tcmalloc释放内存之后top显示还是有剩余

[qq_16432347]

随机申请内存，统一释放，可执行程序链接tamlloc，执行完成后top物理内存有剩余
tcmalloc内存状态

buffer=------------------------------------------------
MALLOC: 1442288 ( 1.4 MiB) Bytes in use by application
MALLOC: + 0 ( 0.0 MiB) Bytes in page heap freelist
MALLOC: + 9301224 ( 8.9 MiB) Bytes in central cache freelist
MALLOC: + 27725824 ( 26.4 MiB) Bytes in transfer cache freelist
MALLOC: + 803112 ( 0.8 MiB) Bytes in thread cache freelists
MALLOC: + 12316832 ( 11.7 MiB) Bytes in malloc metadata
MALLOC: ------------
MALLOC: = 51589280 ( 49.2 MiB) Actual memory used (physical + swap)
MALLOC: + 2660810752 ( 2537.5 MiB) Bytes released to OS (aka unmapped)
MALLOC: ------------
MALLOC: = 2712400032 ( 2586.7 MiB) Virtual address space used
MALLOC:
MALLOC: 4067 Spans in use
MALLOC: 1 Thread heaps in use
MALLOC: 8192 Tcmalloc page size
------------------------------------------------
物理内存剩余29.4M

top物理内存剩余87m
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
7743 root 18 0 2642m 87m 1300 S 0.0 0.4 0:21.28 test

不确定剩余的内存到哪里去了，求大神解释。