Messages in this thread | | | Date | Thu, 30 Dec 2021 17:31:45 +0800 | Subject | Re: [PATCH v2 0/2] Add a new scheme to support demotion on tiered memory system | From | Baolin Wang <> |
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On 12/28/2021 4:44 PM, SeongJae Park wrote: > Hello, > > On Mon, 27 Dec 2021 11:09:56 +0800 "Huang, Ying" <ying.huang@intel.com> wrote: > >> Hi, SeongJae, >> >> SeongJae Park <sj@kernel.org> writes: >> >>> Hi, >>> >>> On Thu, 23 Dec 2021 15:51:18 +0800 "Huang, Ying" <ying.huang@intel.com> 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] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=fda504fade7f124858d7022341dc46ff35b45274 >>> >>> 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. > > 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
Agree. It will be easier for us to deploy it to the products for the common scenarios.
> 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
Yes, I am still trying to set up the effective measurement environment and get the performance number in the next version.
> 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] https://github.com/awslabs/damoos
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