Re: [External] Re: [RFC PATCH 1/3] zram: charge the compressed RAM to the page's memcgroup

Yosry Ahmed
2023-06-16T07:38:31Z

On Thu, Jun 15, 2023 at 9:41 PM 贺中坤 <hezhongkun.hzk@bytedance.com> wrote:
>
> > Thanks Fabian for tagging me.
> >
> > I am not familiar with #1, so I will speak to #2. Zhongkun, There are
> > a few parts that I do not understand -- hopefully you can help me out
> > here:
> >
> > (1) If I understand correctly in this patch we set the active memcg
> > trying to charge any pages allocated in a zspage to the current memcg,
> > yet that zspage will contain multiple compressed object slots, not
> > just the one used by this memcg. Aren't we overcharging the memcg?
> > Basically the first memcg that happens to allocate the zspage will pay
> > for all the objects in this zspage, even after it stops using the
> > zspage completely?
>
> It will not overcharge. As you said below, we are not using
> __GFP_ACCOUNT and charging the compressed slots to the memcgs.
>
> >
> > (2) Patch 3 seems to be charging the compressed slots to the memcgs,
> > yet this patch is trying to charge the entire zspage. Aren't we double
> > charging the zspage? I am guessing this isn't happening because (as
> > Michal pointed out) we are not using __GFP_ACCOUNT here anyway, so
> > this patch may be NOP, and the actual charging is coming from patch 3
> > only.
>
> YES， the actual charging is coming from patch 3. This patch just
> delivers the BIO page's memcg to the current task which is not the
> consumer.
>
> >
> > (3) Zswap recently implemented per-memcg charging of compressed
> > objects in a much simpler way. If your main interest is #2 (which is
> > what I understand from the commit log), it seems like zswap might be
> > providing this already? Why can't you use zswap? Is it the fact that
> > zswap requires a backing swapfile?
>
> Thanks for your reply and review. Yes, the zswap requires a backing
> swapfile. The I/O path is very complex, sometimes it will throttle the
> whole system if some resources are short , so we hope to use zram.

Is the only problem with zswap for you the requirement of a backing swapfile?

If yes, I am in the early stages of developing a solution to make
zswap work without a backing swapfile. This was discussed in LSF/MM
[1]. Would this make zswap usable in for your use case?

[1]https://lore.kernel.org/linux-mm/CAJD7tkYb_sGN8mfGVjr2JxdB8Pz8Td=yj9_sBCMrmsKQo56vTg@mail.gmail.com/

>
> >
> > Thanks!
> >