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SubjectRe: [PATCH v2 0/2] Add a new scheme to support demotion on tiered memory system
SeongJae Park <> writes:

> Hello,
> On Mon, 27 Dec 2021 11:09:56 +0800 "Huang, Ying" <> wrote:
>> Hi, SeongJae,
>> SeongJae Park <> writes:
>> > Hi,
>> >
>> > On Thu, 23 Dec 2021 15:51:18 +0800 "Huang, Ying" <> wrote:
>> [snip]
>> >> It's good to avoid to change the source code of an application to apply
>> >> some memory management optimization (for example, use DAMON +
>> >> madvise()). But it's much easier to run a user space daemon to optimize
>> >> for the application. (for example, use DAMON + other information +
>> >> process_madvise()).
>> >>
>> >> And this kind of per-application optimization is kind of application
>> >> specific policy. This kind of policy may be too complex and flexible to
>> >> be put in the kernel directly. For example, in addition to DAMON, some
>> >> other application specific or system knowledge may be helpful too, so we
>> >> have process_madvise() for that before DAMON. Some more complex
>> >> algorithm may be needed for some applications.
>> >>
>> >> And this kind of application specific policy usually need complex
>> >> configuration. It's hard to export all these policy parameters to the
>> >> user space as the kernel ABI. Now, DAMON schemes parameters are
>> >> exported in debugfs so they are not considered ABI. So they may be
>> >> changed at any time. But applications need some stable and
>> >> well-maintained ABI.
>> >>
>> >> All in all, IMHO, what we need is a user space per-application policy
>> >> daemon with the information from DAMON and other sources.
>> >
>> > I basically agree to Ying, as I also noted in the coverletter of DAMOS
>> > patchset[1]:
>> >
>> > DAMON[1] can be used as a primitive for data access aware memory
>> > management optimizations. For that, users who want such optimizations
>> > should run DAMON, read the monitoring results, analyze it, plan a new
>> > memory management scheme, and apply the new scheme by themselves. Such
>> > efforts will be inevitable for some complicated optimizations.
>> >
>> > [1]
>> >
>> > That is, I believe some programs and big companies would definitely have their
>> > own information and want such kind of complicated optimizations. But, such
>> > optimizations would depend on characteristics of each program and require
>> > investment of some amount of resources. Some other programs and users wouldn't
>> > have such special information, and/or resource to invest for such
>> > optimizations. For them, some amount of benefit would be helpful enough even
>> > though its sub-optimal.
>> >
>> > I think we should help both groups, and DAMOS could be useful for the second
>> > group. And I don't think DAMOS is useless for the first group. They could use
>> > their information-based policy in prallel to DAMOS in some cases. E.g., if
>> > they have a way to predict the data access pattern of specific memory region
>> > even without help from DAMON, they can use their own policy for the region but
>> > DAMOS for other regions.
>> >
>> > Someone could ask why not implement a user-space implementation for the second
>> > group, then. First of all, DAMOS is not only for the user-space driven virtual
>> > memory management optimization, but also for kernel-space programs and any
>> > DAMOS-supportable address spaces including the physical address space. And,
>> > another important goal of DAMOS for user space driven use case in addition to
>> > reducing the redundant code is minimizing the user-kernel context switch
>> > overhead for passing the monitoring results information and memory management
>> > action requests.
>> >
>> > In summary, I agree the user space per-application policy daemon will be useful
>> > for the specialized ultimate optimizations, but we also need DAMOS for another
>> > common group of cases.
>> >
>> > If I'm missing something, please feel free to let me know.
>> I guess that most end-users and quite some system administrators of
>> small companies have no enough capability to take advantage of the
>> per-application optimizations. How do they know the appropriate region
>> number and proactive reclaim threshold?
>> So per my understanding, Linux kernel
>> need provide,
>> 1. An in-kernel general policy that is obviously correct and benefits
>> almost all users and applications, at least no regression. No
>> complex configuration or deep knowledge is needed to take advantage
>> of it.
>> 2. Some way to inspect and control system and application behavior, so
>> that some advanced and customized user space policy daemons can be
>> built to satisfy some advanced users who have the enough knowledge
>> for the applications and systems, for example, oomd.
> Agreed, and I think that's the approach that DAMON is currently taking. In
> specific, we provide DAMON debugfs interface for users who want to inspect and
> control their system and application behavior. Using it, we also made a PoC
> level user space policy daemon[1].
> For the in-kernel policies, we are developing DAMON-based kernel components one
> by one, for specific usages. DAMON-based proactive reclamation module
> (DAMON_RECLAIM) is one such example. Such DAMON-based components will remove
> complex tunables that necessary for the general inspection and control of the
> system but unnecessary for their specific purpose (e.g., proactive reclamation)
> to allow users use it in a simple manner. Also, those will use conservative
> default configs to not incur visible regression. For example, DAMON_RECLAIM
> uses only up to 1% of single CPU time for the reclamation by default.

I don't think DAMON schemes are the in-kernel general policy I mentioned
above (1.). For example, NUMA balancing is a general policy to optimize
performance. It tries to provide a general policy that works for all
users with as few as possible tunables. If some tunables are needed,
they will be provided as ABI.

Best Regards,
Huang, Ying

> In short, I think we're on the same page, and adding DEMOTION scheme action
> could be helpful for the users who want to efficiently inspect and control the
> system/application behavior for their tiered memory systems. It's unclear how
> much benefit this could give to users, though. I assume Baolin would come back
> with some sort of numbers in the next spin. Nevertheless, I personally don't
> think that's a critical blocker, as this patch is essentially just adding a way
> for using the pre-existing primitive, namely move_pages(), in a little bit more
> efficient manner, for the access pattern-based use cases.
> If I'm missing something, please feel free to let me know.
> [1]
> Thanks,
> SJ
>> Best Regards,
>> Huang, Ying

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