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

[Andrea Arcangeli <andrea@suse.de>]

On Tue, Mar 16, 2004 at 09:05:32AM +1100, Nick Piggin wrote:
> 
> What I meant by unfairness is that low zone scanning in response
> to low zone pressure will not put any pressure on higher zones.
> Thus pages in higher zones have an advantage.

Ok I see what you mean now, in this sense the unfariness is the same
with the global lru too.

> We do scan lowmem in response to highmem pressure.

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.

Obviously I expected that you would can the lowmem zones too, otherwise
you couldn't allocate in cache more than 100M or so in a 1G box.

shrink_caches(struct zone **zones, int priority, int *total_scanned,
		int gfp_mask, int nr_pages, struct page_state *ps)
{
	int ret = 0;
	int i;

	for (i = 0; zones[i] != NULL; i++) {
		int to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX);
		struct zone *zone = zones[i];
		int nr_mapped = 0;
		int max_scan;

you seem to start always from zones[0] (that zone ** thing is the
zonelist so it starts with highmem, the normal then dma, depending on
the classzone you're shrinking). That will generate the waste of cache
in a 2G box that I described.

I'm reading 2.4.4 mainline here.

to really fix it, you need a global information keeping track of the
last zone shrinked to keep going in round robin.

Either that or you can choose to do some overwork and to shrink from all
the zones removing this break:

		if (ret >= nr_pages)
			break;

but as far as I can tell, the 50% waste of cache in a 2G box can happen
in 2.6.4 and it won't happen in 2.4.x.