From time to time machines use some swap space due to heavy load. But Linux doesn't free the swap space itself if enough memory is available.

An easy method to flush all disk swap to memory is to disable and re-enable the swap. Eg:

# free -tm
             total       used       free     shared    buffers     cached
Mem:          4096       3269        826          0         31         76
-/+ buffers/cache:       3161        934
Swap:          509        114        395
Total:        4605       3384       1221

# swapoff /dev/xvdb1
# swapon /dev/xvdb1

# free -tm
             total       used       free     shared    buffers     cached
Mem:          4096       3375        720          0         31         78
-/+ buffers/cache:       3265        830
Swap:          509          0        509
Total:        4605       3375       1230