On Wed, Feb 23, 2022 at 10:27 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: > > 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. That's great. I'll make sure it works in the next version.
