Re: [PATCH RFC] mm: count zeromap read and set for swapout and swapin

[Usama Arif <usamaarif642@gmail.com>]

On 28/10/2024 20:42, Barry Song wrote:
> On Tue, Oct 29, 2024 at 4:00 AM Usama Arif <usamaarif642@gmail.com> wrote:
>>
>>
>>
>> On 28/10/2024 19:54, Barry Song wrote:
>>> On Tue, Oct 29, 2024 at 1:20 AM Usama Arif <usamaarif642@gmail.com> wrote:
>>>>
>>>>
>>>>
>>>> On 28/10/2024 17:08, Yosry Ahmed wrote:
>>>>> On Mon, Oct 28, 2024 at 10:00 AM Usama Arif <usamaarif642@gmail.com> wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>> On 28/10/2024 16:33, Nhat Pham wrote:
>>>>>>> On Mon, Oct 28, 2024 at 5:23 AM Usama Arif <usamaarif642@gmail.com> wrote:
>>>>>>>>
>>>>>>>> I wonder if instead of having counters, it might be better to keep track
>>>>>>>> of the number of zeropages currently stored in zeromap, similar to how
>>>>>>>> zswap_same_filled_pages did it. It will be more complicated then this
>>>>>>>> patch, but would give more insight of the current state of the system.
>>>>>>>>
>>>>>>>> Joshua (in CC) was going to have a look at that.
>>>>>>>
>>>>>>> I don't think one can substitute for the other.
>>>>>>
>>>>>> Yes agreed, they have separate uses and provide different information, but
>>>>>> maybe wasteful to have both types of counters? They are counters so maybe
>>>>>> dont consume too much resources but I think we should still think about
>>>>>> it..
>>>>>
>>>>> Not for or against here, but I would say that statement is debatable
>>>>> at best for memcg stats :)
>>>>>
>>>>> Each new counter consumes 2 longs per-memcg per-CPU (see
>>>>> memcg_vmstats_percpu), about 16 bytes, which is not a lot but it can
>>>>> quickly add up with a large number of CPUs/memcgs/stats.
>>>>>
>>>>> Also, when flushing the stats we iterate all of them to propagate
>>>>> updates from per-CPU counters. This is already a slowpath so adding
>>>>> one stat is not a big deal, but again because we iterate all stats on
>>>>> multiple CPUs (and sometimes on each node as well), the overall flush
>>>>> latency becomes a concern sometimes.
>>>>>
>>>>> All of that is not to say we shouldn't add more memcg stats, but we
>>>>> have to be mindful of the resources.
>>>>
>>>> Yes agreed! Plus the cost of incrementing similar counters (which ofcourse is
>>>> also not much).
>>>>
>>>> Not trying to block this patch in anyway. Just think its a good point
>>>> to discuss here if we are ok with both types of counters. If its too wasteful
>>>> then which one we should have.
>>>
>>> Hi Usama,
>>> my point is that with all the below three counters:
>>> 1. PSWPIN/PSWPOUT
>>> 2. ZSWPIN/ZSWPOUT
>>> 3. SWAPIN_SKIP/SWAPOUT_SKIP or (ZEROSWPIN, ZEROSWPOUT what ever)
>>>
>>> Shouldn't we have been able to determine the portion of zeromap
>>> swap indirectly?
>>>
>>
>> Hmm, I might be wrong, but I would have thought no?
>>
>> What if you swapout a zero folio, but then discard it?
>> zeromap_swpout would be incremented, but zeromap_swapin would not.
> 
> I understand. It looks like we have two issues to tackle:
> 1. We shouldn't let zeromap swap in or out anything that vanishes into
> a black hole
> 2. We want to find out how much I/O/memory has been saved due to zeromap so far
> 
> From my perspective, issue 1 requires a "fix", while issue 2 is more
> of an optimization.

Hmm I dont understand why point 1 would be an issue.

If its discarded thats fine as far as I can see.

As a reference, memory.stat.zswapped != memory.stat.zswapout - memory.stat.zswapin.
Because zswapped would take into account swapped out anon memory freed, MADV_FREE,
shmem truncate, etc as Yosry said about zeromap, But zswapout and zswapin dont.


> 
> I consider issue 1 to be more critical because, after observing a phone
> running for some time, I've been able to roughly estimate the portion
> zeromap can
> help save using only PSWPOUT, ZSWPOUT, and SWAPOUT_SKIP, even without a
> SWPIN counter. However, I agree that issue 2 still holds significant value
> as a separate patch.
> 
> Thanks
> Barry