linux-kernel - Re: [PATCH v2] zswap: Zero-filled pages handling

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Date:   Thu, 6 Jul 2017 14:19:59 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Seth Jennings <sjenning@...hat.com>
Cc:     srividya.dr@...sung.com, "ddstreet@...e.org" <ddstreet@...e.org>,
        "penberg@...nel.org" <penberg@...nel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Dinakar Reddy Pathireddy <dinakar.p@...sung.com>,
        SHARAN ALLUR <sharan.allur@...sung.com>,
        SUNEEL KUMAR SURIMANI <suneel@...sung.com>,
        JUHUN KIM <juhunkim@...sung.com>,
        "srividya.desireddy@...il.com" <srividya.desireddy@...il.com>
Subject: Re: [PATCH v2] zswap: Zero-filled pages handling

On (07/02/17 20:28), Seth Jennings wrote:
> On Sun, Jul 2, 2017 at 9:19 AM, Srividya Desireddy
> > Zswap is a cache which compresses the pages that are being swapped out
> > and stores them into a dynamically allocated RAM-based memory pool.
> > Experiments have shown that around 10-20% of pages stored in zswap
> > are zero-filled pages (i.e. contents of the page are all zeros), but
> > these pages are handled as normal pages by compressing and allocating
> > memory in the pool.
> 
> I am somewhat surprised that this many anon pages are zero filled.
> 
> If this is true, then maybe we should consider solving this at the
> swap level in general, as we can de-dup zero pages in all swap
> devices, not just zswap.
> 
> That being said, this is a fair small change and I don't see anything
> objectionable.  However, I do think the better solution would be to do
> this at a higher level.

zero-filled pages are just 1 case. in general, it's better
to handle pages that are memset-ed with the same value (e.g.
memset(page, 0x01, page_size)). which includes, but not
limited to, 0x00. zram does it.

	-ss