> The box is a 500MHz PIII with 128MB RAM and an IDE disk.  What I did was
> fire up a parallel compile to the point where the CPU idle time goes to
> 0%, while using X applications on the console.  Under NetBSD you can
> bearly notice the machine is under load.  Under Linux the machine becomes
> unbearably sluggish.  
> 
> I believe what happens under Linux is that since the buffer cache is
> competing with the VM system, application code and data pages are being
> reclaimed by the buffer cache and swapped out.

Your concern is worth considering, but there are other points that
Linux may be more sluggish than NetBSD. Since Linux has only two 
levels (cli and sti) to block bottom half, Linux may block more
events from interrupt than NetBSD under load.
Did you really compared paging activity between the Linux box and
the NetBSD box by vmstat or something?

I think this issue (limit memory usage of filesystem cache in UBC) is 
already discussed in this mailing list, and as far as I remember
correctly, Chuck has a plan about this.
--
soda