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

[David Hildenbrand <david@redhat.com>]

On 16.06.23 09:37, Yosry Ahmed wrote:
> 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?

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