linux-kernel - Re: [PATCH v6 11/12] zsmalloc: page migration support

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Date:	Thu, 26 May 2016 09:59:26 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 11/12] zsmalloc: page migration support

Hello Minchan,

On (05/26/16 09:32), Minchan Kim wrote:
[..]
> Unfortunately, I don't have now. However, I don't feel we need a data for
> that because *unbounded work* within VM interaction context is bad. ;-)

fair enough, even though the shrinker doesn't put any constraints here.

> > hm, probably it makes sense to change it. but if the change will
> > replace "1 full pool scan" to "2 scans of 1/2 of pool's classes",
> > then I'm less sure.
> 
> Other shrinker is on same page. They have *cache* which is helpful for
> performance but if it's not hot, it can lose performance if memory
> pressure is severe. For the balance, comprimise approach is shrinker.
> 
> We can see fragement space in zspage as wasted memory which is bad
> on the other hand we can see it as cache to store upcoming compressed
> page. So, too much freeing can hurt the performance so, let's obey
> VM's rule. If memory pressure goes severe, they want to reclaim more
> pages with proportional of memory pressure.

I agree that the unused memory has a dual nature here. we don't really
do the "compact too much" thing, in a sense that we very rarely compact
classes to be ZS_FULL. ~2-3% of the cases (if I recall the testing
results correctly). but reducing the class ->lock pressure is a nice
thing on its own. so yeah, let's do it!

btw, I've uploaded zram-fio test script to
 https://github.com/sergey-senozhatsky/zram-perf-test

it's very minimalistic and half baked, but can be used
to some degree. open to patches, improvements, etc.

	-ss