So I've experimented with a somewhat orthogonal change, which is a bit
simpler and is intended only to restore the `two-handed clock
algorithm' behavior from Mach.  To review:

* One hand sweeps through the active pages and `deactivates' them.

* The other hand sweeps through the inactive pages, and either
  reactivates them, frees them, or starts paging them out.

Due to the UVM behavior of revoking mappings when a page is
deactivated, the reactivate case currently happens by simply faulting
the page again.  As I said before, this is suboptimal, but it does
work.

So, the problem this change addresses is only to not deactivate a page
if it has been recently used.

This appears to improve behavior *significantly* when reading large
volumes of data -- or, more precisely, when the read rate much higher
than the write rate -- including reading (but not copying) data off a
CD or DVD, or compiling.

It does *not* completely fix the case where massive amounts of data
are being written, though.  E.g.:

	dd if=/dev/zero of=foo bs=64k
	cat title-key /dvd/video_ts/vts_01_?.vob | efdtt >movie.vob
	cdd -a

In these cases, it appears that the data to be written backs up very
badly and causes process (e.g. the X server and my Emacs) to lose
pages.  Some sort of flow control is badly needed to repair this.


Index: uvm_page_i.h
===================================================================
RCS file: /cvsroot/syssrc/sys/uvm/uvm_page_i.h,v
retrieving revision 1.14
diff -c -2 -r1.14 uvm_page_i.h
*** uvm_page_i.h	2000/11/27 07:47:42	1.14
--- uvm_page_i.h	2001/01/25 03:18:40
***************
*** 218,222 ****
  		pg->pqflags |= PQ_INACTIVE;
  		uvmexp.inactive++;
- 		pmap_clear_reference(pg);
  		if (pmap_is_modified(pg))
  			pg->flags &= ~PG_CLEAN;
--- 218,221 ----
Index: uvm_pdaemon.c
===================================================================
RCS file: /cvsroot/syssrc/sys/uvm/uvm_pdaemon.c,v
retrieving revision 1.27
diff -c -2 -r1.27 uvm_pdaemon.c
*** uvm_pdaemon.c	2001/01/25 00:10:03	1.27
--- uvm_pdaemon.c	2001/01/25 03:18:40
***************
*** 1100,1107 ****
  		/*
  		 * deactivate this page if there's a shortage of
! 		 * inactive pages.
  		 */
  
! 		if (inactive_shortage > 0) {
  			pmap_page_protect(p, VM_PROT_NONE);
  			/* no need to check wire_count as pg is "active" */
--- 1100,1108 ----
  		/*
  		 * deactivate this page if there's a shortage of
! 		 * inactive pages and it hasn't been touched since the
! 		 * last scan.
  		 */
  
! 		if (inactive_shortage > 0 && !pmap_clear_reference(p)) {
  			pmap_page_protect(p, VM_PROT_NONE);
  			/* no need to check wire_count as pg is "active" */