SeongJae Park <sj@xxxxxxxxxx> writes: > On Wed, 29 Dec 2021 09:33:56 +0800 "Huang, Ying" <ying.huang@xxxxxxxxx> wrote: > >> SeongJae Park <sj@xxxxxxxxxx> writes: >> >> > Hello, >> > >> > On Mon, 27 Dec 2021 11:09:56 +0800 "Huang, Ying" <ying.huang@xxxxxxxxx> wrote: >> > >> >> Hi, SeongJae, >> >> >> >> SeongJae Park <sj@xxxxxxxxxx> writes: >> >> >> >> > Hi, >> >> > >> >> > On Thu, 23 Dec 2021 15:51:18 +0800 "Huang, Ying" <ying.huang@xxxxxxxxx> 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. >> >> 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. > > Exactly. What I'm saying is, DAMON schemes that exposed to user space via the > debugfs interface is for inspection of system and development of user space > daemon (2.). It requires some level of tuning and doesn't provide stable ABI > but the debugfs interface. Meanwhile, DAMON-based kernel components like > DAMON_RECLAIM can be used for the in-kernel general policy (1.). For example, > DAMON_RECLAIM also tries to be beneficial or at least incur no regression for > almost every users, provides as few as possible tunables, and provides those > via its ABI (module parameters), not debugfs. Thanks for your detailed explanation. Per my understanding, DAMON schemes are kind of building blocks of some kernel feature such as DAMON_RECLAIM. Whether do we need a new scheme depends on whether it's useful as part of some kernel feature. Do you agree? Best Regards, Huang, Ying > Thanks, > SJ > >> >> 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] https://github.com/awslabs/damoos >> > >> > >> > Thanks, >> > SJ >> > >> >> >> >> Best Regards, >> >> Huang, Ying >>
