Command organises ps output by rss
RSS stands for Resident Set Size
This is a actual number in kilobytes of how much RAM the current process is using.

ps -Fe --sort:-rss

ps -Fe --sort:-rss | head -11

Find the ram usage of a specific service:

ps --no-headers -o "rss,cmd" -C httpd | awk '{ sum+=$1 }
END { printf ("\nRAM statistics\n--------------\n") }
END { printf ("Total RAM:           %d%s\n", sum/1024, "M") }
END { printf ("Total processes:     %d\n", NR) }
END { printf ("Average RAM/process: %d%s\n", sum/NR/1024, "M\n") }'

Description

-e = select all processes
-F = full format
--sort:-rss = sort the results by resident set size (real memory size in bytes)

Once you have the output of the command you will need to investigate the processes 'State'