Linux缓存回收

# free
             total       used       free     shared    buffers     cached
Mem:         26892      13296      13596       2068        420       3320
-/+ buffers/cache:       9556      17336
Swap:            0          0          0

其中cached是内存中缓存，其中有一部分是可以用命令回收的。（比如/dev/shm里也占着空间，但无法被回收）

注意清Cache、swap并不是个必要的操作，可能会带来麻烦。

江湖传言，保险起见sync要执行两次！因为这命令不可靠

Linux MemFree与MemAvailable的区别¶

via: https://blog.51cto.com/xujpxm/1961072

2、MemFree：空闲内存数

表示系统尚未使用的内存。MemUsed=MemTotal-MemFree就是已被用掉的内存。

3、MemAvailable：可用内存数

应用程序可用内存数。系统中有些内存虽然已被使用但是可以回收的，比如cache/buffer、slab都有一部分可以回收，所以MemFree不能代表全部可用的内存，这部分可回收的内存加上MemFree才是系统可用的内存，即：MemAvailable≈MemFree+Buffers+Cached，它是内核使用特定的算法计算出来的，是一个估计值。它与MemFree的关键区别点在于，MemFree是说的系统层面，MemAvailable是说的应用程序层面。

How to Clear RAM Memory Cache, Buffer and Swap Space on Linux¶

via: https://www.tecmint.com/clear-ram-memory-cache-buffer-and-swap-space-on-linux/

How to Clear Cache in Linux?¶

1. Clear PageCache only.

# sync; echo 1 > /proc/sys/vm/drop_caches

2. Clear dentries and inodes.

# sync; echo 2 > /proc/sys/vm/drop_caches

3. Clear PageCache, dentries and inodes.

# sync; echo 3 > /proc/sys/vm/drop_caches

Explanation of above command.

sync will flush the file system buffer. Command Separated by “;” run sequentially. The shell wait for each command to terminate before executing the next command in the sequence. As mentioned in kernel documentation, writing to drop_cache will clean cache without killing any application/service, command echo is doing the job of writing to file.

If you have to clear the disk cache, the first command is safest in enterprise and production as “...echo 1 > ….” will clear the PageCache only. It is not recommended to use third option above “...echo 3 >” in production until you know what you are doing, as it will clear PageCache, dentries and inodes.

How to Clear Swap Space in Linux?¶

you want to clear Swap space, you may like to run the below command.

# swapoff -a && swapon -a

Also you may add above command to a cron script above, after understanding all the associated risk.

Now we will be combining both above commands into one single command to make a proper script to clear RAM Cache and Swap Space.

# echo 3 > /proc/sys/vm/drop_caches && swapoff -a && swapon -a && printf '\n%s\n' 'Ram-cache and Swap Cleared'

OR

$ su -c "echo 3 >'/proc/sys/vm/drop_caches' && swapoff -a && swapon -a && printf '\n%s\n' 'Ram-cache and Swap Cleared'" root