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From:   Shakeel Butt <shakeelb@google.com>
Date:   Thu, 20 Aug 2020 08:44:33 -0700
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Subject: Re: [RFC v7 00/10] DAMON: Support Physical Memory Address Space Monitoring
To:     SeongJae Park <sjpark@amazon.com>
Cc:     SeongJae Park <sjpark@amazon.de>, Jonathan.Cameron@huawei.com,
        Andrea Arcangeli <aarcange@redhat.com>, acme@kernel.org,
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On Thu, Aug 20, 2020 at 12:11 AM SeongJae Park <sjpark@amazon.com> wrote:
>
> On Wed, 19 Aug 2020 18:21:44 -0700 Shakeel Butt <shakeelb@google.com> wrote:
>
> > On Tue, Aug 18, 2020 at 12:25 AM SeongJae Park <sjpark@amazon.com> wrote:
> > >
> > > From: SeongJae Park <sjpark@amazon.de>
> > >
> > > Changes from Previous Version
> > > =============================
> > >
> > > - Use 42 as the fake target id for paddr instead of -1
> > > - Fix a typo
> > >
> > > Introduction
> > > ============
> > >
> > > DAMON[1] programming interface users can extend DAMON for any address space by
> > > configuring the address-space specific low level primitives with appropriate
> > > ones including their own implementations.  However, because the implementation
> > > for the virtual address space is only available now, the users should implement
> > > their own for other address spaces.  Worse yet, the user space users who rely
> > > on the debugfs interface and user space tool, cannot implement their own.
> > >
> > > This patchset implements another reference implementation of the low level
> > > primitives for the physical memory address space.  With this change, hence, the
> > > kernel space users can monitor both the virtual and the physical address spaces
> > > by simply changing the configuration in the runtime.  Further, this patchset
> > > links the implementation to the debugfs interface and the user space tool for
> > > the user space users.
> > >
> > > Note that the implementation supports only the user memory, as same to the idle
> > > page access tracking feature.
> > >
> > > [1] https://lore.kernel.org/linux-mm/20200706115322.29598-1-sjpark@amazon.com/
> > >
> >
> > I am still struggling to find the benefit of this feature the way it
> > is implemented i.e. region based physical address space monitoring.
> > What exactly am I supposed to do for a given hot (or cold) physical
> > region? In a containerized world, that region can contain pages from
> > any cgroup. I can not really do anything about the accesses PHY-DAMON
> > provides me for a region.
>
> Technically speaking, this patchset introduces an implementation of DAMON's low
> level primitives for physical address space of LRU-listed pages.  In other
> words, it is not designed for cgroups case.

So, this RFC is for a system running a single workload which comprises
multiple processes. Instead of registering each process with DAMON,
just monitor the whole physical memory, right?

Though I am still not sure how the output from DAMON can be used in
this case. DAMON told me a physical region is cold, how do I find out
processes that have mapped the pages in that region to do
process_madvise(PAGEOUT) on them?