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

[Srividya Desireddy]

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