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

[Barry Song <21cnbao@gmail.com>]

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?

Thanks
Barry