Re: [PATCH v5 0/6] workload-specific and memory pressure-driven zswap writeback

[Yosry Ahmed]

On Fri, Nov 17, 2023 at 8:23 AM Nhat Pham <nphamcs@xxxxxxxxx> wrote:
>
> On Thu, Nov 16, 2023 at 4:57 PM Chris Li <chrisl@xxxxxxxxxx> wrote:
> >
> > Hi Nhat,
> >
> > I want want to share the high level feedback we discussed here in the
> > mailing list as well.
> >
> > It is my observation that each memcg LRU list can't compare the page
> > time order with other memcg.
> > It works great when the leaf level memcg hits the memory limit and you
> > want to reclaim from that memcg.
> > It works less well on the global memory pressure you need to reclaim
> > from all memcg. You kind of have to
> > scan each all child memcg to find out the best page to shrink from. It
> > is less effective to get to the most desirable page quickly.
> >
> > This can benefit from a design similar to MGLRU. This idea is
> > suggested by Yu Zhao, credit goes to him not me.
> > In other words, the current patch is similar to the memcg page list
> > pre MGLRU world. We can have a MRLRU
> > like per memcg zswap shrink list.
>
> I was gonna summarize the points myself :P But thanks for doing this.
> It's your idea so you're more qualified to explain this anyway ;)
>
> I absolutely agree that having a generation-aware cgroup-aware
> NUMA-aware LRU is the future way to go. Currently, IIUC, the reclaim logic
> selects cgroups in a round-robin-ish manner. It's "fair" in this perspective,
> but I also think it's not ideal. As we have discussed, the current list_lru
> infrastructure only take into account intra-cgroup relative recency, not
> inter-cgroup relative recency. The recently proposed time-based zswap
> reclaim mechanism will provide us with a source of information, but the
> overhead of using this might be too high - and it's very zswap-specific.
>
> Maybe after this, we should improve zswap reclaim (and perhaps all
> list_lru users) by adding generations to list_lru then take generations
> into account in the vmscan code. This patch series could be merged
> as-is, and once we make list_lru generation-aware, zswap shrinker
> will automagically be improved (along with all other list_lru/shrinker
> users).
>
> I don't know enough about the current design of MGLRU to comment
> too much further, but let me know if this makes sense, and if you have
> objections/other ideas.
>
> And if you have other documentations for MGLRU than its code, could
> you please let me know? I'm struggling to find more details about this.
>

This could be a good place to start:
https://www.youtube.com/watch?v=9HvJfN21H9Y