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

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  Re: [PATCH v2] zswap: Zero-filled pages handling Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2017-07-06 07:30 +0200
    Re: [PATCH v2] zswap: Zero-filled pages handling Srividya Desireddy <srividya.dr@samsung.com> - 2017-07-06 11:30 +0200
      Re: [PATCH v2] zswap: Zero-filled pages handling Dan Streetman <ddstreet@ieee.org> - 2017-07-06 12:10 +0200

#1682050 — 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

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#1682240

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.

- Srividya

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#1682304

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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