Will swap useage auto cleanup?

[lhmwzy]

I just have a question
When swap space is used more and more, will the system automatically free
up the used the swap space as time go on?
If yes,when and how.

top show as fllowing:

Memory: 645M Active, 768M Inact, 378M Wired, 13M Cache, 211M Buf, 153M Free
Swap: 4096M Total, 42M Used, 4054M Free, 1% Inuse

the swap used space is growing larger and larger.
Server days go on and could not be freed up.

sorry for my pool english,I hope you can understand what I have said.
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