本節簡單介紹了Linux記憶體管理中兩個容易混淆的概念:Buffer和Cache.

在Linux中,一般使用free命令檢視記憶體的使用情況
```
[root@localhost ~]# free
              total        used        free      shared  buff/cache   available
Mem:        3873760      143548     3322324        8788      407888     3453708
Swap:       1048572           0     1048572
[root@localhost ~]# 
```
透過命令man free可檢視以上輸出項所代表的意思
```
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, available on kernels
              2.6.32, displayed as zero if not available)

buffers
              Memory used by kernel buffers (Buffers in /proc/meminfo)

cache  Memory used by the page cache and slabs (Cached and Slab 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+, oth‐
              erwise the same as free)
```
free命令顯示了空閒+物理使用+交換區等資訊,透過解析/proc/meminfo獲得,該檔案中的內容如下:
```
[root@localhost ~]# cat /proc/meminfo
MemTotal:        3873760 kB
MemFree:         3322052 kB
MemAvailable:    3453436 kB
Buffers:            3252 kB
Cached:           320316 kB
SwapCached:            0 kB
Active:           256968 kB
Inactive:         124468 kB
Active(anon):      58264 kB
Inactive(anon):     8392 kB
Active(file):     198704 kB
Inactive(file):   116076 kB
Unevictable:           0 kB
Mlocked:               0 kB
SwapTotal:       1048572 kB
SwapFree:        1048572 kB
Dirty:                 0 kB
Writeback:             0 kB
AnonPages:         57868 kB
Mapped:            27384 kB
Shmem:              8788 kB
Slab:              84320 kB
SReclaimable:      42580 kB
SUnreclaim:        41740 kB
KernelStack:        3120 kB
PageTables:         4492 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:     2985452 kB
Committed_AS:     293568 kB
VmallocTotal:   34359738367 kB
VmallocUsed:      180372 kB
VmallocChunk:   34359326716 kB
HardwareCorrupted:     0 kB
AnonHugePages:      4096 kB
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB
DirectMap4k:       81728 kB
DirectMap2M:     3063808 kB
DirectMap1G:     3145728 kB
[root@localhost ~]# 
```
total - 總記憶體大小
used - 使用的記憶體大小
free - 未使用記憶體大小
shared - tmpfs使用的記憶體
buffers - 核心快取大小(/proc/meminfo中的Buffers)
cache - 頁面快取和slabs(/proc/meminfo中的Cached和Slab)
buff/cache - buffers + cache
available - 可用記憶體大小(剔除交換區)

buffers和cache在man free中仍沒有詳細解釋清楚,透過檢視kernel doc進一步獲得相關的解釋
```
...
Buffers: Relatively temporary storage for raw disk blocks
         shouldn't get tremendously large (20MB or so)
Cached: in-memory cache for files read from the disk (the
        pagecache).  Doesn't include SwapCached
Slab: in-kernel data structures cache
SReclaimable: Part of Slab, that might be reclaimed, such as caches
SUnreclaim: Part of Slab, that cannot be reclaimed on memory pressure
```
Buffers - 裸磁碟塊的臨時儲存(通常不會不大,20MB左右)
Cached - 從磁碟上讀取檔案的快取(即頁面快取).不包括SwapCached
Slab - 核心中的資料結構快取
SReclaimable - Slab中的一部分,可回收 
SUnreclaim - Slab中的另一部分,不能回收

使用vmstat可監控記憶體的使用情況
```
[root@localhost ~]# echo 3 > /proc/sys/vm/drop_caches
[root@localhost ~]# vmstat 1
procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 1  1      0 3648060      0  96964    0    0     4  5158  151  174  5  6 89  0  0
 0  0      0 3648248      0  97424    0    0   296     0  311  331  0  1 99  0  0
...

```

**參考資料**
[THE /proc FILESYSTEM](https://www.kernel.org/doc/Documentation/filesystems/proc.txt)
[Linux Cache VS. Buffer](https://jin-yang.github.io/post/linux-memory-buffer-vs-cache-details.html)

PostgreSQL DBA(89) - Linux(Buffer vs Cache)

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