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

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Date:	Thu, 18 Jun 2015 12:01:36 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	Minchan Kim <minchan@...nel.org>,
	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 11:41), Sergey Senozhatsky wrote:
[..]
> > My concern is not a compacion overhead but higher memory footprint
> > consumed by zram in reserved memory.
> > It might hang system if zram used up reserved memory of system with
> > ALLOC_NO_WATERMARKS. With auto-compaction, userspace has a higher chance
> > to use more memory with uncompressible pages or file-backed pages
> > so zram-swap can use more reserved memory. We need to evaluate it, I think.
> > 

a couple of _not really related_ ideas that I want to voice.

(a) I'm thinking of extending zramX/compact attr. right now it's WO,
  and I think it makes sense to make it RW:
    ->write will trigger compaction
    ->read will return estimated number of bytes
  "zs_can_compact() * pages per zspage * page_size" that can be freed.
  so user-space will have at least minimal idea whether compaction is
  reasonable. but sure, this is racy and in general case things may
  change between `cat compact` and `echo 1 > compact`.


(b) adding a knob (yeah, like we don't have enough knobs already :-))
that will allow 'enable/disable auto compaction'.

	-ss
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