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

[Yosry Ahmed <yosryahmed@google.com>]

On Mon, Oct 28, 2024 at 12:54 PM Barry Song <21cnbao@gmail.com> 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?

What about swap entries that get freed without being swapped in (e.g.
swapped out anon memory freed, MADV_FREE, shmem truncate, etc)?

>
> Thanks
> Barry