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

$ free -m             total       used       free     shared    buffers     cachedMem:          7976       6459       1517          0        865       2248-/+ buffers/cache:       3344       4631Swap:         1951          0       1951

The 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

The next way to check memory usage is to read the /proc/meminfo file. Know that the /proc file system does not contain real files. They are rather virtual files that contain dynamic information about the kernel and the system.

$ cat /proc/meminfoMemTotal:        8167848 kBMemFree:         1409696 kBBuffers:          961452 kBCached:          2347236 kBSwapCached:            0 kBActive:          3124752 kBInactive:        2781308 kBActive(anon):    2603376 kBInactive(anon):   309056 kBActive(file):     521376 kBInactive(file):  2472252 kBUnevictable:        5864 kBMlocked:            5880 kBSwapTotal:       1998844 kBSwapFree:        1998844 kBDirty:              7180 kBWriteback:             0 kBAnonPages:       2603272 kBMapped:           788380 kBShmem:            311596 kBSlab:             200468 kBSReclaimable:     151760 kBSUnreclaim:        48708 kBKernelStack:        6488 kBPageTables:        78592 kBNFS_Unstable:          0 kBBounce:                0 kBWritebackTmp:          0 kBCommitLimit:     6082768 kBCommitted_AS:    9397536 kBVmallocTotal:   34359738367 kBVmallocUsed:      420204 kBVmallocChunk:   34359311104 kBHardwareCorrupted:     0 kBAnonHugePages:         0 kB                                                                                                                           HugePages_Total:       0HugePages_Free:        0HugePages_Rsvd:        0HugePages_Surp:        0Hugepagesize:       2048 kBDirectMap4k:       62464 kBDirectMap2M:     8316928 kB

Check the values of MemTotal, MemFree, Buffers, Cached, SwapTotal, SwapFree.
They indicate same values of memory usage as the free command.

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 - 15:20:30 up  6:57,  5 users,  load average: 0.64, 0.44, 0.33Tasks: 265 total,   1 running, 263 sleeping,   0 stopped,   1 zombie%Cpu(s):  7.8 us,  2.4 sy,  0.0 ni, 88.9 id,  0.9 wa,  0.0 hi,  0.0 si,  0.0 stKiB Mem:   8167848 total,  6642360 used,  1525488 free,  1026876 buffersKiB Swap:  1998844 total,        0 used,  1998844 free,  2138148 cached  PID USER      PR  NI  VIRT  RES  SHR S  %CPU %MEM    TIME+  COMMAND                                                                                  2986 enlighte  20   0  584m  42m  26m S  14.3  0.5   0:44.27 yakuake                                                                                  1305 root      20   0  448m  68m  39m S   5.0  0.9   3:33.98 Xorg                                                                                     7701 enlighte  20   0  424m  17m  10m S   4.0  0.2   0:00.12 kio_thumbnail

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.

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.

$ sudo dmidecode -t 17# dmidecode 2.11SMBIOS 2.4 present.Handle 0x0015, DMI type 17, 27 bytesMemory Device        Array Handle: 0x0014        Error Information Handle: Not Provided        Total Width: 64 bits        Data Width: 64 bits        Size: 2048 MB        Form Factor: DIMM        Set: None        Locator: J1MY        Bank Locator: CHAN A DIMM 0        Type: DDR2        Type Detail: Synchronous        Speed: 667 MHz        Manufacturer: 0xFF00000000000000        Serial Number: 0xFFFFFFFF        Asset Tag: Unknown        Part Number: 0x524D32474235383443412D36344643FFFFFF

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.

5 commands to check memory usage on Linux