RAM usage in Linux

1 . using command top

prayag@Prayag:~$ top | grep java
 PID USER      PR  NI  VIRT  RES  SHR S  %CPU %MEM    TIME+  COMMAND
 4216 prayag    20   0  429m 139m 8808 S   1.0  7.9   1:04.51 java                                                                                     
 4216 prayag    20   0  429m 139m 8808 S   0.3  7.9   1:04.52 java                                                                                     
 4216 prayag    20   0  429m 139m 8808 S   0.7  7.9   1:04.54 java                                                                                     
 4216 prayag    20   0  429m 139m 8808 S   0.3  7.9   1:04.55 java                                                                                     
 4216 prayag    20   0  429m 139m 8808 S   0.7  7.9   1:04.57 java

 6035 prayag    20   0  406m 107m 7812 S   1.0  6.1   1:02.63 java                                                                                    
 7249 prayag    20   0  857m 223m 7436 S   0.3 12.8   0:28.04 java                                                                                    
 7249 prayag    20   0  857m 223m 7436 S   0.3 12.8   0:28.05 java

#type h for help, q to quit

RES   = RESident Size (how much of your program is in actual, physical RAM.)
VIRT = VIRTual Memory Footprint (how much virtual memory, RAM + Swap, your program has allocated).
In my case, Java eats 429 MB - 857 MB.

2. free --help

prayag@Prayag:~$ free --mega
             total       used       free     shared    buffers     cached
Mem:          1754       1563        190          0          8        301
-/+ buffers/cache:       1253        500
Swap:         2859        867       1992

3. vmstat --help

prayag@Prayag:~$ vmstat
procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
 1  0 887604 192400   8996 310300   77  111   311   181  211 1096  8  2 82  8

References
Tomcat VIRT memory usage. 2012. http://serverfault.com/a/296721/113713

Why (on Linux) am I seeing so much RAM usage?. 2013.
http://www.chrisjohnston.org/ubuntu/why-on-linux-am-i-seeing-so-much-ram-usage