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? >> >> > +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? It's a new feature to check whether there is still space on the slow node. We can look at that later. Best Regards, Huang, Ying
