linux-kernel - Re: [PATCH 08/41] mm: introduce CONFIG_PER_VMA

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Message-ID: <CAJuCfpE0_xvgoGqpaRoqp=pKujocXLPqU0rBAh80_vUP6d3jyQ@mail.gmail.com>
Date:   Wed, 11 Jan 2023 10:09:18 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     David Laight <David.Laight@...lab.com>,
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Subject: Re: [PATCH 08/41] mm: introduce CONFIG_PER_VMA_LOCK

On Wed, Jan 11, 2023 at 10:03 AM Michal Hocko <mhocko@...e.com> wrote:
>
> On Wed 11-01-23 09:49:08, Suren Baghdasaryan wrote:
> > On Wed, Jan 11, 2023 at 9:37 AM Michal Hocko <mhocko@...e.com> wrote:
> > >
> > > On Wed 11-01-23 09:04:41, Suren Baghdasaryan wrote:
> > > > On Wed, Jan 11, 2023 at 8:44 AM Michal Hocko <mhocko@...e.com> wrote:
> > > > >
> > > > > On Wed 11-01-23 08:28:49, Suren Baghdasaryan wrote:
> > > > > [...]
> > > > > > Anyhow. Sounds like the overhead of the current design is small enough
> > > > > > to remove CONFIG_PER_VMA_LOCK and let it depend only on architecture
> > > > > > support?
> > > > >
> > > > > Yes. Further optimizations can be done on top. Let's not over optimize
> > > > > at this stage.
> > > >
> > > > Sure, I won't optimize any further.
> > > > Just to expand on your question. Original design would be problematic
> > > > for embedded systems like Android. It notoriously has a high number of
> > > > VMAs due to anonymous VMAs being named, which prevents them from
> > > > merging.
> > >
> > > What is the usual number of VMAs in that environment?
> >
> > I've seen some games which had over 4000 VMAs but that's on the upper
> > side. In my calculations I used 40000 VMAs as a ballpark number and
> > rough calculations before size optimization would increase memory
> > consumption by ~2M (depending on the lock placement in vm_area_struct
> > it would vary a bit). In Android, the performance team flags any
> > change that exceeds 500KB, so it would raise questions.
>
> Thanks, that is a useful information! This is just slightly off-topic
> but I ak wondering how much memory those vma names consume. Are there
> that many unique names or they just happen to be alternating so that
> neighboring ones tend to be different.

Good question. I don't have the ready answer to that but will try to
collect some stats. I know that many names are standardized but
haven't looked at how they are distributed in the address space. Will
followup once I collect the data.
Thanks,
Suren.

> --
> Michal Hocko
> SUSE Labs