本节简单介绍了Linux内存管理中两个容易混淆的概念:Buffer和Cache. 在Linux中,一般使用free命令查看内存的使用情况 ``` [root@localhost ~]# free total used free shared buff/cache available Mem: 3873760 143548 3322324 8788 407888 3453708 Swap: 1048572 0 1048572 [root@localhost ~]# ``` 通过命令man free可查看以上输出项所代表的意思 ``` 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命令显示了空闲+物理使用+交换区等信息,通过解析/proc/meminfo获得,该文件中的内容如下: ``` [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 - 总内存大小 used - 使用的内存大小 free - 未使用内存大小 shared - tmpfs使用的内存 buffers - 内核缓存大小(/proc/meminfo中的Buffers) cache - 页面缓存和slabs(/proc/meminfo中的Cached和Slab) buff/cache - buffers + cache available - 可用内存大小(剔除交换区) buffers和cache在man free中仍没有详细解释清楚,通过查看kernel doc进一步获得相关的解释 ``` ... 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 - 裸磁盘块的临时存储(通常不会不大,20MB左右) Cached - 从磁盘上读取文件的缓存(即页面缓存).不包括SwapCached Slab - 内核中的数据结构缓存 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可监控内存的使用情况 ``` [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 ... ``` **参考资料** [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)