语法
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free [-bkmotV][-s <间隔秒数>]
参数说明：
- -b 以 Byte 为单位显示内存使用情况。
- -k 以 KB 为单位显示内存使用情况。
- -m 以 MB 为单位显示内存使用情况。
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-h 以合适的单位显示内存使用情况，最大为三位数，自动计算对应的单位值。单位有：
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B = bytes K = kilos M = megas G = gigas T = teras
- -o 不显示缓冲区调节列。
- -s <间隔秒数> 持续观察内存使用状况。
- -t 显示内存总和列。
- -V 显示版本信息。
实例
显示内存使用情况
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# free //显示内存使用信息
total used free shared buffers cached
Mem: 254772 184568 70204 0 5692 89892
-/+ buffers/cache: 88984 165788
Swap: 524280 65116 459164
以总和的形式显示内存的使用信息
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# free -t //以总和的形式查询内存的使用信息
total used free shared buffers cached
Mem: 254772 184868 69904 0 5936 89908
-/+ buffers/cache: 89024 165748
Swap: 524280 65116 459164
Total: 779052 249984 529068
周期性的查询内存使用信息
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# free -s 10 //每10s 执行一次命令
total used free shared buffers cached
Mem: 254772 187628 67144 0 6140 89964
-/+ buffers/cache: 91524 163248
Swap: 524280 65116 459164
total used free shared buffers cached
Mem: 254772 187748 67024 0 6164 89940
-/+ buffers/cache: 91644 163128
Swap: 524280 65116 459164
输出解释
free 是用来查看当前系统的内存使用情况的。 在解释之前，先要熟悉几个概念。 共享链接库：它的意思是一些共享的库文件，不需要每个进程都加载到内存中，每个进程可以共享它们，从而达到节省内存的目的，比如 libc.so。
然后是 Cache Pages 和 Buffer，以下是从 Redhat 的网站上抓出来的。 这里需要注意一点的是：它们的系统来使用的，在新版本的 free 中是 used 列中，是没有计算出来的，原因就是在系统的可用内存没有的时候，OS 会释放一些 Cached 或 Buffered 的内存，给应用程序。
Linux always tries to use RAM to speed up disk operations by using available memory for buffers (file system metadata) and cache (pages with actual contents of files or block devices). This helps the system to run faster because disk information is already in memory which saves I/O operations. If space is needed by programs or applications like Oracle, then Linux will free up the buffers and cache to yield memory for the applications. If your system runs for a while you will usually see a small number under the field “free” on the first line.
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Cache Pages:
A cache is the part of the memory which transparently stores data so that future requests for that data can be served faster. This memory is utilized by the kernel to cache disk data and improve i/o performance.
The Linux kernel is built in such a way that it will use as much RAM as it can to cache information from your local and remote filesystems and disks. As the time passes over various reads and writes are performed on the system, kernel tries to keep data stored in the memory for the various processes which are running on the system or the data that of relevant processes which would be used in the near future. The cache is not reclaimed at the time when process get stop/exit, however when the other processes requires more memory then the free available memory, kernel will run heuristics to reclaim the memory by storing the cache data and allocating that memory to new process.
When any kind of file/data is requested then the kernel will look for a copy of the part of the file the user is acting on, and, if no such copy exists, it will allocate one new page of cache memory and fill it with the appropriate contents read out from the disk.
The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere in the disk. When some data is requested, the cache is first checked to see whether it contains that data. The data can be retrieved more quickly from the cache than from its source origin.
SysV shared memory segments are also accounted as a cache, though they do not represent any data on the disks. One can check the size of the shared memory segments using ipcs -m command and checking the bytes column.
Buffers :
Buffers are the disk block representation of the data that is stored under the page caches. Buffers contains the metadata of the files/data which resides under the page cache. Example: When there is a request of any data which is present in the page cache, first the kernel checks the data in the buffers which contain the metadata which points to the actual files/data contained in the page caches. Once from the metadata the actual block address of the file is known, it is picked up by the kernel for processing.
我们来看看 free 的输出：
| - | total | used | free | shared | buff/cache | available |
|---|---|---|---|---|---|---|
| Mem: | 65697148 | 28995804 | 15909532 | 1587368 | 20791812 | 32578364 |
total ： 表示系统的总内存 used ： 表示应用程序已经使用的内存 free ： 表示当前还没有被使用的内存 shared ：表示共享链接库使用的内存 buff/cache ： 表示系统的 page cache 和 buffer 使用到的内存 available ： 表示应用程序还可以申请到的内存
系统当前使用到的内存是：used + buff/cache，used 中包含了 shared。
所以 total = used + buff/cache + free = 28995804 +20791812 + 15909532 = 65697148。
available（32578364） <= free + buff/cache（15909532 + 20791812 = 36701344），为什么是小于呢？因为系统的一些 page 或 cache 是不能回收的。
As of the new available field, for Linux kernels older than 2.6.27, its value is the same as the free value, but for the later versions of the Kernel, its a bit different:
Estimation of how much memory is available for starting new applications, without swapping. Unlike the data provided by the cache or free fields, this field takes into account page cache and also that not all reclaimable memory slabs will be reclaimed due to items being in use (MemAvailable in /proc/meminfo, available on kernels 3.14, emulated on kernels 2.6.27+, otherwise the same as free)
cache过高处理
先执行 sync 强制将缓存刷到磁盘
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echo 3 > /proc/sys/vm/drop_caches #释放所有缓存
drop_caches 的值可以是 0-3 之间的数字，代表不同的含义： 0：不释放（系统默认值） 1：释放页缓存 2：释放 dentries 和 inodes 3：释放所有缓存
如果系统 buffer&cache 经常性使用高，可以 crontab 中定时执行来释放空间。
