linux-kernel - Re: [PATCH v1 0/7] writeback incompressible pages to storage

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Date:   Thu, 29 Jun 2017 17:47:26 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Juneho Choi <juno.choi@....com>,
        kernel-team <kernel-team@....com>
Subject: Re: [PATCH v1 0/7] writeback incompressible pages to storage

Hi Sergey,

On Thu, Jun 29, 2017 at 12:41:57AM +0900, Sergey Senozhatsky wrote:
> Hello,
> 
> On (06/26/17 15:52), Minchan Kim wrote:
> [..]
> > zRam is useful for memory saving with compressible pages but sometime,
> > workload can be changed and system has lots of incompressible pages
> > which is very harmful for zram.
> 
> could do. that makes zram quite complicated, to be honest. no offense,
> but the whole zram's "good compression" margin looks to me completely
> random and quite unreasonable. building a complex logic atop of random
> logic is a bit tricky. but I see what problem you are trying to address.
> 
> > This patch supports writeback feature of zram so admin can set up
> > a block device and with it, zram can save the memory via writing
> > out the incompressile pages once it found it's incompressible pages
> > (1/4 comp ratio) instead of keeping the page in memory.
> 
> hm, alternative idea. just an idea. can we try compressing the page
> with another algorithm? example: downcast from lz4 to zlib? we can
> set up a fallback "worst case" algorithm, so each entry can contain
> additional flag that would tell if the src page was compressed with
> the fast or slow algorithm. that sounds to me easier than "create a
> new block device and bond it to zram, etc". but I may be wrong.

We tried it although it was static not dynamic adatation you suggested.
However problem was media-stream data so zlib, lzam added just pointless
overhead.

Thanks.