Linux vmstat command used to check memory usage on linux .

# vmstat
procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 4  0  42116 15386132 1304192 18180176    0    0     4    22    0    1  2  1 97  0  0	

r column

r: The number of processes waiting for run time or placed in run queue or are already executing (running)

Whenever the value of the ‘ r ‘ exceeds the number of CPUs on the server, tasks are forced to wait for execution.

To find number of CPU cores on a server, run

cat /proc/cpuinfo | grep processor | wc -l

b column

b: The number of processes in uninterruptible sleep. (b=blocked queue, waiting for resource (e.g. filesystem I/O blocked, inode lock))

si/so column

This is for data written to swap.

if server is using swap, this is because too many process running or server under load. This is not good, on a healthy server, value of this column should be 0.

bi/bo column

This is Block Input/Output, that is data written and read from Hard disk.

The bi (bytes in) and bo (bytes out) column show disk I/O (including swapping memory to/from disk) on your server.

CPU Usage: (us sy id)

The us (user), sy (system) and id (idle) show the amount of CPU your server is using. The higher the idle value, the better.