On Fri, Nov 17, 2023 at 8:23 AM Nhat Pham nphamcs@gmail.com wrote:
On Thu, Nov 16, 2023 at 4:57 PM Chris Li chrisl@kernel.org 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