在Linux中，有很多命令或工具用来查看内存使用情况，今天我们来简单讨论一下如何查看进程消耗、占用的内存情况，Linux的内存管理和相关概念要比Windows复杂一些。在此之前，我们需要了解一下Linux系统下面有关内存的专用名词和专业术语概念：
物理内存和虚拟内存
物理内存：就是系统硬件提供的内存大小，是真正的内存，一般叫做内存条。也叫随机存取存储器(random access memory，RAM)又称作“随机存储器”，是与CPU直接交换数据的内部存储器，也叫主存(内存)。
虚拟内存：相对于物理内存，在Linux下还有一个虚拟内存的概念，虚拟内存就是为了满足物理内存的不足而提出的策略，它是利用磁盘空间虚拟出的一块逻辑内存，用作虚拟内存的磁盘空间被称为交换空间(Swap Space)。Linux会在物理内存不足时，使用虚拟内存，内核会把暂时不用的内存块信息写到虚拟内存，这样物理内存就得到了释放，这块儿内存就可以用于其他目的，而需要用到这些内容的时候，这些信息就会被重新从虚拟内存读入物理内存。
Linux的buffers与cached
在Linux系统中经常发现空闲的内存很少，似乎所有的内存都被消耗殆尽了，表面上看是内存不够用了，很多新手看到内存被“消耗殆尽”非常紧张，其实这个是因为Linux系统将空闲的内存用来做磁盘文件数据的缓存。这个导致你的系统看起来处于内存非常紧急的状况。但是实际上不是这样。这个区别于Windows的内存管理。Linux会利用空闲的内存来做cached & buffers。
buffers是指用来给块设备做的缓冲大小(块设备的读写缓冲区)，它只记录文件系统的metadata以及 tracking in-flight pages.
Buffers are associated with a specific block device, and cover caching of filesystem metadata as well as tracking in-flight pages. The cache only contains parked file data. That is, the buffers remember what's in directories, what file permissions are, and keep track of what memory is being written from or read to for a particular block device. The cache only contains the contents of the files themselves.
cached是作为page cache的内存, 文件系统的cache。你读写文件的时候，Linux内核为了提高读写性能与速度，会将文件在内存中进行缓存，这部分内存就是Cache Memory(缓存内存)。即使你的程序运行结束后，Cache Memory也不会自动释放。这就会导致你在Linux系统中程序频繁读写文件后，你会发现可用物理内存会很少。其实缓存内存(Cache Memory)在你需要使用内存的时候会自动释放，所以你不必担心没有内存可用
Cached is the size of the page cache. Buffers is the size of in-memory block I/O buffers. Cached matters; Buffers is largely irrelevant.
Cached is the size of the Linux page cache, minus the memory in the swap cache, which is represented by SwapCached (thus the total page cache size is Cached + SwapCached). Linux performs all file I/O through the page cache. Writes are implemented as simply marking as dirty the corresponding pages in the page cache; the flusher threads then periodically write back to disk any dirty pages. Reads are implemented by returning the data from the page cache; if the data is not yet in the cache, it is first populated. On a modern Linux system, Cached can easily be several gigabytes. It will shrink only in response to memory pressure. The system will purge the page cache along with swapping data out to disk to make available more memory as needed.
Buffers are in-memory block I/O buffers. They are relatively short-lived. Prior to Linux kernel version 2.4, Linux had separate page and buffer caches. Since 2.4, the page and buffer cache are unified and Buffers is raw disk blocks not represented in the page cache—i.e., not file data. The Buffers metric is thus of minimal importance. On most systems, Buffers is often only tens of megabytes.
