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SubjectRe: [PATCH 02/12] mm: supporting variables and functions for balanced zone aging
Wu Fengguang <wfg@mail.ustc.edu.cn> wrote:
>
> 865 if (sc->nr_to_reclaim <= 0)
> 866 break;
> 867 }
> 868 }
>
> Line 843 is the core of the scan balancing logic:
>
> priority 12 11 10
>
> On each call nr_scan_inactive is increased by:
> DMA(2k pages) +1 +2 +3
> Normal(64k pages) +17 +33 +65
>
> Round it up to SWAP_CLUSTER_MAX=32, we get (scan batches/accumulate rounds):
> DMA 1/32 1/16 2/11
> Normal 2/2 2/1 3/1
> DMA:Normal ratio 1:32 1:32 2:33
>
> This keeps the scan rate roughly balanced(i.e. 1:32) in low vm pressure.
>
> But lines 865-866 together with line 846 make most shrink_zone() invocations
> only run one batch of scan.

Yes, this seems to be the problem. Sigh. By the time 2.6.8 came around I
just didn't have time to do the amount of testing which any page reclaim
tweak necessitates.



From: Andrew Morton <akpm@osdl.org>

Revert a patch which went into 2.6.8-rc1. The changelog for that patch was:

The shrink_zone() logic can, under some circumstances, cause far too many
pages to be reclaimed. Say, we're scanning at high priority and suddenly
hit a large number of reclaimable pages on the LRU.

Change things so we bale out when SWAP_CLUSTER_MAX pages have been
reclaimed.

Problem is, this change caused significant imbalance in inter-zone scan
balancing by truncating scans of larger zones.

Suppose, for example, ZONE_HIGHMEM is 10x the size of ZONE_NORMAL. The zone
balancing algorithm would require that if we're scanning 100 pages of
ZONE_HIGHMEM, we should scan 10 pages of ZONE_NORMAL. But this logic will
cause the scanning of ZONE_HIGHMEM to bale out after only 32 pages are
reclaimed. Thus effectively causing smaller zones to be scanned relatively
harder than large ones.

Now I need to remember what the workload was which caused me to write this
patch originally, then fix it up in a different way...

Signed-off-by: Andrew Morton <akpm@osdl.org>
---
mm/vmscan.c | 8 --------
1 files changed, 8 deletions(-)
diff -puN mm/vmscan.c~vmscan-balancing-fix mm/vmscan.c
--- devel/mm/vmscan.c~vmscan-balancing-fix 2005-12-01 21:20:44.000000000 -0800
+++ devel-akpm/mm/vmscan.c 2005-12-01 21:21:38.000000000 -0800
@@ -63,9 +63,6 @@ struct scan_control {

unsigned long nr_mapped; /* From page_state */

- /* How many pages shrink_cache() should reclaim */
- int nr_to_reclaim;
-
/* Ask shrink_caches, or shrink_zone to scan at this priority */
unsigned int priority;

@@ -901,7 +898,6 @@ static void shrink_cache(struct zone *zo
if (current_is_kswapd())
mod_page_state(kswapd_steal, nr_freed);
mod_page_state_zone(zone, pgsteal, nr_freed);
- sc->nr_to_reclaim -= nr_freed;

spin_lock_irq(&zone->lru_lock);
/*
@@ -1101,8 +1097,6 @@ shrink_zone(struct zone *zone, struct sc
else
nr_inactive = 0;

- sc->nr_to_reclaim = sc->swap_cluster_max;
-
while (nr_active || nr_inactive) {
if (nr_active) {
sc->nr_to_scan = min(nr_active,
@@ -1116,8 +1110,6 @@ shrink_zone(struct zone *zone, struct sc
(unsigned long)sc->swap_cluster_max);
nr_inactive -= sc->nr_to_scan;
shrink_cache(zone, sc);
- if (sc->nr_to_reclaim <= 0)
- break;
}
}

_
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