On Thu, Jan 27, 2011 at 01:40:58PM +0000, Mel Gorman wrote:

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On Wed, Jan 26, 2011 at 05:42:37PM +0000, Mel Gorman wrote:

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On Wed, Jan 26, 2011 at 04:36:55PM +0000, Mel Gorman wrote:

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But the wmarks don't
seem the real offender, maybe it's something related to the tiny

pci32

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zone that materialize on 4g systems that relocate some little

memory

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over 4g to make space for the pci32 mmio. I didn't yet finish to

debug

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it.

This has to be it. What I think is happening is that we're in

balance_pgdat(),

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the "Normal" zone is never hitting the watermark and we constantly

call

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"goto loop_again" trying to "rebalance" all zones.

Confirmed.
<SNIP>

How about the following? Functionally it would work but I am concerned
that the logic in balance_pgdat() and kswapd() is getting out of hand
having being adjusted to work with a number of corner cases already. In
the next cycle, it could do with a "do-over" attempt to make it easier
to follow.

That number 8 is the problem, I don't think anybody was ever supposed
to free 8*highwmark pages. kswapd must work in the hysteresis range
low->high area and then sleep wait low to hit again before it gets
wakenup. Not sure how that number 8 ever come up... but to be it looks
like the real offender and I wouldn't work around it.

totally untested... I will test....

====
Subject: vmscan: kswapd must not free more than high_wmark pages

From: Andrea Arcangeli <redacted>

When the min_free_kbytes is set with `hugeadm
--set-recommended-min_free_kbytes" or with THP enabled (which runs the
equivalent of "hugeadm --set-recommended-min_free_kbytes" to activate
anti-frag at full effectiveness automatically at boot) the high wmark
of some zone is as high as ~88M. 88M free on a 4G system isn't
horrible, but 88M*8 = 704M free on a 4G system is definitely
unbearable. This only tends to be visible on 4G systems with tiny
over-4g zone where kswapd insists to reach the high wmark on the
over-4g zone but doing so it shrunk up to 704M from the normal zone by
mistake.

Signed-off-by: Andrea Arcangeli <redacted>
---

diff --git a/mm/vmscan.c b/mm/vmscan.c
index f5d90de..9e3c78e 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c

@@ -2407,7 +2407,7 @@ loop_again:
                        * zone has way too many pages free already.
                        */
                        if (!zone_watermark_ok_safe(zone, order,
-                                       8*high_wmark_pages(zone), end_zone,

0))
+                                       high_wmark_pages(zone), end_zone,
0))
                                shrink_zone(priority, zone, &sc);
                       reclaim_state->reclaimed_slab = 0;
                       nr_slab = shrink_slab(sc.nr_scanned, GFP_KERNEL,


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