In this proposal, I'd like to discuss userspace-managed memory tiering and the kernel support that it needs. New memory technologies and interconnect standard make it possible to have memory with different performance and cost on the same machine (e.g. DRAM + PMEM, DRAM + cost-optimized memory attached via CXL.mem). We can expect heterogeneous memory systems that have performance implications far beyond classical NUMA to become increasingly common in the future. One of important use cases of such tiered memory systems is to improve the data center and cloud efficiency with better performance/TCO. Because different classes of applications (e.g. latency sensitive vs latency tolerant, high priority vs low priority) have different requirements, richer and more flexible memory tiering policies will be needed to achieve the desired performance target on a tiered memory system, which would be more effectively managed by a userspace agent, not by the kernel. Moreover, we (Google) are explicitly trying to avoid adding a ton of heuristics to enlighten the kernel about the policy that we want on multi-tenant machines when the userspace offers more flexibility. To manage memory tiering in userspace, we need the kernel support in the three key areas: - resource abstraction and control of tiered memory; - API to monitor page accesses for making memory tiering decisions; - API to migrate pages (demotion/promotion). Userspace memory tiering can work on just NUMA memory nodes, provided that memory resources from different tiers are abstracted into separate NUMA nodes. The userspace agent can create a tiering topology among these nodes based on their distances. An explicit memory tiering abstraction in the kernel is preferred, though, because it can not only allow the kernel to react in cases where it is challenging for userspace (e.g. reclaim-based demotion when the system is under DRAM pressure due to usage surge), but also enable tiering controls such as per-cgroup memory tier limits. This requirement is mostly aligned with the existing proposals [1] and [2]. The userspace agent manages all migratable user memory on the system and this can be transparent from the point of view of applications. To demote cold pages and promote hot pages, the userspace agent needs page access information. Because it is a system-wide tiering for user memory, the access information for both mapped and unmapped user pages is needed, and so are the physical page addresses. A combination of page table accessed-bit scanning and struct page scanning should be needed. Such page access monitoring should be efficient as well because the scans can be frequent. To return the page-level access information to the userspace, one proposal is to use tracepoint events. The userspace agent can then use BPF programs to collect such data and also apply customized filters when necessary. The userspace agent can also make use of hardware PMU events, for which the existing kernel support should be sufficient. The third area is the API support for migrating pages. The existing move_pages() syscall can be a candidate, though it is virtual-address based and cannot migrate unmapped pages. Is a physical-address based variant (e.g. move_pfns()), an acceptable proposal? [1] https://lore.kernel.org/lkml/9cd0dcde-f257-1b94-17d0-f2e24a3ce979@xxxxxxxxx/ [2] https://lore.kernel.org/patchwork/cover/1408180/ Thanks, Wei
