Take the 2-minute tour ×  I have something around 200 GB worth
data stored in a mongo cluster. The physical memory on one of
the instances which runs mongo is 8GB. Nothing else of any
consequence runs on this instance. As near as I can understand
based on Mongo's docs (like this one: http://www.mongodb.org/dis
play/DOCS/Checking+Server+Memory+Usage) this means that the
mongod process should be using about 100% of the available
physical memory. But if you look at the following output from
the top command you'll see that the mongod instance is only
using 2GB of resident memory and theres a full 2GB of free
physical memory available which isn't being used at all.  Can
someone explain this behavior to me? Why is there 2GB of free
memory?  top output:  top - 23:19:43 up 89 days, 20:05,  2
users,  load average: 0.41, 0.55, 0.59 Tasks: 101 total,   1
running, 100 sleeping,   0 stopped,   0 zombie Cpu(s):  2.0%us,
1.3%sy,  0.0%ni, 93.9%id,  2.6%wa,  0.0%hi,  0.1%si,  0.0%st
Mem:   8163664k total,  6131764k used,  203