VM performance on 64Mb in pre9-4/5

Hi Juan,
Thanks for all the tuning! pre9-4 definately performs better. It is
almost as good as pre7-4 at swapping, and probably better at other things. Very usable :)

	I think pre7-6 broke (compared to pre7-4) because swap_cnt==RSS, or

RSS==0. This means that swap pages are ONLY taken from processes with large RSS. Pre9-4 is fixed because it decrements swap_cnt even if pte_young(pte) is true. More on this later.

Hard data:
Yes, xfs-xtt, and other damons are swapped out. They EVENTUALLY reach
an RSS of 4k, but it takes a long time (e.g. 4 hours of use) before they reach this level, and other processes like netscape are swapped out in the meantime. However, netscape, X, emacs are also partially swapped out. I must admit though, that many of the pages swapped out of these processes stay out.

Also, this kernel does a much better job at swapping ENOUGH pages out:
telomere:~> free
total used free shared buffers
cached
Mem: 62740 53424 9316 0 412
25348

-/+ buffers/cache:      27664      35076
Swap:       128484      36400      92084

OK, back to swap_cnt. I finally read the code and looked at the
swap-out algorithm. 2 things struck me (which I should have known):
1) in pre-8, swap-out is called on PROCESSES, not PAGES. swap_cnt is
used to guess which processes have lots of freeable pages. However, in pre-8, swap_cnt == RSS or swap_cnt==0. This means that many processes will rarely have swap_out_mm called on them.
2) try_to_swap_out does important things besides actually swapping out,
so if it is not working we have problems even on machines that don't use swap. For example, with pre9-3, I had almost 60MB of used memory running only gnome and X! That is crazy. But it is not surprising if swap_out_mm is never called on small processes.

	If swap_cnt==RSS, then swap_cnt is fundamentally broken.  In that case,
we don't need this line in swap_out_pmd:
		if (!mm->swap_cnt)
			return 0;
	But in pre9-4, swap_cnt is decreased when pages are scanned, even if

but nothing is swapped out. This means that if a large process, like netscape, has no unused pages, other process can still have a turn in the swap_out code, because netscape's swap_cnt will go down even if its RSS is high.

	In fact I wonder what would happen if you tried this...

	if (pte_young(pte)) {
	   if (mm->swap_cnt)
	     mm->swap_cnt--;
	   ...
	}
	if (PageReferenced(page))
		mm->swap_cnt--;

	This will encourage the VM subsystem not to spend all its time scanning

processes that should really be in memory. I'm not really sure that it would be a good idea, but what is certain is that a better algorithm for swap_cnt is desperately needed. Right now there is (I think) no mechanism to ensure that all process get turns in swap_out_mm.

OK, I hope this is helpful!
-BenRI

"I want to be in the light, as He is in the Light,
 I want to shine like the stars in the heavens." - DC Talk, "In the
Light"
Benjamin Redelings I      <><     http://www.bol.ucla.edu/~bredelin/
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