Memory purge

When InDesignServer (CS3) is continuously running for a couple of days on our test server, we get the following output messages: (every few minutes)

WARN  [server] Memory Purge: 889 MB available, 670 MB physical, 244 MB available physical bytes

From this point IDServer is also very slow.

Can someone explain? Does it mean that there's not enough memory available? It seems odd that this can happen because every script always closes the InDesign files before completion..

Thanks in advance.