Message-ID: <20110721162417.GF1713@barrios-desktop>
Date:	Fri, 22 Jul 2011 01:24:17 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	P?draig Brady <P@...igBrady.com>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Colin King <colin.king@...onical.com>,
	Andrew Lutomirski <luto@....edu>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/4] Stop kswapd consuming 100% CPU when highest zone is
 small

On Thu, Jul 21, 2011 at 05:09:59PM +0100, Mel Gorman wrote:
> On Fri, Jul 22, 2011 at 12:37:22AM +0900, Minchan Kim wrote:
> > On Fri, Jun 24, 2011 at 03:44:53PM +0100, Mel Gorman wrote:
> > > (Built this time and passed a basic sniff-test.)
> > > 
> > > During allocator-intensive workloads, kswapd will be woken frequently
> > > causing free memory to oscillate between the high and min watermark.
> > > This is expected behaviour.  Unfortunately, if the highest zone is
> > > small, a problem occurs.
> > > 
> > > This seems to happen most with recent sandybridge laptops but it's
> > > probably a co-incidence as some of these laptops just happen to have
> > > a small Normal zone. The reproduction case is almost always during
> > > copying large files that kswapd pegs at 100% CPU until the file is
> > > deleted or cache is dropped.
> > > 
> > > The problem is mostly down to sleeping_prematurely() keeping kswapd
> > > awake when the highest zone is small and unreclaimable and compounded
> > > by the fact we shrink slabs even when not shrinking zones causing a lot
> > > of time to be spent in shrinkers and a lot of memory to be reclaimed.
> > > 
> > > Patch 1 corrects sleeping_prematurely to check the zones matching
> > > 	the classzone_idx instead of all zones.
> > > 
> > > Patch 2 avoids shrinking slab when we are not shrinking a zone.
> > > 
> > > Patch 3 notes that sleeping_prematurely is checking lower zones against
> > > 	a high classzone which is not what allocators or balance_pgdat()
> > > 	is doing leading to an artifical believe that kswapd should be
> > > 	still awake.
> > > 
> > > Patch 4 notes that when balance_pgdat() gives up on a high zone that the
> > > 	decision is not communicated to sleeping_prematurely()
> > > 
> > > This problem affects 2.6.38.8 for certain and is expected to affect
> > > 2.6.39 and 3.0-rc4 as well. If accepted, they need to go to -stable
> > > to be picked up by distros and this series is against 3.0-rc4. I've
> > > cc'd people that reported similar problems recently to see if they
> > > still suffer from the problem and if this fixes it.
> > > 
> > 
> > Good!
> > This patch solved the problem.
> > But there is still a mystery.
> > 
> > In log, we could see excessive shrink_slab calls.
> 
> Yes, because shrink_slab() was called on each loop through
> balance_pgdat() even if the zone was balanced.
> 
> 
> > And as you know, we had merged patch which adds cond_resched where last of the function
> > in shrink_slab. So other task should get the CPU and we should not see
> > 100% CPU of kswapd, I think.
> > 
> 
> cond_resched() is not a substitute for going to sleep.

Of course, it's not equal with sleep but other task should get CPU and conusme their time slice
So we should never see 100% CPU consumption of kswapd.
No?

> 
> -- 
> Mel Gorman
> SUSE Labs

-- 
Kind regards,
Minchan Kim
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