Re: [PATCH 0/4] Stop kswapd consuming 100% CPU when highest zone is small

[Mel Gorman <mgorman@suse.de>]

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.

-- 
Mel Gorman
SUSE Labs