Linux free 查看内存使用情况常用命令

客官°小女子只卖身不卖艺 2022-12-30 13:55 281阅读 0赞

## Intro ##

*  常用命令

`free`  
`free -h` 以更易读(human-readable)的格式输出  
`free -s 3` 每3秒输出一次

*  示例

wuyujin1997@budy:~$ free -h
                  total        used        free      shared  buff/cache   available
    Mem:           985M        253M        124M        2.7M        607M        566M
    Swap:            0B          0B          0B
    wuyujin1997@budy:~$

*  行列意义

先区分行：

*  `Mem` 真正的物理内存
 *  `Swap` 交换空间(物理量内存过小时利用了硬盘空间进行“中转”的空间)

再区分列：

*  `total` 机器的整体内存大小(物理内存 + 处于硬盘区的交换空间)
 *  `used` 已使用的内存大小，计算方式： `total - free - buffers - cache`
 *  `free` 未使用/空闲的内存
 *  `shared` 共享内存(tmpfs使用的内存)
 *  `buff/cache` 内核使用的缓冲块buffer、页面缓存cache。
 *  `available` 估算在不使用交换空间的前提下，有多少内存可用于新启动的程序运行。
 *  表格分析

total        used        free      shared  buff/cache   available
    Mem:           985M        253M        124M        2.7M        607M        566M
    Swap:            0B          0B          0B

由表格可见，我的机器：  
内存大小`total`为：`985M`  
已使用内存`used`为： `253M`  
空闲内存`free`为：`124M`  
`buff/cache`之和为`607M`  
而：已使用内存 + 空闲内存 + `buff/cache` = 253 + 124 + 607 = 984M 约等于整体内存`985M`

两个核心指标：  
`total` 机器整体内存有`985M`  
`available` 新运行程序的可用内存大小为`566M`(而非free的`124M`)

available是大于free值的，因为如果新启动的程序内存不够用，可以去回收`buff/cache`的大小来运行新的程序。  
可得`available`的值上限：`free`\+`buff/cache`  
即：`0 < available < (free + buff/cache)` 毕竟`buff/cache`使用的内存是**不能被全部回收**的。

## 程序文档 ##

*  查看free命令的使用手册

`man free`

NAME
           free - Display amount of free and used memory in the system
    
    SYNOPSIS
           free [options]
    
    DESCRIPTION
           free  displays  the total amount of free and used physical and swap memory in the system, as well as the buffers
           and caches used by the kernel. The information is gathered by parsing /proc/meminfo. The displayed columns are:
    
           total  Total installed memory (MemTotal and SwapTotal in /proc/meminfo)
    
           used   Used memory (calculated as total - free - buffers - cache)
    
           free   Unused memory (MemFree and SwapFree in /proc/meminfo)
    
           shared Memory used (mostly) by tmpfs (Shmem in /proc/meminfo)
    
           buffers
                  Memory used by kernel buffers (Buffers in /proc/meminfo)
    
           cache  Memory used by the page cache and slabs (Cached and SReclaimable in /proc/meminfo)
    
           buff/cache
                  Sum of buffers and cache
    
           available
                  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)
    ...

可见：free命令的输出信息是基于`/proc/meminfo`文件的

*  查看系统文件

`more /proc/meminfo`

wuyujin1997@budy:~$ more /proc/meminfo
    MemTotal:        1008720 kB
    MemFree:          123096 kB
    MemAvailable:     575792 kB
    Buffers:          145400 kB
    Cached:           398732 kB
    SwapCached:            0 kB
    Active:           622084 kB
    Inactive:         141012 kB
    Active(anon):     219436 kB
    Inactive(anon):     2348 kB
    Active(file):     402648 kB
    Inactive(file):   138664 kB
    Unevictable:           0 kB
    Mlocked:               0 kB
    SwapTotal:             0 kB
    SwapFree:              0 kB
    Dirty:               372 kB
    Writeback:             0 kB
    AnonPages:        219020 kB
    Mapped:            92012 kB
    Shmem:              2820 kB
    Slab:              93564 kB
    SReclaimable:      77848 kB
    SUnreclaim:        15716 kB
    KernelStack:        2988 kB
    PageTables:         7156 kB
    NFS_Unstable:          0 kB
    Bounce:                0 kB
    WritebackTmp:          0 kB
    CommitLimit:      504360 kB
    Committed_AS:     906428 kB
    VmallocTotal:   34359738367 kB
    VmallocUsed:           0 kB
    VmallocChunk:          0 kB
    HardwareCorrupted:     0 kB
    AnonHugePages:         0 kB
    ShmemHugePages:        0 kB
    ShmemPmdMapped:        0 kB
    CmaTotal:              0 kB
    CmaFree:               0 kB
    HugePages_Total:       0
    HugePages_Free:        0
    HugePages_Rsvd:        0
    HugePages_Surp:        0
    Hugepagesize:       2048 kB
    DirectMap4k:       85888 kB
    DirectMap2M:      962560 kB
    DirectMap1G:           0 kB
    wuyujin1997@budy:~$