On Tue, May 3, 2022 at 12:12 PM Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> wrote: > > On Fri, 2022-04-29 at 19:10 -0700, Wei Xu wrote: > > The current kernel has the basic memory tiering support: Inactive > > pages on a higher tier NUMA node can be migrated (demoted) to a lower > > tier NUMA node to make room for new allocations on the higher tier > > NUMA node. Frequently accessed pages on a lower tier NUMA node can be > > migrated (promoted) to a higher tier NUMA node to improve the > > performance. > > > > A tiering relationship between NUMA nodes in the form of demotion path > > is created during the kernel initialization and updated when a NUMA > > node is hot-added or hot-removed. The current implementation puts all > > nodes with CPU into the top tier, and then builds the tiering hierarchy > > tier-by-tier by establishing the per-node demotion targets based on > > the distances between nodes. > > Thanks for making this proposal. It has many of the elements needed > for the tiering support. > > > > > The current memory tiering interface needs to be improved to address > > several important use cases: > > > > * The current tiering initialization code always initializes > > each memory-only NUMA node into a lower tier. But a memory-only > > NUMA node may have a high performance memory device (e.g. a DRAM > > device attached via CXL.mem or a DRAM-backed memory-only node on > > a virtual machine) and should be put into the top tier. > > > > * The current tiering hierarchy always puts CPU nodes into the top > > tier. But on a system with HBM (e.g. GPU memory) devices, these > > memory-only HBM NUMA nodes should be in the top tier, and DRAM nodes > > with CPUs are better to be placed into the next lower tier. > > > > * Also because the current tiering hierarchy always puts CPU nodes > > into the top tier, when a CPU is hot-added (or hot-removed) and > > triggers a memory node from CPU-less into a CPU node (or vice > > versa), the memory tiering hierarchy gets changed, even though no > > memory node is added or removed. This can make the tiering > > hierarchy much less stable. > > > > * A higher tier node can only be demoted to selected nodes on the > > next lower tier, not any other node from the next lower tier. This > > strict, hard-coded demotion order does not work in all use cases > > (e.g. some use cases may want to allow cross-socket demotion to > > another node in the same demotion tier as a fallback when the > > preferred demotion node is out of space), and has resulted in the > > feature request for an interface to override the system-wide, > > per-node demotion order from the userspace. > > > > * There are no interfaces for the userspace to learn about the memory > > tiering hierarchy in order to optimize its memory allocations. > > > > I'd like to propose revised memory tiering kernel interfaces based on > > the discussions in the threads: > > > > - https://lore.kernel.org/lkml/20220425201728.5kzm4seu7rep7ndr@offworld/T/ > > - https://lore.kernel.org/linux-mm/20220426114300.00003ad8@xxxxxxxxxx/t/ > > > > > > Sysfs Interfaces > > ================ > > > > * /sys/devices/system/node/memory_tiers > > > > Format: node list (one tier per line, in the tier order) > > > > When read, list memory nodes by tiers. > > > > When written (one tier per line), take the user-provided node-tier > > assignment as the new tiering hierarchy and rebuild the per-node > > demotion order. It is allowed to only override the top tiers, in > > which cases, the kernel will establish the lower tiers automatically. > > > > > > Kernel Representation > > ===================== > > > > * nodemask_t node_states[N_TOPTIER_MEMORY] > > > > Store all top-tier memory nodes. > > > > * nodemask_t memory_tiers[MAX_TIERS] > > > > Store memory nodes by tiers. > > > > * struct demotion_nodes node_demotion[] > > > > where: struct demotion_nodes { nodemask_t preferred; nodemask_t allowed; } > > > > For a node N: > > > > node_demotion[N].preferred lists all preferred demotion targets; > > > > node_demotion[N].allowed lists all allowed demotion targets > > (initialized to be all the nodes in the same demotion tier). > > > > I assume that the preferred list is auto-configured/initialized based on > NUMA distances. Not sure why "allowed" list is only to the same demotion > tier? For example, I think the default should be tier 0 should > is allowed to demote to tier 1 and tier 2, not just to tier 1. So if we > fail to demote to tier 1, we can demote to tier 2. I agree that we can allow demotion to go to all the lower tiers, not just the immediate next tier. I have mentioned the same idea as well when replying to Dan's comments. > Do you also expose the demotion preferred node and allowed > list via /sys/devices/system/node/memory_tiers, as you have done in the examples? To keep the memory tier sysfs minimal for now, I didn't propose exposing the demotion preferred/allowed list in /sys/devices/system/node/memory_tiers. But now I can see that in the way that the examples were presented, N_TOPTIER_MEMORY and node_demotion[] can be thought as part of the memory_tiers output, which is not the intention. > > Examples > > ======== > > > > * Example 2: > > Node 0 & 1 are DRAM nodes. > > Node 2 is a PMEM node and closer to node 0. > > > > Node 0 has node 2 as the preferred and only demotion target. > > > > Node 1 has no preferred demotion target, but can still demote > > to node 2. > > > > Set mempolicy to prevent cross-socket demotion and memory access, > > e.g. cpuset.mems=0,2 > > Do we expect to later allow configuration of the demotion list explicitly? > Something like: > > echo "demotion 0 1 1-3" > /sys/devices/system/node/memory_tiers > > to set demotion list for node 0, where preferred demote node is 1, > allowed demote node list is 1-3. IMHO, we'd better follow the allocation fallback order for the demotion node order in each tier and avoid userspace override of per-node demotion list. In general, I think we'd better keep the tier assignment of each node stable. If adding/changing one node can redefine the tiers of other nodes, it can make tier-based memory accounting very difficult. Overriding the per-node demotion list can have such undesirable side effects (if the per-node demotion list is used to redefine tiers). > Thanks. > > Tim > >
