Came across a good article regarding SWAP memory

http://linux-mm.org/Low_On_Memory

Negative GC observed in GC logs

Off late in our JEE Web application tests we observed Negative GC in the JBoss GC logs. After some reading we found that the clock of the Servers were not properly Synched up using Network  time Protocol. 

Possibly the time of the server had gone out of synch and NTP kicked in and corrected the time before GC finished. Note the values in the first column indicating the number of seconds from the JVM at which the GC ran. It also shows a similar trend. After 8175 seconds it shows 8137 seconds.

We are yet to validate if this server suffers from going out of synch by checking with the drift file generated.  Learnt that to enable drift file one an use this configuration driftfile /var/lib/ntp/ntp.drift in the ntp configuration file.