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:

quoted

On Fri, Jun 24, 2011 at 03:44:53PM +0100, Mel Gorman wrote:

quoted

(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.

quoted

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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