> 
> I thought this was what LRU was all about; every time the sweep comes
> by, it decrements the usage count by 1, and when it hits zero -- or
> if the system's really busy, if it falls into the high-water mark
> region for usage count -- the page gets dropped from the free list.

You might have thought that, I did.  However I can't see a 'referenced'
count in the 'buf' data structure.   To me it looked like the 'free'
list was simply FIFO.

I did think of adding a count, but there is a complication with some
writes that seem to require the buf LRU list to be ordered.  (there
is a flag and pointer into the list...)
When (if) I work out what they are for I might try it.

    David