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

Lokesh Gidra
2024-08-27T00:12:20Z

On Mon, Aug 26, 2024 at 12:55 PM Barry Song <21cnbao@gmail.com> wrote:
>
> 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?
>
Every app is a separate process and therefore has its own private ART
heap. Those numbers that you are asking vary drastically. But here's
what I can tell you:

Max heap size for an app is 512MB typically. But it is rarely entirely
used. Typical heap usage is 50MB to 250MB. But as I said, not all of
it is MADV_FREE'ed. Only those pages which are freed after GC
compaction are.
> > > >
> > > > 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?
>
That's definitely an important question to get answered. But putting
my users hat on again, by explicitly MADV_FREE'ing we ask for that
behavior. IMHO, MADV_FREE'ed pages should be the first ones to be
reclaimed on memory pressure.
> > >
> > > 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