共享内存就是进程之间可以共享的一段内存，通过一个唯一的KEY值绑定
shmget()创建或获取
shmat()连接
shmdt()断连
共享内存的访问涉及同步措施，通常与信号灯一起使用
用ipcs -m命令可以查看系统中已有的共享内存段
/proc/sys/kernel/shm开头的几个文件是共享内存的参数
共享内存的访问涉及同步措施，通常与信号灯一起使用
用ipcs -m命令可以查看系统中已有的共享内存段
ipcs -m 看Shared Memory 共享内存
ipcs -q 看Message Queues 消息队列
ipcs -s 看Semaphore Arrays 信号量
使用命令：ipcs -al
------ Shared Memory Limits --------
max number of segments = 4096
max seg size (kbytes) = 4091826
max total shared memory (pages) = 2097152
min seg size (bytes) = 1
by VIVEK GITE on APRIL 6, 2006 · 36 COMMENTS
How do I check used and free RAM memory usage under Linux operating systems using command line and GUI tools?
Linux comes with different set of commands to check memory usage. The free command displays the total amount of free and used physical and swap memory in the system, as well as the buffers used by the kernel. The vmstat command reports information about processes, memory, paging, block IO, traps, and cpu activity. Finally, you can use the top command which provides a dynamic real-time view of a running system. It can display system summary information as well as a list of tasks currently being managed by the Linux kernel.
Display free memory size in MB:
$ free -mOutput:
total used free shared buffers cached
Mem: 750 625 125 0 35 335
-/+ buffers/cache: 254 496
Swap: 956 0 956 Displays a line containing the totals memory in MB:
$ free -t -m
Output:
total used free shared buffers cached
Mem: 750 625 125 0 35 335
-/+ buffers/cache: 253 496
Swap: 956 0 956
Total: 1707 625 1082 Type vmstat command at shell prompt:
$ vmstat
Output:
procs -----------memory---------- ---swap-- -----io---- --system-- ----cpu----
r b swpd free buff cache si so bi bo in cs us sy id wa
1 0 0 131620 35432 341496 0 0 42 82 737 1364 15 3 81 1 Type top command at the shell prompt:
$ top
Sample outputs:
Fig.01: top command displaying used memory
The Gnome 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. You can start System Monitor by visiting System menu > Choose Administration > System Monitor option. Or type the following command at the shell prompt:
gnome-system-monitor
Sample outputs:
Fig.02: Linux See Memory Usage With GUI System Monitor Tool
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Shared memory is responsible for the smooth operation of your system; it acts as a space that multiple programs can access at once. This enables those programs to communicate in sync, while avoiding file redundancy. Shared memory can exist in the system's physical RAM or the swap file (which is virtual memory). In Linux operating systems, check the details of shared memory by typing in a few simple commands.
Power on your system and log in.
Open a "Terminal" window.
Type in "cat /proc/sys/kernel/shmall" and press "Enter." You'll see the total amount of shared memory available displayed in bytes. The max size of a shared memory segment is displayed with "cat /proc/sys/kernel/shmmax", and the total number of segments in the system is displayed with "cat /proc/sys/kernel/shmmni".
Read more: How to Check Shared Memory on Linux | eHow.com http://www.ehow.com/how_6849046_check-shared-memory-linux.html#ixzz19eUg5rtL
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McIDAS-X 200x uses shared memory for a variety of tasks including frame allocation. By default, the shared memory facility is activated and should have sufficient maximum segment size on HP, IBM and SGI systems. However, this is not true for Sun systems running Solaris 2.x or Digital systems running True64. On Linux systems, the amount of available shared memory appears is equivalent to the combination of physical RAM and swap.
The system administrator should complete the steps below the first time McIDAS-X 200x is installed on Sun Solaris or Digital True64 workstations or if they desire to increase shared memory defaults on HP, IBM, or SGI systems.
Sun Solaris 2.X
SGI IRIX 6.x
HP HP-UX 10, 11
IBM AIX
Compaq Tru64 UNIX
RedHat Linux
FreeBSD 4.[789]
MacOS-X 7.4.0
The system administrator must complete the steps below the first time McIDAS-X is installed on a Sun workstation running Solaris 2. x .
1. Force the kernel to activate the shared memory system at boot time by adding the line below to the appropriate location in the file
/etc/system .
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2. Check the maximum size of a shared memory segment using the command below. The value is indicated in bytes; the example lists a 512 MB maximum shared memory segment.
If your value is less than 512 MB, go to step 3. If your value is 512 MB or greater, go to step 4.
3. Set the shared memory size to 512 MB by adding the line below to the appropriate location in the file /etc/system .
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4. Reboot the system so the forceload entry and 512 MB maximum shared memory size take effect.
On Irix, SHMMAX defaults to 512 MB (0x20000000 bytes), so tuning this parameter is unlikely to be needed. SHMMAX is found in /var/sysgen/mtune/shm.
On HP-UX 10 and 11, the maximum value for SHMMAX is 1024 MB (0x40000000; the default value for 64 MB (0x04000000). SHMMAX is found in /etc/conf/master.d/core-hpux.
The procedure for changing the parameter is similar, if not identical, to the one described for HP-UX 9.
One way to check the current value of SHMMAX is to enter the following command:
Type: grep SHMMAX /etc/master
If the grep does not print anything, you are using the default value of 64 MB. If the grep prints something, the output might look like this:
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The value is bytes, so here the maximum segment size has been set to 512 MB.
The shmget(2) man page says that the maximum shared-memory segment size is 256 MB (0x10000000); this limit does not appear to be tunable.
The easiest way to increase shared-memory allocation on True64 is to use the GUI called dxkerneltuner, but one can also use the sysconfigdb command-line interface.
Running dxkerneltuner and increasing the shared-memory size to 64 MB will result in the following entry in the file /etc/sysconfigtab:
ipc:
shm-max=67108864
The following are the out-of-the-box defaults for the ipc subsystem for OSF/1 Version 4.0. They may be helpful for True64:
% sysconfig -q ipc
ipc:
msg-max = 8192
msg-mnb = 16384
msg-mni = 64
msg-tql = 40
shm-max = 4194304
shm-min = 1
shm-mni = 128
shm-seg = 32
sem-mni = 16
sem-msl = 25
sem-opm = 10
sem-ume = 10
sem-vmx = 32767
sem-aem = 16384
num-of-sems = 60
max-kernel-ports = 22487
port-hash-max-num = 1124350
port-reserved-max-num = 22487
set-max-num = 1029
ssm-threshold = 8388608
ssm-enable-core-dump = 1
There appears to be two ways of (re)defining the amount of shared memory in your system (tips thanks to Dave Glowacki of SSEC):
echo shared_memory_size > /proc/sys/kernel/shmmax
(where shared_memory_size is the amount of shared memory you want to declare in bytes) and reboot.
Note: since this will be the last thing run before your system comes up, your McIDAS session won't get a larger shared memory segment if it's started during the boot process).
Shared memory parameters are only read when the system goes from single user to multi user mode.
Add setting of shared memory max to /etc/sysctl.conf
# $FreeBSD: src/etc/sysctl.conf,v 1.1.2.3 2002/04/15 00:44:13 dougb Exp $
#
# This file is read when going to multi-user and its contents piped thru
# ``sysctl'' to adjust kernel values. ``man 5 sysctl.conf'' for details.
#
kern.ipc.shmmax=536870912
and then reboot.
In the Panther release of MacOS-X, shared memory is set in the file /etc/rc. The default settings will look something like:
# System tuning
sysctl -w kern.maxvnodes=$(echo $(sysctl -n hw.physmem) '33554432 / 512 * 1024 +p'|dc)
sysctl -w kern.sysv.shmmax=4194304
sysctl -w kern.sysv.shmmin=1
sysctl -w kern.sysv.shmmni=32
sysctl -w kern.sysv.shmseg=8
sysctl -w kern.sysv.shmall=1024
We recommend changing this to:
# System tuning
sysctl -w kern.maxvnodes=$(echo $(sysctl -n hw.physmem) '33554432 / 512 * 1024 +p'|dc)
sysctl -w kern.sysv.shmmax=536870912
sysctl -w kern.sysv.shmmin=1
sysctl -w kern.sysv.shmmni=4096
sysctl -w kern.sysv.shmseg=4096
sysctl -w kern.sysv.shmall=131072
After making these changes, the system will need to be rebooted for the changes to take effect.