Re: [RFC] 2.3.39 zone balancing

[Linus Torvalds <torvalds@transmeta.com>]

On Thu, 13 Jan 2000, Alan Cox wrote:
>
> > doc. If you have a large number of free regular pages, and the dma
> > zone is completely exhausted, the 2.2 decision of balacing the dma
> > zone might never fetch an "yes" answer, because it is based on total
> > number of free pages, not also the per zone free pages. Right? Things 
> > will get worse the more non-dma pages there are.
> 
> We might not make good choices to free ISA DMA pages, you are correct yes

What I think needs to happen is something like
 - global page table aging logic (it would be surreal to try to age the
   page table entries on a per-zone basis, because page tables do not have
   zones)
 - per-zone page freeing logic

Right now we do neither. Out page table aging thing (swap_out()) looks at
the zone (which I don't think it should), while our shrink_mmap() is often
completely zone-unaware (ie kswapd uses a NULL zone).

The reason swap_out() looks at the zone is that a long time ago the logic
was that you should avoid swapping normal pages out if you really only
needed DMA pages. I think that logic is broken in the larger picture (when
there are multiple kinds of zones), and is unnecessary even in the old
sense, because these days the swap cache works just fine for us, and
should make the impact of "wrong zone" swapouts be insignificant.

So, I'd like somebody to _try_ to (a) rip out the zone-awareness from
swap_out() completely and (b) make kswapd do something more like

	more_work = 0;
	for (i = 0; i < NR_ZONES; i++) {
		more_work |= balance_zone(zone+i)
	}		
	if (!more_work)
		sleep()

where "balance_zone()" would really be a per-zone "shrink_mmap()" with the
free page logic taken into account.

Sounds reasonable?

		Linus

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