Re: too big min_free_kbytes

[Andrea Arcangeli <aarcange@redhat.com>]

On Thu, Jan 27, 2011 at 01:40:58PM +0000, Mel Gorman wrote:
> On Wed, Jan 26, 2011 at 05:42:37PM +0000, Mel Gorman wrote:
> > On Wed, Jan 26, 2011 at 04:36:55PM +0000, Mel Gorman wrote:
> > > > But the wmarks don't
> > > > seem the real offender, maybe it's something related to the tiny pci32
> > > > zone that materialize on 4g systems that relocate some little memory
> > > > over 4g to make space for the pci32 mmio. I didn't yet finish to debug
> > > > it.
> > > > 
> > > 
> > > This has to be it. What I think is happening is that we're in balance_pgdat(),
> > > the "Normal" zone is never hitting the watermark and we constantly call
> > > "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 <aarcange@redhat.com>

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 <aarcange@redhat.com>
---


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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