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

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Date:   Wed, 11 Jan 2023 19:02:59 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Suren Baghdasaryan <surenb@...gle.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 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.
-- 
Michal Hocko
SUSE Labs