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

From: Dan Streetman <ddstreet@ieee.org>
To: srividya.dr@samsung.com
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	Seth Jennings <sjenning@redhat.com>,
	"penberg@kernel.org" <penberg@kernel.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Dinakar Reddy Pathireddy <dinakar.p@samsung.com>,
	SHARAN ALLUR <sharan.allur@samsung.com>,
	SUNEEL KUMAR SURIMANI <suneel@samsung.com>,
	JUHUN KIM <juhunkim@samsung.com>,
	"srividya.desireddy@gmail.com" <srividya.desireddy@gmail.com>
Subject: Re: [PATCH v2] zswap: Zero-filled pages handling
Date: Thu, 6 Jul 2017 06:02:58 -0400	[thread overview]
Message-ID: <CALZtONDqz0GZUxhBt0QXmup2=5zw+xsSvNrNGOFN4KAZRb9urg@mail.gmail.com> (raw)
In-Reply-To: <20170706092919epcms5p53dae183bd95cd2fa5b050f496f32aa73@epcms5p5>

On Thu, Jul 6, 2017 at 5:29 AM, Srividya Desireddy
<srividya.dr@samsung.com> wrote:
> On Wed, Jul 6, 2017 at 10:49 AM, Sergey Senozhatsky wrote:
>> 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.
>>
>
> Thank you for your suggestion. It is a better solution to handle
> zero-filled pages before swapping-out to zswap. Since, Zram is already
> handles Zero pages internally, I considered to handle within Zswap.
> In a long run, we can work on it to commonly handle zero-filled anon
> pages.
>
>> 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
>
> It is a good solution to extend zero-filled pages handling to same value
> pages. I will work on to identify the percentage of same value pages
> excluding zero-filled pages in Zswap and will get back.

Yes, this sounds like a good modification to the patch.  Also, unless
anyone else disagrees, it may be good to control this with a module
param - in case anyone has a use case that they know won't be helped
by this, and the extra overhead of checking each page is wasteful.
Probably should default to enabled.

>
> - Srividya

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