From: Mel Gorman <mgorman@suse.de>
To: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] mm: page allocator: Reconsider zones for allocation after direct reclaim
Date: Thu, 14 Jul 2011 07:10:49 +0100 [thread overview]
Message-ID: <20110714061049.GK7529@suse.de> (raw)
In-Reply-To: <4E1E6086.4060902@jp.fujitsu.com>
On Thu, Jul 14, 2011 at 12:20:38PM +0900, KOSAKI Motohiro wrote:
> (2011/07/13 20:10), Mel Gorman wrote:
> > On Wed, Jul 13, 2011 at 09:42:39AM +0900, KOSAKI Motohiro wrote:
> >> (2011/07/11 22:01), Mel Gorman wrote:
> >>> With zone_reclaim_mode enabled, it's possible for zones to be considered
> >>> full in the zonelist_cache so they are skipped in the future. If the
> >>> process enters direct reclaim, the ZLC may still consider zones to be
> >>> full even after reclaiming pages. Reconsider all zones for allocation
> >>> if direct reclaim returns successfully.
> >>>
> >>> Signed-off-by: Mel Gorman <mgorman@suse.de>
> >>
> >> Hmmm...
> >>
> >> I like the concept, but I'm worry about a corner case a bit.
> >>
> >> If users are using cpusets/mempolicy, direct reclaim don't scan all zones.
> >> Then, zlc_clear_zones_full() seems too aggressive operation.
> >
> > As the system is likely to be running slow if it is in direct reclaim
> > that the complexity of being careful about which zone was cleared was
> > not worth it.
> >
> >> Instead, couldn't we turn zlc->fullzones off from kswapd?
> >>
> >
> > Which zonelist should it clear (there are two) and when should it
> > happen? If it clears it on each cycle around balance_pgdat(), there
> > is no guarantee that it'll be cleared between when direct reclaim
> > finishes and an attempt is made to allocate.
>
> Hmm..
>
> Probably I'm now missing the point of this patch. Why do we need
> to guarantee tightly coupled zlc cache and direct reclaim?
Because direct reclaim may free enough memory such that the zlc cache
stating the zone is full is wrong.
> IIUC,
> zlc cache mean "to avoid free list touch if they have no free mem".
> So, any free page increasing point is acceptable good, I thought.
> In the other hand, direct reclaim finishing has no guarantee to
> zones of zonelist have enough free memory because it has bailing out logic.
>
It has no guarantee but there is a reasonable expectation that direct
reclaim will free some memory that means we should reconsider the
zone for allocation.
> So, I think we don't need to care zonelist, just kswapd turn off
> their own node.
>
I don't understand what you mean by this.
> And, just curious, If we will have a proper zlc clear point, why
> do we need to keep HZ timeout?
>
Yes because we are not guaranteed to call direct reclaim either. Memory
could be freed by a process exiting and I'd rather not add cost to
the free path to find and clear all zonelists referencing the zone the
page being freed belongs to.
--
Mel Gorman
SUSE Labs
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next prev parent reply other threads:[~2011-07-14 6:10 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-07-11 13:01 [RFC PATCH 0/3] Reduce frequency of stalls due to zone_reclaim() on NUMA Mel Gorman
2011-07-11 13:01 ` [PATCH 1/3] mm: vmscan: Do use use PF_SWAPWRITE from zone_reclaim Mel Gorman
2011-07-12 9:27 ` Minchan Kim
2011-07-12 9:40 ` KOSAKI Motohiro
2011-07-12 9:55 ` Minchan Kim
2011-07-12 15:43 ` Christoph Lameter
2011-07-13 10:40 ` Mel Gorman
2011-07-12 10:14 ` Mel Gorman
2011-07-13 0:34 ` KOSAKI Motohiro
2011-07-11 13:01 ` [PATCH 2/3] mm: page allocator: Initialise ZLC for first zone eligible for zone_reclaim Mel Gorman
2011-07-13 1:15 ` KOSAKI Motohiro
2011-07-13 11:02 ` Mel Gorman
2011-07-14 1:20 ` KOSAKI Motohiro
2011-07-14 6:11 ` Mel Gorman
2011-07-11 13:01 ` [PATCH 3/3] mm: page allocator: Reconsider zones for allocation after direct reclaim Mel Gorman
2011-07-13 0:42 ` KOSAKI Motohiro
2011-07-13 11:10 ` Mel Gorman
2011-07-14 3:20 ` KOSAKI Motohiro
2011-07-14 6:10 ` Mel Gorman [this message]
2011-07-21 9:35 ` KOSAKI Motohiro
2011-07-21 10:31 ` Mel Gorman
-- strict thread matches above, loose matches on Subject: below --
2011-07-15 14:59 [PATCH 0/2] Reduce frequency of stalls due to zone_reclaim() on NUMA v2 Mel Gorman
2011-07-15 14:59 ` [PATCH 3/3] mm: page allocator: Reconsider zones for allocation after direct reclaim Mel Gorman
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