On Wed, 2021-04-14 at 13:14 -0600, Yu Zhao wrote:
> On Wed, Apr 14, 2021 at 9:59 AM Rik van Riel <riel@surriel.com>
> wrote:
> > On Wed, 2021-04-14 at 08:51 -0700, Andi Kleen wrote:
> > > >    2) It will not scan PTE tables under non-leaf PMD entries
> > > > that
> > > > do not
> > > >       have the accessed bit set, when
> > > >       CONFIG_HAVE_ARCH_PARENT_PMD_YOUNG=y.
> > > 
> > > This assumes  that workloads have reasonable locality. Could
> > > there
> > > be a worst case where only one or two pages in each PTE are used,
> > > so this PTE skipping trick doesn't work?
> > 
> > Databases with large shared memory segments shared between
> > many processes come to mind as a real-world example of a
> > worst case scenario.
> 
> Well, I don't think you two are talking about the same thing. Andi
> was
> focusing on sparsity. Your example seems to be about sharing, i.e.,
> ihgh mapcount. Of course both can happen at the same time, as I
> tested
> here:
> https://lore.kernel.org/linux-mm/YHFuL%2FDdtiml4biw@google.com/#t
> 
> I'm skeptical that shared memory used by databases is that sparse,
> i.e., one page per PTE table, because the extremely low locality
> would
> heavily penalize their performance. But my knowledge in databases is
> close to zero. So feel free to enlighten me or just ignore what I
> said.

A database may have a 200GB shared memory segment,
and a worker task that gets spun up to handle a
query might access only 1MB of memory to answer
that query.

That memory could be from anywhere inside the
shared memory segment. Maybe some of the accesses
are more dense, and others more sparse, who knows?

A lot of the locality
will depend on how memory
space inside the shared memory segment is reclaimed
and recycled inside the database.

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