How to reduce cached memory?

[Joshua C.]

I have kde 4.2 and f9. After 2+ hours on my computer it has about
1,4GB cached memory. I don't see him releasing this memory after time.
I know it comes from the apps being open but it seems the kernel
"never" releases the memory after the app terminates. How to make the
kernel free this memory? I've seen similar cases with kde 3.5.10 and
f8 and f7. I know kde eats memory but this is too much. Who caches so
much of it? Why?

The mashine has 2,6 GB RAM (still x86) and 512mb swap which is rarely
used (~ 50mb at most) and combined cached + app memory totals about 2
gb.