How to monitor Linux server memory usage by using 

vmstat?

During performance testing monitoring, we are supposed to check how memory is being utilized for your Linux server, but how?
If you can use simple vmstat (have a look at attached SS). You may not get accurate used memory utilization, if you use vmstat –s will facilities this need but we need to iterate this to get memory usage during entire tenure of your performance tests.

Below small shell script will print the used memory along with time stamp.

for i in {1..120}
do
vmstat -s | sed "s/$/$(date)/" | awk '/used memory/'
sleep 30
done

Note: this will take the reading for every 30 seconds and  for 120 times.
If you want to print the output into a file use the below command.
./sampleloop.sh > UsedMemory1.txt
Where sampleloop.sh is your shell script name (copy past the above code in the file named as sampleloop.sh) and UsedMemory1.txt is the text file where you want to print the output.

Sample Output:
      1674660  used memoryWed Jun 18 09:52:46 UTC 2014
      1504104  used memoryWed Jun 18 09:53:16 UTC 2014
      1623140  used memoryWed Jun 18 09:53:46 UTC 2014
      1675956  used memoryWed Jun 18 09:54:16 UTC 2014
      1675624  used memoryWed Jun 18 09:54:46 UTC 2014
      1591800  used memoryWed Jun 18 09:55:16 UTC 2014
      1676364  used memoryWed Jun 18 09:55:46 UTC 2014
      1685188  used memoryWed Jun 18 09:56:16 UTC 2014
      1558236  used memoryWed Jun 18 09:56:46 UTC 2014