[PATCH 3/4] zswap: Zero-filled pages handling

[Srividya Desireddy]

On 17 August 2016 at 18:02, Pekka Enberg <penberg@xxxxxxxxxx> wrote:
> On Wed, Aug 17, 2016 at 1:18 PM, Srividya Desireddy
> <srividya.dr@xxxxxxxxxxx> wrote:
>>> This patch adds a check in zswap_frontswap_store() to identify zero-filled
>>> page before compression of the page. If the page is a zero-filled page, set
>>> zswap_entry.zeroflag and skip the compression of the page and alloction
>>> of memory in zpool. In zswap_frontswap_load(), check if the zeroflag is
>>> set for the page in zswap_entry. If the flag is set, memset the page with
>>> zero. This saves the decompression time during load.
>>>
>>> The overall overhead caused due to zero-filled page check is very minimal
>>> when compared to the time saved by avoiding compression and allocation in
>>> case of zero-filled pages. The load time of a zero-filled page is reduced
>>> by 80% when compared to baseline.
>
> On Wed, Aug 17, 2016 at 3:25 PM, Pekka Enberg <penberg@xxxxxxxxxx> wrote:
>> AFAICT, that's an overall improvement only if there are a lot of
>> zero-filled pages because it's just overhead for pages that we *need*
>> to compress, no? So I suppose the question is, are there a lot of
>> zero-filled pages that we need to swap and why is that the case?
>
> I suppose reading your cover letter would have been helpful before
> sending out my email:
>
> "Experiments have shown that around 10-15% of pages stored in zswap are
> duplicates which results in 10-12% more RAM required to store these
> duplicate compressed pages."
>
> But I still don't understand why we have zero-filled pages that we are
> swapping out.
>
> - Pekka

Zero-filled pages exists in memory because applications may be
initializing the allocated pages with zeros and not using them; or
the actual content written to the memory pages during execution 
itself is zeros.
The existing page reclamation path in kernel does not check for
zero-filled pages in the anonymous LRU lists before swapping out.

- Srividya