Re: [PATCH v7 2/2] mm: support large folios swap-in for sync io devices

[Usama Arif <usamaarif642@gmail.com>]

On 23/09/2024 00:57, Barry Song wrote:
> On Thu, Sep 5, 2024 at 7:36 AM Yosry Ahmed <yosryahmed@google.com> wrote:
>>
>> [..]
>>>>
>>>> On the other hand, if you read the code of zRAM, you will find zRAM has
>>>> exactly the same mechanism as zeromap but zRAM can even do more
>>>> by same_pages filled. since zRAM does the job in swapfile layer, there
>>>> is no this kind of consistency issue like zeromap.
>>>>
>>>> So I feel for zRAM case, we don't need zeromap at all as there are duplicated
>>>> efforts while I really appreciate your job which can benefit all swapfiles.
>>>> i mean, zRAM has the ability to check "zero"(and also non-zero but same
>>>> content). after zeromap checks zeromap, zRAM will check again:
>>>>
>>>
>>> Yes, so there is a reason for having the zeromap patches, which I have outlined
>>> in the coverletter.
>>>
>>> https://lore.kernel.org/all/20240627105730.3110705-1-usamaarif642@gmail.com/
>>>
>>> There are usecases where zswap/zram might not be used in production.
>>> We can reduce I/O and flash wear in those cases by a large amount.
>>>
>>> Also running in Meta production, we found that the number of non-zero filled
>>> complete pages were less than 1%, so essentially its only the zero-filled pages
>>> that matter.
>>>
>>> I believe after zeromap, it might be a good idea to remove the page_same_filled
>>> check from zram code? Its not really a problem if its kept as well as I dont
>>> believe any zero-filled pages should reach zram_write_page?
>>
>> I brought this up before and Sergey pointed out that zram is sometimes
>> used as a block device without swap, and that use case would benefit
>> from having this handling in zram. That being said, I have no idea how
>> many people care about this specific scenario.
> 
> Hi Usama/Yosry,
> 
> We successfully gathered page_same_filled data for zram on Android.
> Interestingly,
> our findings differ from yours on zswap.
> 
> Hailong discovered that around 85-86% of the page_same_filled data
> consists of zeros,
> while about 15% are non-zero. We suspect that on Android or similar
> systems, some
> graphics or media data might be duplicated at times, such as a red
> block displayed
> on the screen.
> 
> Does this suggest that page_same_filled could still provide some
> benefits in zram
> cases?

Hi Barry,

Thanks for the data, its very interesting to know this from mobile side.
Eventhough its not 99% that I observed, I do feel 85% is still quite high. 

The 2 main reasons for the patcheset to store zero pages to be
swapped out in a bitmap were for applications that use swap but
not zswap/zram (reducing I/O and flash wear), and simplifying zswap
code. It also meant fewer zswap_entry structs in memory which would
offset the memory usage by bitmap.

Yosry mentioned that Sergey pointed out that zram is sometimes
used as a block device without swap, and that use case would benefit
from having this handling in zram. Will that case also not go through
swap_writepage and therefore be takencare of by swap_info_struct->zeromap?

Also just curious if there are cases in mobile where only swap is used,
but not zswap/zram?

I think even with 85% zero-filled pages, we could get rid of page_same_filled
especially if the zram without swap case is handled by swap_info_struct->zeromap.
But don't have strong preference.

Thanks,
Usama

> 
> Thanks
> Barry