Often memory pages are paged out to swap space when the system requires 
more memory than is currently available. However, this is often needed 
for just a short while. However, the pages that have been paged out 
remain in swap until they're used again, while there are loads of free 
memory and the system is idle. This appears to be the default behaviour 
on any Un*x system I've seen or used.

This does not seem logical to me. When the pages in swap are required 
again, there is a huge performance penalty when paging them back in. 
Wouldn't it be better to page swap pages back in during idle time if 
the amount of free memory permits that? I do see one disadvantage, it 
may be required to page them back out before the pages are used again, 
but it may be possible to reduce that disadvantage with a timeout

Are there perhaps disadvantages in this approach that I have not 
foreseen? Has it ever been considered? It seems better to me.

-- 
Best Regards,

Reinier Jonker