精确度量Linux下进程占用多少内存的方法

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背景

在Linux中，要了解进程的信息，莫过于从 proc 文件系统中入手去看。

proc的详细介绍，可以参考内核文档的解读，里面有很多内容

yum install -y kernel-doc
cat /usr/share/doc/kernel-doc-3.10.0/Documentation/filesystems/proc.txt

proc主要内容

Table of Contents
-----------------  

  0     Preface
  0.1   Introduction/Credits
  0.2   Legal Stuff  

  1     Collecting System Information
  1.1   Process-Specific Subdirectories
  1.2   Kernel data
  1.3   IDE devices in /proc/ide
  1.4   Networking info in /proc/net
  1.5   SCSI info
  1.6   Parallel port info in /proc/parport
  1.7   TTY info in /proc/tty
  1.8   Miscellaneous kernel statistics in /proc/stat
  1.9 Ext4 file system parameters  

  2     Modifying System Parameters  

  3     Per-Process Parameters
  3.1   /proc/<pid>/oom_adj & /proc/<pid>/oom_score_adj - Adjust the oom-killer
                                                                score
  3.2   /proc/<pid>/oom_score - Display current oom-killer score
  3.3   /proc/<pid>/io - Display the IO accounting fields
  3.4   /proc/<pid>/coredump_filter - Core dump filtering settings
  3.5   /proc/<pid>/mountinfo - Information about mounts
  3.6   /proc/<pid>/comm  & /proc/<pid>/task/<tid>/comm
  3.7   /proc/<pid>/task/<tid>/children - Information about task children
  3.8   /proc/<pid>/fdinfo/<fd> - Information about opened file  

  4     Configuring procfs
  4.1   Mount options

和进程内存相关的几个信息

 maps           Memory maps to executables and library files    (2.4)
 statm          Process memory status information
 status         Process status in human readable form
 smaps          a extension based on maps, showing the memory consumption of
                each mapping and flags associated with it

详解

status

这里可以看到概貌的内存统计

程序启动后，进程的内存占用可能包括程序本身的空间，共享的内存空间，mmap，malloc 的等

 VmPeak                      peak virtual memory size
 VmSize                      total program size
 VmLck                       locked memory size
 VmHWM                       peak resident set size ("high water mark")
 VmRSS                       size of memory portions
 VmData                      size of data, stack, and text segments
 VmStk                       size of data, stack, and text segments
 VmExe                       size of text segment
 VmLib                       size of shared library code
 VmPTE                       size of page table entries
 VmSwap                      size of swap usage (the number of referred swapents)

statm

内存统计信息，单位为PAGE ，通过getconf可以获得操作系统的page大小

getconf PAGE_SIZE

 Field    Content
 size     total program size (pages)            (same as VmSize in status)
 resident size of memory portions (pages)       (same as VmRSS in status)
 shared   number of pages that are shared       (i.e. backed by a file)
 trs      number of pages that are 'code'       (not including libs; broken,
                                                        includes data segment)
 lrs      number of pages of library            (always 0 on 2.6)
 drs      number of pages of data/stack         (including libs; broken,
                                                        includes library text)
 dt       number of dirty pages                 (always 0 on 2.6)

maps

进程与可执行程序或动态库文件相关的映射信息

address           perms offset  dev   inode      pathname  

08048000-08049000 r-xp 00000000 03:00 8312       /opt/test
08049000-0804a000 rw-p 00001000 03:00 8312       /opt/test
0804a000-0806b000 rw-p 00000000 00:00 0          [heap]
a7cb1000-a7cb2000 ---p 00000000 00:00 0
a7cb2000-a7eb2000 rw-p 00000000 00:00 0
a7eb2000-a7eb3000 ---p 00000000 00:00 0
a7eb3000-a7ed5000 rw-p 00000000 00:00 0          [stack:1001]
a7ed5000-a8008000 r-xp 00000000 03:00 4222       /lib/libc.so.6
a8008000-a800a000 r--p 00133000 03:00 4222       /lib/libc.so.6
a800a000-a800b000 rw-p 00135000 03:00 4222       /lib/libc.so.6
a800b000-a800e000 rw-p 00000000 00:00 0
a800e000-a8022000 r-xp 00000000 03:00 14462      /lib/libpthread.so.0
a8022000-a8023000 r--p 00013000 03:00 14462      /lib/libpthread.so.0
a8023000-a8024000 rw-p 00014000 03:00 14462      /lib/libpthread.so.0
a8024000-a8027000 rw-p 00000000 00:00 0
a8027000-a8043000 r-xp 00000000 03:00 8317       /lib/ld-linux.so.2
a8043000-a8044000 r--p 0001b000 03:00 8317       /lib/ld-linux.so.2
a8044000-a8045000 rw-p 0001c000 03:00 8317       /lib/ld-linux.so.2
aff35000-aff4a000 rw-p 00000000 00:00 0          [stack]
ffffe000-fffff000 r-xp 00000000 00:00 0          [vdso]

1. where "address" is the address space in the process that it occupies, "perms"

is a set of permissions:

 r = read
 w = write
 x = execute
 s = shared
 p = private (copy on write)

2. "offset" is the offset into the mapping,

3. "dev" is the device (major:minor),

4. "inode" is the inode on that device.

0 indicates that  no inode is associated with the memory region, as the case would be with BSS (uninitialized data).

5. The "pathname" shows the name associated file for this mapping.

If the mapping is not associated with a file:  

 [heap]                   = the heap of the program
 [stack]                  = the stack of the main process
 [stack:1001]             = the stack of the thread with tid 1001
 [vdso]                   = the "virtual dynamic shared object",
                            the kernel system call handler  

 or if empty, the mapping is anonymous.

smaps

对应每个映射的内存开销详情

08048000-080bc000 r-xp 00000000 03:02 13130      /bin/bash
Size:               1084 kB
Rss:                 892 kB
Pss:                 374 kB
Shared_Clean:        892 kB
Shared_Dirty:          0 kB
Private_Clean:         0 kB
Private_Dirty:         0 kB
Referenced:          892 kB
Anonymous:             0 kB
Swap:                  0 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
Locked:              374 kB
VmFlags: rd ex mr mw me de  

1. the size of the mapping(size),
2. the amount of the mapping that is currently resident in RAM (RSS),
3. the process' proportional share of this mapping (PSS),
4. the number of clean and dirty private pages in the mapping.
Note that even a page which is part of a MAP_SHARED mapping, but has only a single pte mapped,
i.e.  is currently used by only one process, is accounted as private and not as shared.
5. "Referenced" indicates the amount of memory currently marked as referenced or accessed.
6. "Anonymous" shows the amount of memory that does not belong to any file.
Even a mapping associated with a file may contain anonymous pages:
when MAP_PRIVATE and a page is modified, the file page is replaced by a private anonymous copy.
7. "Swap" shows how much would-be-anonymous memory is also used, but out on
swap.
8. "VmFlags" field deserves a separate description.
This member represents the kernel flags associated with the particular virtual memory area in two letter encoded manner.
The codes are the following:
    rd  - readable
    wr  - writeable
    ex  - executable
    sh  - shared
    mr  - may read
    mw  - may write
    me  - may execute
    ms  - may share
    gd  - stack segment growns down
    pf  - pure PFN range
    dw  - disabled write to the mapped file
    lo  - pages are locked in memory
    io  - memory mapped I/O area
    sr  - sequential read advise provided
    rr  - random read advise provided
    dc  - do not copy area on fork
    de  - do not expand area on remapping
    ac  - area is accountable
    nr  - swap space is not reserved for the area
    ht  - area uses huge tlb pages
    nl  - non-linear mapping
    ar  - architecture specific flag
    dd  - do not include area into core dump
    mm  - mixed map area
    hg  - huge page advise flag
    nh  - no-huge page advise flag
    mg  - mergable advise flag

一般来说，业务进程使用的内存主要有以下几种情况：

（1）用户空间的匿名映射页（Anonymous pages in User Mode address spaces），比如调用malloc分配的内存，以及使用MAP_ANONYMOUS的mmap；当系统内存不够时，内核可以将这部分内存交换出去；

（2）用户空间的文件映射页（Mapped pages in User Mode address spaces），包含map file和map tmpfs；前者比如指定文件的mmap，后者比如IPC共享内存；当系统内存不够时，内核可以回收这些页，但回收之前可能需要与文件同步数据；

（3）文件缓存（page in page cache of disk file）；发生在程序通过普通的read/write读写文件时，当系统内存不够时，内核可以回收这些页，但回收之前可能需要与文件同步数据；

（4）buffer pages，属于page cache；比如读取块设备文件。

进程RSS, 进程使用的所有物理内存（file_rss＋anon_rss），即Anonymous pages＋Mapped apges（包含共享内存）

Resident Set Size:
  number of pages the process has in real memory.
  This is just the pages which count toward text, data, or stack space.
  This does not include pages which have not been demand-loaded in,
  or which are swapped out.

显然如果把所有进程RSS的值相加，可能会超过实际的内存大小，原因是RSS统计存在一定的重复部分，例如在共享内存的计算方面，不同的进程会有重复的现象。

通过smaps可以非常方便的将重复的部分消除掉。

例如有多个进程加载了同样的库文件，那么会在这些进程间均摊这部分内存，均摊后的共享部分加上进程私有的内存记为Pss。

Pss:                 374 kB

私有的内存则在Private里面计算

Private_Clean:         0 kB
Private_Dirty:         0 kB

在linux中有一个工具叫smem，其实就是通过smaps来统计的。

PSS是Pss的相加

USS则是Private的相加

yum install -y smem smemstat  

smem can report proportional set size (PSS), which is a more meaningful representation of the amount of memory used by libraries and applications in a virtual memory system.  

Because large portions of physical memory are typically shared among multiple applications, the standard measure of memory usage known as resident set size (RSS) will significantly overestimate memory usage. PSS instead measures each application's "fair share" of each shared area to give a realistic measure.

例子

smem
  PID User     Command                         Swap      USS      PSS      RSS
23716 digoal   postgres: postgres postgres        0     4924     5387     7040   

对应的RSS, PSS, USS分别等于以下相加.
# cat /proc/23716/smaps | grep Rss
# cat /proc/23716/smaps | grep Pss
# cat /proc/23716/smaps | grep Private_

其他参考文章

https://www.selenic.com/smem/

http://hustcat.github.io/memory-usage-in-process-and-cgroup/

http://blog.hellosa.org/2010/02/26/pmap-process-memory.html

首先 ps 看一下我的系统跑着哪些process

$ ps aux  

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
...
czbug     1980  0.0  1.7 180472 34416 ?        Sl   Feb25   0:01 /usr/bin/yakuake
...

我拿 yakuake 这个小程序作例子。

其中，关于内存的是 VSZ 和 RSS 这两个

man ps 看看它们的含义：

rss       RSS    resident set size, the non-swapped physical memory that a task has used (in kiloBytes). (alias rssize, rsz).  

vsz       VSZ    virtual memory size of the process in KiB (1024-byte units). Device mappings are currently excluded; this is subject to change. (alias vsize).

简单一点说，RSS 就是这个process 实际占用的物理内存，VSZ 就是process 的虚拟内存，就是process 现在没有使用但未来可能会分配的内存大小。

其实这里的ps 出来的结果，是有点不正确的，如果把所有程序的 RSS 加起来，恐怕比你的实际内存还要大呢。为什么呢？？因为 ps 的结果，RSS 那部分，是包括共享内存的。这里我用 pmap 来看看。

$ pmap -d 1980  

1980:   /usr/bin/yakuake
Address   Kbytes Mode  Offset           Device    Mapping
00110000    2524 r-x-- 0000000000000000 008:00002 libkio.so.5.3.0
00387000       4 ----- 0000000000277000 008:00002 libkio.so.5.3.0
00388000      32 r---- 0000000000277000 008:00002 libkio.so.5.3.0
00390000      16 rw--- 000000000027f000 008:00002 libkio.so.5.3.0
00394000     444 r-x-- 0000000000000000 008:00002 libQtDBus.so.4.5.2
00403000       4 ----- 000000000006f000 008:00002 libQtDBus.so.4.5.2
00404000       4 r---- 000000000006f000 008:00002 libQtDBus.so.4.5.2
00405000       4 rw--- 0000000000070000 008:00002 libQtDBus.so.4.5.2
00407000     228 r-x-- 0000000000000000 008:00002 libkparts.so.4.3.0
00440000       8 r---- 0000000000039000 008:00002 libkparts.so.4.3.0
00442000       4 rw--- 000000000003b000 008:00002 libkparts.so.4.3.0
00443000    3552 r-x-- 0000000000000000 008:00002 libkdeui.so.5.3.0
007bb000      76 r---- 0000000000377000 008:00002 libkdeui.so.5.3.0
007ce000      24 rw--- 000000000038a000 008:00002 libkdeui.so.5.3.0
007d4000       4 rw--- 0000000000000000 000:00000   [ anon ]
....
mapped: 180472K    writeable/private: 19208K    shared: 20544K

我略去了一部分输出，都是差不多的，重点在最后那行输出。

linux 会把一些shared libraries 载入到内存中，在pmap 的输出中，这些shared libraries 的名字通常是 lib*.so 。如 libX11.so.6.2.0 。这个 libX11.so.6.2.0 会被很多process load 到自己的运行环境中，同时，ps 输出的RSS 结果中，每个process 都包含了这个libX11.so.6.2.0 ，而事实上它只被load 了一次，如果单纯把ps 的结果相加，这样就重复计算了。

而 pmap 的输出中，writeable/private: 19208K ，这个就是yakuake 这个程序真正占用的物理内存，不包含shared libraries 。在这里，它只有19208K，而ps 的RSS 是34416K。

我在看这方面的资料时，还看到一些关于virtual memory 的，再记录下。

以下两个命令均可查看 vmsize 。

$ cat /proc/<pid>/stat | awk '{print $23 / 1024}'
$ cat /proc/<pid>/status | grep -i vmsize

一般来说，得出来的值，是和 ps 的 VSZ 是一样的，但有一种情况例外，就是查看X server 的时候。

举个例：

$ ps aux|grep /usr/bin/X|grep -v grep | awk '{print $2}'   # 得出X server 的 pid   ...
1076  

$ cat /proc/1076/stat | awk '{print $23 / 1024}'
139012  

$ cat /proc/1076/status | grep -i vmsize
VmSize:      106516 kB

而 ps 的 VSZ 为 106516 ，与后者是一致的。

据说是因为

VmSize = memory + memory-mapped hardware (e.g. video card memory).

参考：https://github.com/digoal/blog/blob/master/201606/20160608_01.md