Linux cached and buffers memory
我们都知道Linux 的free memory是有两种算法的，一种是系统实际剩余的物理内存，一种是加上系统的cached住的临时缓存后的总剩余内存数，如下面标示的两个free值。
total used free shared buffers cached Mem: 1986376 891920 1094456 0 44352 462224 -/+ buffers/cache: 385344 1601032 Swap: 3695604 0 3695604
内核通过一组内核线程pdflush去管理缓存，有三种方式去释放缓存区。
1. 当脏缓存页保留一段时间后，内核就会显式地开始把脏页写到磁盘。
2. 缓存区达到一定阀值时，pdflush会去主动回收。
3. 使用sync、fsync和fdatasync系统调用将缓存区刷新到磁盘。
所以那些buffers、cached中的缓存空间，在系统需要的时候，它会主动让出空间来做它用。
buffers和cached的区别主要是， buffers是块设备的读写缓存区，是物理级的，如你对裸盘的读写。cached是作为page caches，是针对文件系统的页缓存，文件的读写都是缓存在这里面的。当page cache的数据需要刷新时，page cache中的数据交给buffer cache（这种操作在2.6内核后变的简单了，具体怎么简单？）。
手动回收cache的方法：
方法一：
# And free up caches
#
echo Freeing the page cache:
echo 1 > /proc/sys/vm/drop_caches
echo Free dentries and inodes:
echo 2 > /proc/sys/vm/drop_caches
echo Free the page cache, dentries and the inodes:
echo 3 > /proc/sys/vm/drop_caches
当然这种方法也适用与sysctl。
方法二：
fcntl(fd, F_SETFL, O_DIRECT)
对文件描述符设置O_DRECT，会避免cache该文件的内容，直接读写磁盘。在大量数据传输的时候，会避免占用大量缓存，而且传输效率也不会下降什么。
方法三：
降低如下阀值的数值，有利于回收缓存，但会影响性能，这些设置要根据具体环境来调优。
/proc/sys/vm/dirty_ratio
/proc/sys/vm/dirty_background_ratio
/proc/sys/vm/dirty_expire_centisecs
方法四：
sysctl -w vm.vfs_cache_pressure=n (n > 100)
这会使Kernel更勤于回收cache。
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