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

[Andrew Lutomirski <luto@mit.edu>]

On Thu, Jul 21, 2011 at 12:24 PM, Minchan Kim <minchan.kim@gmail.com> wrote:
> 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?

If the rest of the system is idle, then kswapd will happily use 100%
CPU.  (Or on a multi-core system, kswapd will use close to 100% of one
CPU even if another task is using the other one.  This is bad enough
on a desktop, but on a laptop you start to notice when your battery
dies.)

--Andy

>
>>
>> --
>> Mel Gorman
>> SUSE Labs
>
> --
> Kind regards,
> Minchan Kim
>

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