释放缓存区内存的方法
a）清理pagecache（页面缓存）
# echo 1 > /proc/sys/vm/drop_caches     或者 # sysctl -w vm.drop_caches=1

b）清理dentries（目录缓存）和inodes
# echo 2 > /proc/sys/vm/drop_caches     或者 # sysctl -w vm.drop_caches=2

c）清理pagecache、dentries和inodes
# echo 3 > /proc/sys/vm/drop_caches     或者 # sysctl -w vm.drop_caches=3


[root@test4_haili_dev hadoop-2.7.3]# free -m
             total       used       free     shared    buffers     cached
Mem:          7809       7159        650          0        324       2444
-/+ buffers/cache:       4390       3419 
Swap:          511         89        422 
[root@test4_haili_dev hadoop-2.7.3]# cat /proc/sys/vm/drop_caches
1
[root@test4_haili_dev hadoop-2.7.3]# echo 1 > /proc/sys/vm/drop_caches  
[root@test4_haili_dev hadoop-2.7.3]# free -m
             total       used       free     shared    buffers     cached
Mem:          7809       4345       3464          0          0         36
-/+ buffers/cache:       4308       3500 
Swap:          511         89        422 
[root@test4_haili_dev hadoop-2.7.3]# cat /proc/sys/vm/drop_caches 
1
[root@test4_haili_dev hadoop-2.7.3]# echo 2 > /proc/sys/vm/drop_caches
[root@test4_haili_dev hadoop-2.7.3]# cat /proc/sys/vm/drop_caches 
2
[root@test4_haili_dev hadoop-2.7.3]# free -m
             total       used       free     shared    buffers     cached
Mem:          7809       4303       3506          0          2         36
-/+ buffers/cache:       4265       3544 
Swap:          511         89        422 
[root@test4_haili_dev hadoop-2.7.3]# cat /proc/sys/vm/drop_caches
2
[root@test4_haili_dev hadoop-2.7.3]# echo 3 > /proc/sys/vm/drop_caches
[root@test4_haili_dev hadoop-2.7.3]# free -m
             total       used       free     shared    buffers     cached
Mem:          7809       4301       3508          0          0         36
-/+ buffers/cache:       4264       3544 
Swap:          511         89        422