Yu Zhao <yuzhao@xxxxxxxxxx> writes: > On Wed, Feb 23, 2022 at 8:32 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: >> >> Yu Zhao <yuzhao@xxxxxxxxxx> writes: >> >> > On Wed, Feb 23, 2022 at 5:59 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: >> >> >> >> Yu Zhao <yuzhao@xxxxxxxxxx> writes: >> >> >> >> > On Wed, Feb 23, 2022 at 1:28 AM Huang, Ying <ying.huang@xxxxxxxxx> wrote: >> >> >> >> >> >> Hi, Yu, >> >> >> >> >> >> Yu Zhao <yuzhao@xxxxxxxxxx> writes: >> >> >> >> >> >> > To avoid confusions, the terms "promotion" and "demotion" will be >> >> >> > applied to the multigenerational LRU, as a new convention; the terms >> >> >> > "activation" and "deactivation" will be applied to the active/inactive >> >> >> > LRU, as usual. >> >> >> >> >> >> In the memory tiering related commits and patchset, for example as follows, >> >> >> >> >> >> commit 668e4147d8850df32ca41e28f52c146025ca45c6 >> >> >> Author: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> >> >> >> Date: Thu Sep 2 14:59:19 2021 -0700 >> >> >> >> >> >> mm/vmscan: add page demotion counter >> >> >> >> >> >> https://lore.kernel.org/linux-mm/20220221084529.1052339-1-ying.huang@xxxxxxxxx/ >> >> >> >> >> >> "demote" and "promote" is used for migrating pages between different >> >> >> types of memory. Is it better for us to avoid overloading these words >> >> >> too much to avoid the possible confusion? >> >> > >> >> > Given that LRU and migration are usually different contexts, I think >> >> > we'd be fine, unless we want a third pair of terms. >> >> >> >> This is true before memory tiering is introduced. In systems with >> >> multiple types memory (called memory tiering), LRU is used to identify >> >> pages to be migrated to the slow memory node. Please take a look at >> >> can_demote(), which is called in shrink_page_list(). >> > >> > This sounds clearly two contexts to me. Promotion/demotion (move >> > between generations) while pages are on LRU; or promotion/demotion >> > (migration between nodes) after pages are taken off LRU. >> > >> > Note that promotion/demotion are not used in function names. They are >> > used to describe how MGLRU works, in comparison with the >> > active/inactive LRU. Memory tiering is not within this context. >> >> Because we have used pgdemote_* in /proc/vmstat, "demotion_enabled" in >> /sys/kernel/mm/numa, and will use pgpromote_* in /proc/vmstat. It seems >> better to avoid to use promote/demote directly for MGLRU in ABI. A >> possible solution is to use "mglru" and "promote/demote" together (such >> as "mglru_promote_*" when it is needed? > > *If* it is needed. Currently there are no such plans. OK. >> >> >> > +static int get_swappiness(struct mem_cgroup *memcg) >> >> >> > +{ >> >> >> > + return mem_cgroup_get_nr_swap_pages(memcg) >= MIN_LRU_BATCH ? >> >> >> > + mem_cgroup_swappiness(memcg) : 0; >> >> >> > +} >> >> >> >> >> >> After we introduced demotion support in Linux kernel. The anonymous >> >> >> pages in the fast memory node could be demoted to the slow memory node >> >> >> via the page reclaiming mechanism as in the following commit. Can you >> >> >> consider that too? >> >> > >> >> > Sure. How do I check whether there is still space on the slow node? >> >> >> >> You can always check the watermark of the slow node. But now, we >> >> actually don't check that (as in demote_page_list()), instead we will >> >> wake up kswapd of the slow node. The intended behavior is something >> >> like, >> >> >> >> DRAM -> PMEM -> disk >> > >> > I'll look into this later -- for now, it's a low priority because >> > there isn't much demand. I'll bump it up if anybody is interested in >> > giving it a try. Meanwhile, please feel free to cook up something if >> > you are interested. >> >> When we introduce a new feature, we shouldn't break an existing one. >> That is, not introducing regression. I think that it is a rule? >> >> If my understanding were correct, MGLRU will ignore to scan anonymous >> page list even if there's demotion target for the node. This breaks the >> demotion feature in the upstream kernel. Right? > > I'm not saying this shouldn't be fixed. I'm saying it's a low priority > until somebody is interested in using/testing it (or making it work). We are interested in this feature and can help to test it. > Regarding regressions, I'm sure MGLRU *will* regress many workloads. > Its goal is to improve the majority of use cases, i.e., total net > gain. Trying to improve everything is methodically wrong because the > problem space is near infinite but the resource is limited. So we have > to prioritize major use cases over minor ones. The bottom line is > users have a choice not to use MGLRU. This is a functionality regression, not performance regression. Without demotion support, some workloads will go OOM when DRAM is used up (while PMEM isn't) if PMEM is onlined in movable zone (as recommended). >> It's a new feature to check whether there is still space on the slow >> node. We can look at that later. > > SGTM. Best Regards, Huang, Ying
