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

[David Hildenbrand]

On 16.06.23 09:37, Yosry Ahmed wrote:
> On Thu, Jun 15, 2023 at 9:41 PM 贺中坤 <hezhongkun.hzk@xxxxxxxxxxxxx> 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?

Out of curiosity, are there any other known pros/cons when using 
zswap-without-swap instead of zram?

I know that zram requires sizing (size of the virtual block device) and 
consumes metadata, zswap doesn't.

--
Cheers,

David / dhildenb