linux-kernel - Re: [RFC][PATCHv2 8/8] zsmalloc: register a shrinker to trigger auto-compaction

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Date:	Thu, 18 Jun 2015 12:58:20 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCHv2 8/8] zsmalloc: register a shrinker to trigger
 auto-compaction

On (06/18/15 12:39), Minchan Kim wrote:
[..]
> > ah, I see.
> > it doesn't hold the lock `until all the pages are done`. it holds it
> > as long as zs_can_compact() returns > 0. hm, I'm not entirely sure that
> > this patch set has increased the locking time (in average).
> 
> I see your point. Sorry for the consusing.
> My point is not average but max time. I bet your patch will increase
> it and it will affect others who want to allocate zspage in parallel on
> another CPU.

makes sense.

[..]
> > > Yes, it's not easy and I believe a few artificial testing are not enough
> > > to prove no regression but we don't have any choice.
> > > Actually, I think this patchset does make sense. Although it might have
> > > a problem on situation heavy memory pressure by lacking of fragment space,
> > 
> > 
> > I tested exactly this scenario yesterday (and sent an email). We leave `no holes'
> > in classes only in ~1.35% of cases. so, no, this argument is not valid. we preserve
> > fragmentation.
> 
> Thanks, Sergey.
> 
> I want to test by myself to simulate worst case scenario to make to use up
> reserved memory by zram. For it, please fix below first and resubmit, please.
> 
> 1. doesn't hold lock until class compation is done.
>    It could prevent another allocation on another CPU.
>    I want to make worst case scenario and it needs it.
> 
> 2. No touch ZS_ALMOST_FULL waterline. It can put more zspages
>    in ZS_ALMOST_FULL list so it couldn't be selected by migration
>    source.
> 
> With new patchset, I want to watch min(free_pages of the system),
> zram.max_used_pages, testing time and so on.
> 
> Really sorry for bothering you, Sergey but I think it's important
> feature on zram so I want to be careful because risk management is
> my role.

ok. will take a day or two to gather new numbers.

	-ss
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