How to Check Memory Usage on CentOS/RHEL:
Linux has different set of commands to check the usage of memory. The free command shows the total amount of free and used physical and swap memory in the system, as well as the buffers used by the kernel. It is important to check memory usage so that resources do not fall short and users are able to access the server. Suppose a website is running form a webserver, then we require enough memory to serve the visitors to the site. If we have not enough memory then the site would become very slow or even go down when there is a traffic spike, simply because memory would fall short.
You can check memory usage is to read the /proc/meminfo file. The same file is used to know the free and other utilities report of free and used memory (both physical and swap) on the system.
# cat /proc/meminfo or # egrep --color 'Mem|Cache|Swap' /proc/meminfo
MemTotal: 7996284 kB MemFree: 5415608 kB Cached: 92416 kB SwapCached: 35924 kB SwapTotal: 8187836 kB SwapFree: 8059332 kB
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.
# free -m
total used free shared buffers cached Mem: 7808 2828 4980 6 37 100 -/+ buffers/cache: 2689 5119 Swap: 7995 124 7871
-b,-k,-m,-g: show output in bytes, KB, MB, or GB
-l: show detailed low and high memory statistics
-o: use old format (no -/+buffers/cache line)
-t: display total for RAM + swap
-s: update every [delay] seconds
-c: update [count] times
vmstat reports information about processes, memory, paging, block IO, traps, and cpu activity.
procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu----- r b swpd free buff cache si so bi bo in cs us sy id wa st 0 2 127504 4907616 57828 114252 0 0 327 507 24 22 3 0 82 15 0
The vmstat command with the s option, lays out the memory usage statistics much like the proc command.
# vmstat -s
7996284 total memory 3028732 used memory 2587708 active memory 253600 inactive memory 4967552 free memory 32212 buffer memory 97732 swap cache 8187836 total swap 127572 used swap 8060264 free swap 2629730 non-nice user cpu ticks 890 nice user cpu ticks 335618 system cpu ticks 80671997 idle cpu ticks 14269700 IO-wait cpu ticks 8 IRQ cpu ticks 12963 softirq cpu ticks 0 stolen cpu ticks 320259348 pages paged in 496267028 pages paged out 40038 pages swapped in 85154 pages swapped out 151875583 interrupts 278983792 CPU context switches 1438090342 boot time 300883 forks
The program atop is an interactive monitor to view the load on a Linux system. This program can display the amount of used and free memory, i.e. cpu, memory, disk and network.
It is similar to top, also allows you to see all the processes running on the system, with their full command lines.
The top command show a real-time view of a running system. It can also display system summary information as well as a list of tasks currently being managed by the Linux kernel.
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