Re: [PATCH 4/4] Memory controller soft limit reclaim on contention (v5)

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> > hm
> > I read past discussion. so, I think we discuss many aspect at once.
> > So, my current thinking is below, 
> > 
> > (1) if the group don't have any soft limit shrinking page, 
> >     mem_cgroup_soft_limit_reclaim() spent time unnecessary.
> >     -> right.
> 
> If the soft limit RB tree is empty, we don't spend any time at all.
> Are you referring to something else? Am I missing something? The tree
> will be empty if no group is over the soft limit.

maybe, I am missing anything.
May I ask your following paragraph meaning?


> I experimented a *lot* with zone reclaim and found it to be not so
> effective. Here is why
> 
> 1. We have no control over priority or how much to scan, that is
> controlled by balance_pgdat(). If we find that we are unable to scan
> anything, we continue scanning with the scan > 0 check, but we scan
> the same pages and the same number, because shrink_zone does scan >>
> priority.

I thought this sentense mean soft-limit-shrinking spent a lot of time.
if not, could you please tell me what makes so slower?

and, you wrote:

> 
> Yes, I sent that reason out as comments to Kame's patches. kswapd or
> balance_pgdat controls the zones, priority and in effect how many
> pages we scan while doing reclaim. I did lots of experiments and found
> that if soft limit reclaim occurred from kswapd, soft_limit_reclaim
> would almost always fail and shrink_zone() would succeed, since it
> looks at the whole zone and is always able to find some pages at all
> priority levels. It also does not allow for targetted reclaim based on
> how much we exceed the soft limit by. 

but, if "soft_limit_reclaim fail and shrink_zone() succeed" don't cause
any performance degression, I don't find why kswapd is wrong.

I guess you and kamezawa-san know it detail. but my understanding don't reach it.
Could you please tell me what so slowness.



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