Re: [PATCH v2] mm: add lazyfree folio to lru tail

[Barry Song <21cnbao@gmail.com>]

On Tue, Aug 27, 2024 at 4:37 AM Lokesh Gidra <lokeshgidra@google.com> wrote:
>
> Thanks Suren for looping in
>
> On Fri, Aug 23, 2024 at 4:39 PM Suren Baghdasaryan <surenb@google.com> wrote:
> >
> > On Wed, Aug 21, 2024 at 2:47 PM Barry Song <21cnbao@gmail.com> wrote:
> > >
> > > On Wed, Aug 21, 2024 at 8:46 PM Michal Hocko <mhocko@suse.com> wrote:
> > > >
> > > > On Fri 16-08-24 07:48:01, gaoxu wrote:
> > > > > Replace lruvec_add_folio with lruvec_add_folio_tail in the lru_lazyfree_fn:
> > > > > 1. The lazy-free folio is added to the LRU_INACTIVE_FILE list. If it's
> > > > >    moved to the LRU tail, it allows for faster release lazy-free folio and
> > > > >    reduces the impact on file refault.
> > > >
> > > > This has been discussed when MADV_FREE was introduced. The question was
> > > > whether this memory has a lower priority than other inactive memory that
> > > > has been marked that way longer ago. Also consider several MADV_FREE
> > > > users should they be LIFO from the reclaim POV?
>
> Thinking from the user's perspective, it seems to me that FIFO within
> MADV_FREE'ed pages makes more sense. As a user I expect the longer a
> MADV_FREE'ed page hasn't been touched, the chances are higher that it
> may not be around anymore.
> > >

Hi Lokesh,
Thanks!

> > > The priority of this memory compared to other inactive memory that has been
> > > marked for a longer time likely depends on the user's expectations - How soon
> > > do users expect MADV_FREE to be reclaimed compared with old file folios.
> > >
> > > art guys moved to MADV_FREE from MADV_DONTNEED without any
> > > useful performance data and reason in the changelog:
> > > https://android-review.googlesource.com/c/platform/art/+/2633132
> > >
> > > Since art is the Android Java heap, it can be quite large. This increases the
> > > likelihood of packing the file LRU and reduces the chances of reclaiming
> > > anonymous memory, which could result in more file re-faults while helping
> > > anonymous folio persist longer in memory.
>
> Individual heaps of android apps are not big, and even in there we
> don't call MADV_FREE on the entire heap.

How do you define "Individual heaps of android apps", do you know the usual
total_size for a phone with memory pressure by running multiple apps and how
much for each app?

> > >
> > > I am really curious why art guys have moved to MADV_FREE if we have
> > > an approach to reach them.
>
> Honestly, it makes little sense as a user that calling MADV_FREE on an
> anonymous mapping will impact file LRU. That was never the intention
> with our ART change.
>

This is just how MADV_FREE is implemented in the kernel, this kind of lazyfree
anon folios are moved to file but *NOT* anon LRU.

> From our perspective, once a set of pages are MADV_FREE'ed, they are
> like a page-cache. It gives an opportunity, without hurting memory
> use, to avoid overhead of page-faults, which happen frequently after
> GC is done on running apps.
>
> IMHO, within LRU_INACTIVE_FILE, MADV_FREE'ed pages should be
> prioritized for reclamation over file ones.

This is exactly what this patch is doing, putting lazyfree anon folios
to the tail of file LRU so that they can be reclaimed earlier than file
folios. But the question is: is the requirement "MADV_FREE'ed pages
should be prioritized for reclamation over file ones" universally true for
all other non-Android users?

> >
> > Adding Lokesh.
> > Lokesh, could you please comment on the reasoning behind the above
> > mentioned change?
>
> Adding Nicolas as well, in case he wants to add something.
> >
> > >
> > > >
> > > > --
> > > > Michal Hocko
> > > > SUSE Labs
> > > >
> > >

Thanks
Barry