CentOS 5這種釋放方法是相當地痛快，而且如果真是non-destructive operation的話那就太好了，jvm 的內存占用一下子減少了接近300m.
#sync
To free pagecache, dentries and inodes:
#echo 3 > /proc/sys/vm/drop_caches
This is a non-destructive operation and will only free things that are completely unused. Dirty objects will continue to be in use until written out to disk and are not freeable. If you run "sync" first to flush them out to disk, these drop operations will tend to free more memory.
再說說 free 命令
# free -m
total used free shared buffers cached
Mem: 497 438 59 0 20 325
-/+ buffers/cache: 92 404
Swap: 511 22 489
其中：
total 內存總數
used 已經使用的內存數
free 空閒的內存數
shared 多個進程共享的內存總額
buffers buffer Cache和cached Page Cache 磁盤緩存的大小
-buffers/cache (已用)的內存數:used - buffers - cached
+buffers/cache(可用)的內存數:free + buffers + cached
可用的memory=free memory+buffers+cached
下面包含一個自動釋放內存的腳本，網上學習的， 加入了crondtab計劃任務了
# vim /root/shTools/freemem.sh
#!/bin/bash used=`free -m | awk 'NR==2' | awk '{print $3}'` free=`free -m | awk 'NR==2' | awk '{print $4}'` echo "===========================" >> /var/log/mem.log date >> /var/log/mem.log echo "Memory usage | [Use：${used}MB][Free：${free}MB]" >> /var/log/mem.log if [ $free -le 100 ] ; then sync && echo 1 > /proc/sys/vm/drop_caches sync && echo 2 > /proc/sys/vm/drop_caches sync && echo 3 > /proc/sys/vm/drop_caches echo "OK" >> /var/log/mem.log else echo "Not required" >> /var/log/mem.log
將腳本添加到crond任務，定時執行。
# echo "*/30 * * * * root /root/shTools/freemem.sh" >> /etc/crondtab
