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Re: [PATCH 05/27] mm, vmscan: Have kswapd only scan based on the highest requested zone

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  Re: [PATCH 05/27] mm, vmscan: Have kswapd only scan based on the  highest requested zone Vlastimil Babka <vbabka@suse.cz> - 2016-06-15 15:20 +0200
    Re: [PATCH 05/27] mm, vmscan: Have kswapd only scan based on the  highest requested zone Mel Gorman <mgorman@techsingularity.net> - 2016-06-16 09:50 +0200

#1423005 — Re: [PATCH 05/27] mm, vmscan: Have kswapd only scan based on the highest requested zone

On 06/09/2016 08:04 PM, Mel Gorman wrote:
> kswapd checks all eligible zones to see if they need balancing even if it was
> woken for a lower zone. This made sense when we reclaimed on a per-zone basis
> because we wanted to shrink zones fairly so avoid age-inversion problems.

Now we reclaim a single lru, but still will skip over pages from the 
higher zones than reclaim_idx, so this is not much different from 
per-zone basis wrt age-inversion?

> Ideally this is completely unnecessary when reclaiming on a per-node basis.
> In theory, there may still be anomalies when all requests are for lower
> zones and very old pages are preserved in higher zones but this should be
> the exceptional case.
>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

I don't see the argument, but agree it should be exceptional in any 
case, and if there's such a case, it's better to focus on pages from the 
zone(s) where a pending (potentially atomic) allocation is restricted 
to. Or rather, this is the only way we can focus reclaim on such pages 
now that there's a single lru list.

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  mm/vmscan.c | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index ab1b28e7e20a..0a619241c576 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3171,11 +3171,8 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
>
>  		sc.nr_reclaimed = 0;
>
> -		/*
> -		 * Scan in the highmem->dma direction for the highest
> -		 * zone which needs scanning
> -		 */
> -		for (i = pgdat->nr_zones - 1; i >= 0; i--) {
> +		/* Scan from the highest requested zone to dma */
> +		for (i = classzone_idx; i >= 0; i--) {
>  			struct zone *zone = pgdat->node_zones + i;
>
>  			if (!populated_zone(zone))
>

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#1423753

On Wed, Jun 15, 2016 at 03:13:13PM +0200, Vlastimil Babka wrote:
> On 06/09/2016 08:04 PM, Mel Gorman wrote:
> >kswapd checks all eligible zones to see if they need balancing even if it was
> >woken for a lower zone. This made sense when we reclaimed on a per-zone basis
> >because we wanted to shrink zones fairly so avoid age-inversion problems.
> 
> Now we reclaim a single lru, but still will skip over pages from the higher
> zones than reclaim_idx, so this is not much different from per-zone basis
> wrt age-inversion?
> 

Yes, but it only applies in the case where the allocation request is zone
restricted. Previously, even with fair zone allocation policy, we had
problems with a high zone with recently allocated pages being reclaimed
simply because the low watermark was reached. Think of bugs in the past
where the normal zone was a small percentage of memory.

-- 
Mel Gorman
SUSE Labs

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