Re: [2.4] heavy-load under swap space shortage

[Andrea Arcangeli <andrea@suse.de>]

On Mon, Mar 15, 2004 at 05:41:54PM -0500, Rik van Riel wrote:
> On Mon, 15 Mar 2004, Andrea Arcangeli wrote:
> 
> > As I told Andrew, you've also to make sure not to start always from the
> > highmemzone, and from the code this seems not the case, so my 2G
> > scenario still applies.
> 
> Agreed, the scenario applies.  However, I don't see how a
> global LRU would fix it in eg. the case of an AMD64 NUMA
> system...

I think I mentioned the per-node lru would be enough for numa, I'm only
talking here about per-zone lru, per-node numa needs are another matter.
For 64bit per-node or per-zone is basically the same in practice.

however after 2.6.4 will be fixed even the per-zone should not generate
loss of caching info, so with that part fixed I'm not against per-zone
even if it's more difficult to be fair.

> And once we fix it right for those NUMA systems, we can
> use the same code to take care of balancing between zones
> on normal PCs, giving us the scalability benefits of the
> per-zone lists and locks.

I argue those scalability benefits of the locks, on a 32G machine or on
a 1G machine those locks benefits are near zero. The only significant
benefit is in terms of computational complexity of the normal-zone
allocations, where we'll only walk on the zone-normal and zone-dma
pages.

> How about AMD64 NUMA systems ?
> What evens out the LRU pressure there in 2.4 ?

by the time you say 64bit you can forget the per-zone per-node
differences.  sure there will be still a difference but it's cosmetical
so I don't care about those per-zone lru issues for 64bit hardware,
infact on 64bit hardware per-zone (even if totally unfair) is the most
optimal just in case somebody asks for ZONE_DMA more than once per day.
But the difference is so small in practice that even global would be ok.

the per-node on numa (not necessairly on amd64, infact in amd64 the
penality is so small that I doubt things like that will payoff big)
still remains but that's not the thing I was discussing here.