求助下大神们，内存去哪了

请教下各位，系统内存32G，使用了28，但是topc出来按内存使用率排序，最多的java进程只用了500M，何解？
[root@localhost ~]# free -g
total used free shared buffers cached
Mem: 31 28 2 0 0 1
-/+ buffers/cache: 26 5
Swap: 7 0 7
[root@localhost ~]# top
top - 20:01:38 up 20 days, 2:54, 1 user, load average: 1.41, 1.19, 1.06
Tasks: 26993 total, 1 running, 26992 sleeping, 0 stopped, 0 zombie
Cpu(s): 11.5%us, 23.9%sy, 0.0%ni, 64.6%id, 0.0%wa, 0.0%hi, 0.0%si, 0.1%st
Mem: 32749888k total, 29998552k used, 2751336k free, 589348k buffers
Swap: 8388604k total, 0k used, 8388604k free, 2095340k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
2288 jboss 20 0 3158m 555m 25m S 99.3 1.7 29045:37 java
1642 consul 20 0 45132 30m 7576 S 3.5 0.1 1117:36 consul
2873 nobody 20 0 36480 22m 540 S 0.0 0.1 13:24.19 nmon
17726 root 20 0 35576 21m 952 R 33.0 0.1 0:27.38 top
1104 root 20 0 243m 5532 1136 S 0.0 0.0 0:36.74 rsyslogd
429 root 20 0 100m 4464 3460 S 0.0 0.0 0:00.04 sshd
463 root 20 0 100m 4464 3460 S 0.0 0.0 0:00.03 sshd
648 root 20 0 100m 4464 3460 S 0.0 0.0 0:00.04 sshd
665 root 20 0 100m 4464 3460 S 0.0 0.0 0:00.04 sshd