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. > >> > +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.
