How do I find out Linux System Memory Utilization?

Q. How do I find out System / Server Memory Utilization under RHEL / CentOS / any other Linux distribution?

A. You need to use free command which, 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 command example

Type the free command at shell prompt:
$ free
$ free -m
Output:

total used free shared buffers cached
Mem: 2010 1965 45 0 152 776
-/+ buffers/cache: 1036 974
Swap: 2047 137 1910

vmstat command

vmstat command provides more information :
$ vmstat
Output:

procs ———–memory———- —swap– —–io—- –system– —–cpu——
r b swpd free buff cache si so bi bo in cs us sy id wa st
0 0 140480 43636 158196 797692 1 0 108 220 1 4 7 5 87 1 0

Understanding vmstat memory options

* swpd: the amount of virtual memory used.
* free: the amount of idle memory.
* buff: the amount of memory used as buffers.
* cache: the amount of memory used as cache.
* inact: the amount of inactive memory. (-a option)
* active: the amount of active memory. (-a option)

$ vmstat -a
Output:

procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu------
r b swpd free inact active si so bi bo in cs us sy id wa st
0 1 140480 37376 109516 1730040 1 0 108 220 1 4 7 5 87 1 0

The following command displays one new line of utilization data every second
$ vmstat 1
The following command displays one new line per 2 second, but only for the next 10 seconds:
$ vmstat 2 10
Output:

procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu------
r b swpd free inact active si so bi bo in cs us sy id wa st
4 0 139216 23508 130644 1723680 1 0 108 220 1 5 7 5 87 1 0
2 0 139216 23252 130668 1723816 0 0 0 410 3242 11472 9 7 84 0 0
1 0 139216 23120 130656 1724012 0 0 0 750 3280 11592 3 6 90 1 0
0 0 139216 22996 130588 1724180 0 0 0 426 3272 11052 2 5 93 0 0
2 0 139216 20988 129932 1726980 0 0 6 1146 3353 12105 14 9 74 2 0
1 0 139216 20244 129900 1727216 0 0 0 392 3238 11752 8 7 85 0 0
1 0 139216 20120 129868 1727352 0 0 0 444 3197 11173 2 5 93 0 0
1 0 139216 25964 129852 1721044 0 0 0 268 3147 9269 1 4 95 0 0
3 0 139216 25964 129748 1721196 0 0 2 132 3199 10540 1 4 95 0 0
1 0 139216 25964 129676 1721332 0 0 0 456 3213 10608 2