Couchbase swapping with free ram

Running couchbase cluster 2.2.0 on Centos 6. It is the only application running on the server.

[root@tracing-1 ~]# free -m
total used free shared buffers cached
Mem: 24152 23890 262 0 10 6281
-/+ buffers/cache: 17598 6554
Swap: 4095 2178 1917

I have 80% of available ram dedicated to couchbase, leaving 4GB free for OS, etc.
Even with the 4GB buffer, the box is still swapping…with 6.5GB of free memory, any idea why that would happen and how do I resolve this?

Please try to tune swappiness property.