.3GB的已分配堆和近1.2GB的已分配线程堆栈(使用许多线程).

然而，当运行ps ax -o pid,rss | grep <mypid>或top时，它只显示RES/rss个驻留内存中的1.6GB.勾选swap表示无正在使用:

free -m
             total       used       free     shared    buffers     cached
Mem:        129180      99348      29831          0       2689      73024
-/+ buffers/cache:      23633     105546
Swap:        15624          0      15624

当只有1.6GB内存驻留时，为什么JVM指示提交了2.7GB内存？其他人go 哪儿了？