Hi Huang, We find the patches in the tiering series are well thought and helpful. For our workloads, we initially started with that series and we find the whole series is too complex and some features do not benefit as expected. Therefore, we have come up with the current basic patches which are essential and help achieve most of the intended behaviors while reducing complexity as much as possible. As we started with your tiering series (with 72 patches), there are overlaps between our patches and the tiering series. We adopt the functionalities from the tiering series, modify, and extend them to make page placement mechanism simpler but workable. Here is the key points for each of the patches in our Transparent Page Placement series. Patch #1: We combine all the promotion and demotion related statistics in this patch Having statistics on both promotion, demotion, and failures help observe the systems behavior and reason about performance behavior. Besides, anon vs file breakdown in both promotion and demotion path help understand application behavior on a tiered memory systems. As applications may have different sensitivity toward the anon and file placements, this breakdown in the migration path is often helpful to assess the effectiveness of the page placement policy. Patch #2: This patch largely overlaps with your current series on NUMA Balancing. https://lore.kernel.org/lkml/20211116013522.140575-1-ying.huang@xxxxxxxxx/ This patch is a combination of your Patch #2 and Patch #3 except the static 10MB free space in the top-tier node to maintain a free headroom for new allocation and promotion. Rather, we find having a user defined demote watermark would make it more generic that we include in our patch#3 Patch #3: This patch has the logic for having a separate demote watermark per node. In the tiering series, that demote watermark is somewhat bound to the cgroup and triggered on per-application basis. Besides, It only supports cgroup-v1. However, we think, instead of cgroup based soft reclamation, a global per-node demote watermark is more meaningful and should be the basic one to start with. In that case, the user does not have to think about per-application setup. Patch #4: This patch includes the code for kswapd based reclamation. As I mentioned earlier, instead of cgroup-based reclamation, here we look whether a node is balanced during each kswapd invocation. For top-tier node, we check whether kswapd reclaimed till DEMOTE_WMARK is satisfied, for other nodes the default mechanism continues. The differences between tiering series and this patch is the cgroup based reclamation vs per-node reclamation. Patch #5: In your patches for promotion, you consider re-fault time for promotion candidate selection. Although the hot-threshold is tunable, from our experiments, we find this not helpful to some extent. For example, if different subset of pages have different re-access time, time-based promotion should not be able to distinguish between them. If you make the time window long enough, then any infrequently accessed pages will also become the promotion candidate, and later be a candidate for the demotion. In this patch, we propose LRU based promotion, which would give anon and files different promotion paths. If pages are used sporadically at high frequency, irregular pages would be eventually moved from the active LRU list. We find that our LRU based approach can reduce up to 11x promotion traffic while retaining the same application throughput for multiple workloads. Besides, with promotion rate limit, if files largely get promoted to top-tier, anon promotion rate often gets hampered as files are taking the large portion of the total rate (which often happen for applications that generates huge caches). In our LRU-based approach, each type has their own separate LRU to check. So for workloads with smaller anons and large file usage, with LRU-based approach, we can see more anons are being promoted rather than the files. I don't mind this patchset being merged to your current patchset under discussion or any later ones. But, I think this series contains the very basic functionalities to have a workable page placement mechanism for tiered-memory. This can obviously be augmented by the other features in you future tiering series. Best, Hasan
