[PATCH v7 05/12] mm: multigenerational LRU: minimal implementation

[Huang, Ying]

Yu Zhao <yuzhao at google.com> writes:

> On Wed, Feb 23, 2022 at 8:32 PM Huang, Ying <ying.huang at intel.com> wrote:
>>
>> Yu Zhao <yuzhao at google.com> writes:
>>
>> > On Wed, Feb 23, 2022 at 5:59 PM Huang, Ying <ying.huang at intel.com> wrote:
>> >>
>> >> Yu Zhao <yuzhao at google.com> writes:
>> >>
>> >> > On Wed, Feb 23, 2022 at 1:28 AM Huang, Ying <ying.huang at intel.com> wrote:
>> >> >>
>> >> >> Hi, Yu,
>> >> >>
>> >> >> Yu Zhao <yuzhao at google.com> 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 at linux.alibaba.com>
>> >> >> 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@intel.com/
>> >> >>
>> >> >> "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