Linux 内存 为什么会一直被占满
在Linux中经常发现空闲内存很少，似乎所有的内存都被系统占用了，表面感觉是内存不够用了，其实不然。这是Linux内存管理的一个优秀特性，在这方面，区别于 Windows的内存管理。主要特点是，无论物理内存有多大，Linux都将其充份利用，将一些程序调用过的硬盘数据读入内存，利用内存读写的高速特性来提高Linux系统的数据访问性能。而Windows是只在需要内存时，才为应用程序分配内存，并不能充分利用大容量的内存空间。换句话说，每增加一些物理内存，Linux都将能充分利用起来，发挥了硬件投资带来的好处，而Windows只将其做为摆设，即使增加8GB甚至更大。
Linux 的这一特性，主要是利用空闲的物理内存，划分出一部份空间，做为 cache 和 buffers，以此提高数据访问性能。
页高速缓存(cache)是Linux内核实现的一种主要磁盘缓存。它主要用来减少对磁盘的I/O操作。具体地讲，是通过把磁盘中的数据缓存到物理内存中，把对磁盘的访问变为对物理内存的访问。
磁盘高速缓存的价值在于两个方面：第一，访问磁盘的速度要远远低于访问内存的速度，因此，从内存访问数据比从磁盘访问速度更快。第二，数据一旦被访问，就很有可能在短期内再次被访问到。
Linux 下查看进程占用的内存
[root@test-01 ~]$ man pmap
PMAP(1) & & & & & & & & & & & & & & & & & & & & & & & & & &Linux User Manual & & & & & & & & & & & & & & & & & & & & & & & & & PMAP(1)
& & & &pmap - report memory map of a process
& & & &pmap [ -x | -d ] [ -q ] pids...
& & & &pmap -V
DESCRIPTION
& & & &The pmap command reports the memory map of a process or processes.
GENERAL OPTIONS
& & & &-x & extended & & & Show the extended format.
& & & &-d & device & & & & Show the device format.
& & & &-q & quiet & & & & &Do not display some header/footer lines.
& & & &-V & show version & Displays version of program.
& & & &ps(1) pgrep(1)
& & & &No standards apply, but pmap looks an awful lot like a SunOS command.
& & & &Albert &Cahalan&
&[root@test-01 ~]$sudo pmap -x 319
319: & &/usr/lib/nfs/nfsmapid
&Address &Kbytes & & RSS & &Anon &Locked Mode & Mapped File
& & &16 & & &16 & & & - & & & - r-x-- &nfsmapid & 代码段
& & & 8 & & & 8 & & & 8 & & & - rwx-- &nfsmapid & &数据段
& &2200 & & 216 & & 216 & & & - rwx-- & &[ heap ] &
FE9FA000 & & & 8 & & & 8 & & & 8 & & & - rw--R & &[ stack tid=4 ]
FEA00000 & &1024 & & &64 & & & - & & & - rwx-- & &[ anon ]&
FEB50000 & & &24 & & &24 & & & - & & & - r-x-- &libgen.so.1
FEB66000 & & & 8 & & & 8 & & & 8 & & & - rwx-- &libgen.so.1
FEB70000 & & &32 & & &24 & & & - & & & - r-x-- &libuutil.so.1
FEB88000 & & & 8 & & & 8 & & & 8 & & & - rwx-- &libuutil.so.1
每列的含义如下：
Address:4000 进程所占的地址空间
Kbytes 该虚拟段的大小
RSS 设备号（主设备：次设备）
Anon 设备的节点号，0表示没有节点与内存相对应
Locked 是否允许swapped
Mode 权限：r=read, w=write, x=execute, s=shared, p=private(copy on write)
后shift + m
top命令用来显示执行中的程序进程，使用权限是所有用户。
2.格式
top [－] [d delay] [q] [c] [S] [s] [i] [n]
3.主要参数
d：指定更新的间隔，以秒计算。
q：没有任何延迟的更新。如果使用者有超级用户，则top命令将会以最高的优先序执行。
c：显示进程完整的路径与名称。
S：累积模式，会将己完成或消失的子行程的CPU时间累积起来。
s：安全模式。
i：不显示任何闲置(Idle)或无用(Zombie)的行程。
n：显示更新的次数，完成后将会退出top。
图1 top命令的显示
在图1中，第一行表示的项目依次为当前时间、系统启动时间、当前系统登录用户数目、平均负载。第二行显示的是所有启动的进程、目前运行的、挂起(Sleeping)的和无用(Zombie)的进程。第三行显示的是目前CPU的使用情况，包括系统占用的比例、用户使用比例、闲置(Idle)比例。第四行显示物理内存的使用情况，包括总的可以使用的内存、已用内存、空闲内存、缓冲区占用的内存。第五行显示交换分区使用情况，包括总的交换分区、使用的、空闲的和用于高速缓存的大小。第六行显示的项目最多，下面列出了详细解释。
PID（Process ID）：进程标示号。
USER：进程所有者的用户名。
PR：进程的优先级别。
NI：进程的优先级别数值。
VIRT：进程占用的虚拟内存值。
RES：进程占用的物理内存值。
SHR：进程使用的共享内存值。
S：进程的状态，其中S表示休眠，R表示正在运行，Z表示僵死状态，N表示该进程优先值是负数。
%CPU：该进程占用的CPU使用率。
%MEM：该进程占用的物理内存和总内存的百分比。
TIME＋：该进程启动后占用的总的CPU时间。
Command：进程启动的启动命令名称，如果这一行显示不下，进程会有一个完整的命令行。
top命令使用过程中，还可以使用一些交互的命令来完成其它参数的功能。这些命令是通过快捷键启动的。
&空格&：立刻刷新。
P：根据CPU使用大小进行排序。
T：根据时间、累计时间排序。
q：退出top命令。
m：切换显示内存信息。
t：切换显示进程和CPU状态信息。
c：切换显示命令名称和完整命令行。
M：根据使用内存大小进行排序。
W：将当前设置写入~/.toprc文件中。这是写top配置文件的推荐方法。
可以看到，top命令是一个功能十分强大的监控系统的工具，对于系统管理员而言尤其重要。但是，它的缺点是会消耗很多系统资源。
更多的请看：/windows/2003/index.html
free命令用来显示内存的使用情况，使用权限是所有用户。
2.格式
free [－b－k－m] [－o] [－s delay] [－t] [－V]
3.主要参数
－b －k －m：分别以字节（KB、MB）为单位显示内存使用情况。
－s delay：显示每隔多少秒数来显示一次内存使用情况。
－t：显示内存总和列。
－o：不显示缓冲区调节列。
4.应用实例
free命令是用来查看内存使用情况的主要命令。和top命令相比，它的优点是使用简单，并且只占用很少的系统资源。通过－S参数可以使用free命令不间断地监视有多少内存在使用，这样可以把它当作一个方便实时监控器。
＃free －b －s5
使用这个命令后终端会连续不断地报告内存使用情况（以字节为单位），每5秒更新一次。
uptime 命令
我曾经看到资料上讲，load avarage &3 系统良好，大于5 则有严重的性能问题。注意，这个值还应当除以CPU数目。
如果load avarage=8 ,CPU=3,8/3=2.666，2.66这个值表示系统状态良好
大于5也不一定是严重性能问题，有可能是的确主机提供的服务超过了他能够提供的能力，需要扩容了。要具体看看。
另外还有 vmstat 与 iostat 这两个很有用的命令。
Linux下看内存和CPU使用率一般都用top命令，但是实际在用的时候，用top查看出来的内存占用率都非常高，如：
Mem:&& 4086496k total, 4034428k used,&&& 52068k free,&& 112620k buffers
Swap: 4192956k total,&& 799952k used, 3393004k free, 1831700k cached
接近98.7%，而实际上的应用程序占用的内存往往并没这么多，
PID USER&&&&& PR NI VIRT RES SHR S %CPU %MEM&&& TIME+ COMMAND&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&
25801 sybase&&& 15&& 0 m 805m S 1.0 20.2 27:56.96 dataserver&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&
12084 oracle&&& 16&& 0 m 719m S 0.0 18.6&& 0:13.50 oracle&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&
27576 xugy&&&&& 25&& 0 986m 210m 1040 S 1.0 5.3 28:51.24 cti&&&&&&&&&
25587 yaoyang&& 17&& 0 m 3792 S 0.0 4.1&& 9:21.14 java
看%MEM这列的数字，按内存排序后，把前几名加起来，撑死了才不过55%，那剩下的内存都干嘛用了？
一般的解释是Linux系统下有一种思想，内存不用白不用，占用了就不释放，听上去有点道理，但如果我一定要知道应用程序还能有多少内存可用呢？
仔细看top关于内存的显示输出，有两个数据buffers和cached，在Linux系统下的buffer指的是磁盘写缓存，而cache则指的是磁盘读缓存。
(A buffer is something that has yet to be &written& to disk.&
A cache is something that has been &read& from the disk and stored for later use.)
而这两块是为了提高系统效率而分配的内存，在内存富余的时候，操作系统将空闲内存利用起来，而有内存需求时，系统会释放这部分的内存供应用程序使用。
这样，真正应用程序可用的内存就是free+buffer+cache，上面的例子就是：
52068k + 112620k + 1831700k = 1996388k
而已用内存则是used-buffer-cache，上面的例子为：
4034428k - 112620k - 1831700k = 2090108k
Linux下查看内存还有一个更方便的命令，free:
&&&&&&&&&&&& total&&&&&& used&&&&&& free&&&& shared&&& buffers&&&& cached
Mem:&&&&&& 4086496&&& 4034044&&&&& 52452&&&&&&&&& 0&&&& 112756&&& 1831564
-/+ buffers/cache:&&& 2089724&&& 1996772
Swap:&&&&& 4192956&&&& 799952&&& 3393004
Mem:这列就是用top命令看到的内存使用情况，而-/+buffers/cache这列就是我们刚刚做的计算结果，used-buffer-cache/free+buffer+cache
也可以加-m或者-g参数查看按MB或者GB换算的结果。
&&&&&&&&&&&& total&&&&&& used&&&&&& free&&&& shared&&& buffers&&&& cached
Mem:&&&&&&&&& 3990&&&&&& 3906&&&&&&&& 83&&&&&&&&& 0&&&&&&&& 90&&&&&& 1786
-/+ buffers/cache:&&&&&& 2029&&&&&& 1961
Swap:&&&&&&&& 4094&&&&&&& 781&&&&&& 3312
这样，真正应用程序的内存使用量就可以得出来了，上面的例子中内存占用率为51.1%。
Need to monitor Linux server performance? Try these built-in commands and a few add-on tools. Most Linux distributions are equipped with tons of monitoring. These tools provide metrics which can be used to get information about system activities. You can
use these tools to find the possible causes of a performance problem. The commands discussed below are some of the most basic commands when it comes to system analysis and debugging server issues such as:
Finding out bottlenecks.Disk (storage) bottlenecks.CPU and memory bottlenecks.Network bottlenecks.
#1: top - Process Activity Command
The top program provides a dynamic real-time view of a running system i.e. actual process activity. By default, it displays the most CPU-intensive tasks running on the server and updates the list every five seconds.
Fig.01: Linux top command
Commonly Used Hot Keys
The top command provides several useful hot keys:
Displays summary information off and on.
Displays memory information off and on.
Sorts the display by top consumers of various system resources. Useful for quick identification of performance-hungry tasks on a system.
Enters an interactive configuration screen for top. Helpful for setting up top for a specific task.
Enables you to interactively select the ordering within top.
Issues renice command.
Issues kill command.
Turn on or off color/mono
=& Related:
How do I Find Out Linux CPU Utilization?
#2: vmstat - System Activity, Hardware and System Information
The command vmstat reports information about processes, memory, paging, block IO, traps, and cpu activity.
# vmstat 3
Sample Outputs:
procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu------
cs us sy id wa st
0 188 5130400
0 188 5130400
0 188 5130400
0 188 5130500
0 188 5130512
0 188 5130588
0 188 5130640
Display Memory Utilization Slabinfo
# vmstat -m
Get Information About Active / Inactive Memory Pages
# vmstat -a
=& Related:
How do I find out Linux Resource utilization to detect system bottlenecks?
#3: w - Find Out Who Is Logged on And What They Are Doing
w command displays information about the users currently on the machine, and their processes.
# w username
Sample Outputs:
17:58:47 up 5 days, 20:28,
load average: 0.36, 0.26, 0.24
10.1.3.145
0.02s vim /etc/resolv.conf
10.1.3.145
#4: uptime - Tell How Long The System Has Been Running
The uptime command can be used to see how long the server has been running. The current time, how long the system has been running, how many users are currently logged on, and the system load averages for the past 1, 5, and 15 minutes.
18:02:41 up 41 days, 23:42,
load average: 0.00, 0.00, 0.00
1 can be considered as optimal load value. The load can change from system to system. For a single CPU system 1 - 3 and SMP systems 6-10 load value might be acceptable.
#5: ps - Displays The Processes
ps command will report a snapshot of the current processes. To select all processes use the -A or -e option:
Sample Outputs:
00:00:02 init
00:00:02 migration/0
00:00:01 ksoftirqd/0
00:00:00 watchdog/0
00:00:00 migration/1
00:00:15 ksoftirqd/1
00:53:28 java
00:00:00 mingetty
00:00:00 mingetty
00:00:00 mingetty
00:00:00 mingetty
00:00:00 mingetty
00:00:00 mingetty
4893 ttyS1
00:00:00 agetty
00:00:00 cifsoplockd
00:00:00 cifsdnotifyd
00:10:34 lighttpd
00:00:00 php-cgi
54981 pts/0
00:00:00 vim
00:00:00 php-cgi
00:00:00 bind9-snmp-stat
55704 pts/1
00:00:00 ps
ps is just like top but provides more information.
Show Long Format Output
To turn on extra full mode (it will show command line arguments passed to process):
To See Threads ( LWP and NLWP)
# ps -AlFH
To See Threads After Processes
# ps -AlLm
Print All Process On The Server
Print A Process Tree
Print Security Information
# ps -eo euser,ruser,suser,fuser,f,comm,label
See Every Process Running As User Vivek
# ps -U vivek -u vivek u
Set Output In a User-Defined Format
# ps -eo pid,tid,class,rtprio,ni,pri,psr,pcpu,stat,wchan:14,comm
# ps axo stat,euid,ruid,tty,tpgid,sess,pgrp,ppid,pid,pcpu,comm
# ps -eopid,tt,user,fname,tmout,f,wchan
Display Only The Process IDs of Lighttpd
# ps -C lighttpd -o pid=
# pgrep lighttpd
# pgrep -u vivek php-cgi
Display The Name of PID 55977
# ps -p 55977 -o comm=
Find Out The Top 10 Memory Consuming Process
# ps -auxf | sort -nr -k 4 | head -10
Find Out top 10 CPU Consuming Process
# ps -auxf | sort -nr -k 3 | head -10
#6: free - Memory Usage
The command free displays the total amount of free and used physical and swap memory in the system, as well as the buffers used by the kernel.
Sample Output:
-/+ buffers/cache:
=& Related: :
Linux Find Out Virtual Memory PAGESIZELinux Limit CPU Usage Per ProcessHow much RAM does my Ubuntu / Fedora Linux desktop PC have?
#7: iostat - Average CPU Load, Disk Activity
The command iostat report Central Processing Unit (CPU) statistics and input/output statistics for devices, partitions and network filesystems (NFS).
Sample Outputs:
Linux 2.6.18-128.1.14.el5 (www03.nixcraft.in)
06/26/2009
%nice %system %iowait
Blk_read/s
Blk_wrtn/s
=& Related: :
Linux Track NFS Directory / Disk I/O Stats
#8: sar - Collect and Report System Activity
The sar command is used to collect, report, and save system activity information. To see network counter, enter:
# sar -n DEV | more
To display the network counters from the 24th:
# sar -n DEV -f /var/log/sa/sa24 | more
You can also display real time usage using sar:
Sample Outputs:
Linux 2.6.18-128.1.14.el5 (www03.nixcraft.in)
06/26/2009
06:45:12 PM
06:45:16 PM
06:45:20 PM
06:45:24 PM
06:45:28 PM
06:45:32 PM
=& Related: :
How to collect Linux system utilization data into a file
#9: mpstat - Multiprocessor Usage
The mpstat command displays activities for each available processor, processor 0 being the first one. mpstat -P ALL to display average CPU utilization per processor:
# mpstat -P ALL
Sample Output:
Linux 2.6.18-128.1.14.el5 (www03.nixcraft.in)
06/26/2009
06:48:11 PM
%sys %iowait
06:48:11 PM
06:48:11 PM
06:48:11 PM
06:48:11 PM
06:48:11 PM
06:48:11 PM
06:48:11 PM
06:48:11 PM
06:48:11 PM
=& Related: :
Linux display each multiple SMP CPU processors utilization individually.
#10: pmap - Process Memory Usage
The command pmap report memory map of a process. Use this command to find out causes of memory bottlenecks.
# pmap -d PID
To display process memory information for pid # 47394, enter:
# pmap -d 47394
Sample Outputs:
/usr/bin/php-cgi
Kbytes Mode
2584 r-x-- 02 php-cgi
140 rw--- 02 php-cgi
52 rw--- a00
76 rw--- a02 php-cgi
1980 rw--- 00
112 r-x-- 02 ld-2.5.so
4 r---- b000 008:00002 ld-2.5.so
4 rw--- c000 008:00002 ld-2.5.so
1328 r-x-- 02 libc-2.5.so
2048 ----- c000 008:00002 libc-2.5.so
4 rw--- 02 xsl.so
40 r-x-- 02 libnss_files-2.5.so
2044 ----- a000 008:00002 libnss_files-2.5.so
4 r---- 02 libnss_files-2.5.so
4 rw--- a000 008:00002 libnss_files-2.5.so
0b1 rw-s- 09 zero (deleted)
00007fffc95fe000
84 rw--- 00007ffffffea000 000:00000
ffffffffff600000
8192 ----- 00
mapped: 933712K
writeable/private: 4304K
shared: 768000K
The last line is very important:
mapped: 933712K total amount of memory mapped to fileswriteable/private: 4304K the amount of private address spaceshared: 768000K the amount of address space this process is sharing with others
=& Related: :
Linux find the memory used by a program / process using pmap command
#11 and #12: netstat and ss - Network Statistics
The command netstat displays network connections, routing tables, interface statistics, masquerade connections, and multicast memberships. ss command is used to dump socket statistics. It allows showing information similar to netstat. See the following resources
about ss and netstat commands:
ss: Display Linux TCP / UDP Network and Socket InformationGet Detailed Information About Particular IP address Connections Using netstat Command
#13: iptraf - Real-time Network Statistics
The iptraf command is interactive colorful IP LAN monitor. It is an ncurses-based IP LAN monitor that generates various network statistics including TCP info, UDP counts, ICMP and OSPF information, Ethernet load info, node stats, IP checksum errors, and
others. It can provide the following info in easy to read format:
Network traffic statistics by TCP connectionIP traffic statistics by network interfaceNetwork traffic statistics by protocolNetwork traffic statistics by TCP/UDP port and by packet sizeNetwork traffic statistics by Layer2 address
Fig.02: General interface statistics: IP traffic statistics by network interface
Fig.03 Network traffic statistics by TCP connection
#14: tcpdump - Detailed Network Traffic Analysis
The tcpdump is simple command that dump traffic on a network. However, you need good understanding of TCP/IP protocol to utilize this tool. For.e.g to display traffic info about DNS, enter:
# tcpdump -i eth1 'udp port 53'
To display all IPv4 HTTP packets to and from port 80, i.e. print only packets that contain data, not, for example, SYN and FIN packets and ACK-only packets, enter:
# tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)&&2)) - ((tcp[12]&0xf0)&&2)) != 0)'
To display all FTP session to 202.54.1.5, enter:
# tcpdump -i eth1 'dst 202.54.1.5 and (port 21 or 20'
To display all HTTP session to 192.168.1.5:
# tcpdump -ni eth0 'dst 192.168.1.5 and tcp and port http'
wireshark to view detailed information about files, enter:
# tcpdump -n -i eth1 -s 0 -w output.txt src or dst port 80
#15: strace - System Calls
Trace system calls and signals. This is useful for debugging webserver and other server problems. See how to use to
trace the process and see What it is doing.
#16: /Proc file system - Various Kernel Statistics
/proc file system provides detailed information about various hardware devices and other Linux kernel information. See
Linux kernel /proc documentations for further details. Common /proc examples:
# cat /proc/cpuinfo
# cat /proc/meminfo
# cat /proc/zoneinfo
# cat /proc/mounts
17#: Nagios - Server And Network Monitoring
Nagios is a popular open source computer system and network monitoring application software. You can easily monitor all your hosts, network equipment and services. It can send alert when things go wrong
and again when they get better.
FAN is &Fully Automated Nagios&. FAN goals are to provide a Nagios installation including most tools provided by the Nagios Community. FAN provides a CDRom image in the standard ISO format, making it easy to easilly install a Nagios server. Added to this,
a wide bunch of tools are including to the distribution, in order to improve the user experience around Nagios.
18#: Cacti - Web-based Monitoring Tool
Cacti is a complete network graphing solution designed to harness the power of RRDTool's data storage and graphing functionality. Cacti provides a fast poller, advanced graph templating, multiple data acquisition methods, and user management features out
of the box. All of this is wrapped in an intuitive, easy to use interface that makes sense for LAN-sized installations up to complex networks with hundreds of devices. It can provide data about network, CPU, memory, logged in users, Apache, DNS servers and
much more. See how
to install and configure Cacti network graphing tool under CentOS / RHEL.
#19: KDE System Guard - Real-time Systems Reporting and Graphing
KSysguard is a network enabled task and system monitor application for KDE desktop. This tool can be run over ssh session. It provides lots of features such as a client/server architecture that enables monitoring of local and remote hosts. The graphical
front end uses so-called sensors to retrieve the information it displays. A sensor can return simple values or more complex information like tables. For each type of information, one or more displays are provided. Displays are organized in worksheets that
can be saved and loaded independently from each other. So, KSysguard is not only a simple task manager but also a very powerful tool to control large server farms.
Fig.05 KDE System Guard {Image credit: Wikipedia}
the KSysguard handbook for detailed usage.
#20: Gnome System Monitor - Real-time Systems Reporting and Graphing
The System Monitor application enables you to display basic system information and monitor system processes, usage of system resources, and file systems. You can also use System Monitor to modify the behavior of your system. Although not as powerful as the
KDE System Guard, it provides the basic information which may be useful for new users:
Displays various basic information about the computer's hardware and software.Linux Kernel versionGNOME versionHardwareInstalled memoryProcessors and speedsSystem StatusCurrently available disk spaceProcessesMemory and swap spaceNetwork usageFile SystemsLists all mounted filesystems along with basic information about each.
Fig.06 The Gnome System Monitor application
Bonus: Additional Tools
A few more tools:
nmap - scan your server for open ports.lsof - list open files, network connections and much more.ntop web based tool - ntop is the best tool to see network usage
in a way similar to what top command does for processes i.e. it is network traffic monitoring software. You can see network status, protocol wise distribution of traffic for UDP, TCP, DNS, HTTP and other protocols.Conky - Another good monitoring tool for the X Window System. It is highly configurable and is able to monitor many system variables including the status of the CPU, memory, swap space, disk storage,
temperatures, processes, network interfaces, battery power, system messages, e-mail inboxes etc.GKrellM - It can be used to monitor the status of CPUs, main memory, hard disks, network interfaces, local and remote mailboxes, and many other things.vnstat - vnStat is a console-based network traffic monitor. It keeps a log of hourly, daily and monthly network traffic for the
selected interface(s).htop - htop is an enhanced version of top, the interactive process viewer, which can display the list of processes in a tree form.mtr - mtr combines the functionality of the traceroute and ping programs in a single network diagnostic tool.
Did I miss something? Please add your favorite system motoring tool in the comments.
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