Re: Re: Re: Re: [PATCH v6 00/14] Introduce Data Access MONitor (DAMON)

[Shakeel Butt <shakeelb@google.com>]

On Thu, Mar 19, 2020 at 2:04 AM SeongJae Park <sjpark@amazon.com> wrote:
>
> On Wed, 18 Mar 2020 12:52:48 -0700 Shakeel Butt <shakeelb@google.com> wrote:
>
> > On Thu, Mar 12, 2020 at 3:44 AM SeongJae Park <sjpark@amazon.com> wrote:
> > >
> > > On Thu, 12 Mar 2020 11:07:59 +0100 SeongJae Park <sjpark@amazon.com> wrote:
> > >
> > > > On Tue, 10 Mar 2020 10:21:34 -0700 Shakeel Butt <shakeelb@google.com> wrote:
> > > >
> > > > > On Mon, Feb 24, 2020 at 4:31 AM SeongJae Park <sjpark@amazon.com> wrote:
> > > > > >
> > > > > > From: SeongJae Park <sjpark@amazon.de>
> > > > > >
> > > > > > Introduction
> > > > > > ============
> > > > > >
> > > [...]
> > > > >
> > > > > I do want to question the actual motivation of the design followed by this work.
> > > > >
> > > > > With the already present Page Idle Tracking feature in the kernel, I
> > > > > can envision that the region sampling and adaptive region adjustments
> > > > > can be done in the user space. Due to sampling, the additional
> > > > > overhead will be very small and configurable.
> > > > >
> > > > > Additionally the proposed mechanism has inherent assumption of the
> > > > > presence of spatial locality (for virtual memory) in the monitored
> > > > > processes which is very workload dependent.
> > > > >
> > > > > Given that the the same mechanism can be implemented in the user space
> > > > > within tolerable overhead and is workload dependent, why it should be
> > > > > done in the kernel? What exactly is the advantage of implementing this
> > > > > in kernel?
> > > >
> > > > First of all, DAMON is not for only user space processes, but also for kernel
> > > > space core mechanisms.  Many of the core mechanisms will be able to use DAMON
> > > > for access pattern based optimizations, with light overhead and reasonable
> > > > accuracy.
> >
> > Which kernel space core mechanisms? I can see memory reclaim, do you
> > envision some other component as well.
>
> In addition to reclmation, I am thinking THP promotion/demotion decision, page
> migration among NUMA nodes on tier-memory configuration, and on-demand virtual
> machine live migration mechanisms could benefit from DAMON, for now.  I also
> believe more use-cases could be found.
>

I am still struggling to see how these use-cases require in-kernel
DAMON. For THP promotion/demotaion, madvise(MADV_[NO]HUGEPAGE) can be
used or we can introduce a new MADV_HUGIFY to synchronously convert
small pages to hugepages. Page migration on tier-memory is similar to
proactive reclaim and we already have migrate_pages/move_pages
syscalls. Basically why userspace DAMON can not perform these
operations?

> >
> > Let's discuss how this can interact with memory reclaim and we can see
> > if there is any benefit to do this in kernel.
>
> For reclaim, I believe we could try the proactive reclamation again using DAMON
> (Yes, I'm a fan of the idea of proactive reclamation).  I already implemented
> and evaluated a simple form of DAMON-based proactive reclamation for the proof
> of the concept (not for production).  In best case (parsec3/freqmine), it
> reduces 22.42% of system memory usage and 88.86% of residential sets while
> incurring only 3.07% runtime overhead.  Please refer to 'Appendix E' of the v7
> patchset[1] of DAMON.  It also describes the implementation and the evaluation
> of a data access monitoring-based THP promotion/demotion policy.
>
> The experimental implementation cannot be directly applied to kernel
> reclamation mechanism, because it requires users to specify the target
> applications.  Nonetheless, I think we can also easily adopt it inside the
> kernel by modifying kswapd to periodically select processes having huge RSS as
> targets, or by creating proactive reclamation type cgroups which selects every
> processes in the cgroup as targets.

Again I feel like these should be done in user space as these are more
policies instead of mechanisms. However if it can be shown that doing
that in userspace is very expensive as compared to in-kernel solution
then we can think about it.

>
> Of course, we can extend DAMON to support physical memory address space instead
> of virtual memory of specific processes.  Actually, this is in our TODO list.
> With the extension, applying DAMON to core memory management mechanisms will be
> even easier.

See below on physical memory monitoring.

>
> Nonetheless, this is only example but not concrete plan.  I didn't make the
> concrete plan yet, but believe that of DAMON will open the gates.
>
> [1] https://lore.kernel.org/linux-mm/20200318112722.30143-1-sjpark@amazon.com/
>
> >
> > > >
> > > > Implementing DAMON in user space is of course possible, but it will be
> > > > inefficient.  Using it from kernel space would make no sense, and it would
> > > > incur unnecessarily frequent kernel-user context switches, which is very
> > > > expensive nowadays.
> > >
> > > Forgot mentioning about the spatial locality.  Yes, it is workload dependant,
> > > but still pervasive in many case.  Also, many core mechanisms in kernel such as
> > > read-ahead or LRU are already using some similar assumptions.
> > >
> >
> > Not sure about the LRU but yes read-ahead in several places does
> > assume spatial locality. However most of those are configurable and
> > the userspace can enable/disable the read-ahead based on the workload.
>
> Sorry for my ambiguous description.  LRU uses temporal locality, which is
> somewhat similar to spatial locality, in terms of workload dependency.
>
> >
> > >
> > > If it is so problematic, you could set the maximum number of regions to the
> > > number of pages in the system so that each region monitors each page.
> > >
> >
> > How will this work in the process context? Number of regions equal to
> > the number of mapped pages?
>
> Suppose that a process has 1024 pages of working set and each of the pages has
> totally different access frequency.  If the maximum number of regions is 1024,
> the adaptive regions adjustment mechanism of DAMON will create each region for
> each page and monitor the access to each page.  So, the output will be same to
> straightforward periodic page-granularity access checking methods, which does
> not depend on the spatial locality.  Nevertheless, the monitoring overhead will
> be also similar to that.
>
> However, if any adjacent pages have similar access frequencies, DAMON will
> group those pages into one region.  This will reduce the total number of PTE
> Accessed bit checks and thus decrease the overhead.  In other words, DAMON do
> its best effort to minimize the overhead while preserving quality.
>
> Also suppose that the maximum number of region is smaller than 1024 in this
> case.  Pages having different access frequency will be grouped in same region
> and thus the output quality will be decreased.  However, the overhead will
> be half, as DAMON does one access check per each region.  This means that you
> can easily trade the monitoring quality with overhead by adjusting the maximum
> number of regions.
>

So, users can select to not merge the regions to keep the monitoring
quality high, right?

> >
> > Basically I am trying to envision the comparison of physical memory
> > based monitoring (using idle page tracking) vs pid+VA based
> > monitoring.
>
> I believe the core mechanisms of DAMON could be easily extended to the physical
> memory.  Indeed, it is in our TODO list, and I believe it would make use of
> DAMON in kernel core mechanisms much easier.
>

How will the sampling and regions representation/resizing work in
physical memory?

> >
> > Anyways I am not against your proposal. I am trying to see how to make
> > it more general to be applicable to more use-cases and one such
> > use-case which I am interested in is monitoring all the user pages on
> > the system for proactive reclaim purpose.
>
> Your questions gave me many insight and shed lights to the way DAMON should go.
> Really appreciate.  If you have any more questions or need my help, please let
> me know.
>
>

Shakeel