[转载]COMMANDS TO CHECK MEMORY USAGE ON LINUX VIA SSH
MEMORY USAGE
On linux, there are commands for almost everything, because the gui might not be always available. When working on servers only shell access is available and everything has to be done from these commands. So today we shall be checking the commands that can be used to check memory usage on a linux system. Memory include RAM and swap.
It is often important to check memory usage and memory used per process on servers so that resources do not fall short and users are able to access the server. For example a website. If you are running a webserver, then the server must have enough memory to serve the visitors to the site. If not, the site would become very slow or even go down when there is a traffic spike, simply because memory would fall short. Its just like what happens on your desktop PC.
1. ‘FREE’ COMMAND
The free command is the most simple and easy to use command to check memory usage on linux. Here is a quick example:
Check RAM Usage
The m (-m) option displays all data in MBs. The total os 7976 MB is the total amount of RAM installed on the system, that is 8GB. The used column shows the amount of RAM that has been used by linux, in this case around 6.4 GB. The output is pretty self explanatory. The catch over here is the cached and buffers column. The second line tells that 4.6 GB is free. This is the free memory in first line added with the buffers and cached amount of memory.
Linux has the habit of caching lots of things for faster performance, so that memory can be freed and used if needed. The last line is the swap memory, which in this case is lying entirely free.
2. /PROC/MEMINFO
Please read more here: http://www.redhat.com/advice/tips/meminfo.html
3. VMSTAT
The vmstat command with the s option, lays out the memory usage statistics much like the proc command. Here is an example:
$ vmstat -s
8167848 K total memory
7449376 K used memory
3423872 K active memory
3140312 K inactive memory
718472 K free memory
1154464 K buffer memory
2422876 K swap cache
1998844 K total swap
0 K used swap
1998844 K free swap
392650 non-nice user cpu ticks
8073 nice user cpu ticks
83959 system cpu ticks
10448341 idle cpu ticks
91904 IO-wait cpu ticks
0 IRQ cpu ticks
2189 softirq cpu ticks
0 stolen cpu ticks
2042603 pages paged in
2614057 pages paged out
0 pages swapped in
0 pages swapped out
42301605 interrupts
94581566 CPU context switches
1382755972 boot time
8567 forks
$
The top few lines indicate total memory, free memory etc and so on.
4. ‘TOP’ COMMAND
The top command is generally used to check memory and cpu usage per process. However it also reports total memory usage and can be used to monitor the total RAM usage. The header on output has the required information. Here is a sample output:
Top Command
Check the KiB Mem and KiB Swap lines on the header. They indicate total, used and free amounts of the memory. The buffer and cache information is present here too, like the free command.
5. HTOP
Similar to the top command, the htop command also shows memory usage along with various other details.
Htop Memory Ram Usage
The header on top shows cpu usage along with RAM and swap usage with the corresponding figures.
RAM INFORMATION
To find out hardware information about the installed RAM, use the demidecode command. It reports lots of information about the installed RAM memory.
RAM Information
Provided information includes the size (2048MB), type (DDR2) , speed(667 Mhz) etc.
SUMMARY
All the above mentioned commands work from the terminal and do not have a gui. When working on a desktop with a gui, it is much easier to use a GUI tool with graphical output. The most common tools are gnome-system-monitor on gnome and ksysguard on KDE. Both provide resource usage information about cpu, ram, swap and network bandwidth in a graphical and easy to understand visual output.
