On Wed, Apr 26, 2023 at 9:30 PM David Rientjes <rientjes@xxxxxxxxxx> wrote: > > Hi everybody, > > As requested, sending along a last minute topic suggestion for > consideration for LSF/MM/BPF 2023 :) > > For a sizable set of emerging technologies, memory tiering presents one of > the most formidable challenges and exicting opportunities for the MM > subsystem today. > > "Memory tiering" can mean many different things based on the user: from > traditional every day NUMA, to swap (to zswap), to NVDIMMs, to HBM, to > locally attached CXL memory, to memory borrowing over PCIe, to memory > pooling with disaggregation, and beyond. > > Just as NUMA started out only being useful for the supercomputers, memory > tiering will likely evolve over the next five years to take on an > expanding set of use cases, and likely with rapidly increasing adoption > even beyond hyperscalers. > > I think a discussion about memory tiering would be highly valuable. A few > key questions that I think can drive this discussion: > > - What are the various form factors that must be supported as short-term > goals as well as need to be supported 5+ years into the future? > > - What incremental changes need to be made on top of NUMA support to > fully support the wide range of use cases that will be coming? (Is > memory tiering support built entirely upon NUMA?) > > - What is the minimum viable *default* support that the MM subsystem > should provide for tiered configs? What are the set of optimizations > that should be left to userspace or BPF to control? > > - What are the various page promotion technqiues that we must plan for > beyond traditional NUMA balancing that will allow us to exploit > hardware innovation? > > (And I'm sure there are more topics of discussion that others would > readily add. It would be great to have additional ideas in replies.) > > A key challenge in all of this is to make memory tiering support in the > upstream kernel compatible with the roadmaps of various CPU vendors. A > key goal is to ensure the end user benefits from all of this rapid > innovation with generalized support that is well abstracted and allows for > extensibility. Thank you for bringing this one up. Memory tiering is a very important topic that should definitely be discussed. I'm especially interested in the userspace control part (which I proposed as a separate topic, but happy to see it addressed as part of this discussion too, as that is where the motivation originally came from). With the increased complexity introduced by memory tiers, is it still possible to provide a one-size-fits-all default? If there is such a default, is it accurately represented by the current model of NUMA nodes, where pages will be demoted to a slower tier as a 'reclaim' operation (e.g. you basically map a global LRU model on to tiers of increased latency)? Are there reasons to break that model, and should applications be able to do that? Is the current mempolicy/madvise model sufficient? - Frank
