On Wed, 2022-04-27 at 11:27 -0700, Wei Xu wrote: > On Tue, Apr 26, 2022 at 10:06 PM Aneesh Kumar K V > <aneesh.kumar@xxxxxxxxxxxxx> wrote: > > > > On 4/25/22 10:26 PM, Wei Xu wrote: > > > On Sat, Apr 23, 2022 at 8:02 PM ying.huang@xxxxxxxxx > > > <ying.huang@xxxxxxxxx> wrote: > > > > > > > > .... > > > > > > 2. For machines with PMEM installed in only 1 of 2 sockets, for example, > > > > > > > > Node 0 & 2 are cpu + dram nodes and node 1 are slow > > > > memory node near node 0, > > > > > > > > available: 3 nodes (0-2) > > > > node 0 cpus: 0 1 > > > > node 0 size: n MB > > > > node 0 free: n MB > > > > node 1 cpus: > > > > node 1 size: n MB > > > > node 1 free: n MB > > > > node 2 cpus: 2 3 > > > > node 2 size: n MB > > > > node 2 free: n MB > > > > node distances: > > > > node 0 1 2 > > > > 0: 10 40 20 > > > > 1: 40 10 80 > > > > 2: 20 80 10 > > > > > > > > We have 2 choices, > > > > > > > > a) > > > > node demotion targets > > > > 0 1 > > > > 2 1 > > > > > > > > b) > > > > node demotion targets > > > > 0 1 > > > > 2 X > > > > > > > > a) is good to take advantage of PMEM. b) is good to reduce cross-socket > > > > traffic. Both are OK as defualt configuration. But some users may > > > > prefer the other one. So we need a user space ABI to override the > > > > default configuration. > > > > > > I think 2(a) should be the system-wide configuration and 2(b) can be > > > achieved with NUMA mempolicy (which needs to be added to demotion). > > > > > > In general, we can view the demotion order in a way similar to > > > allocation fallback order (after all, if we don't demote or demotion > > > lags behind, the allocations will go to these demotion target nodes > > > according to the allocation fallback order anyway). If we initialize > > > the demotion order in that way (i.e. every node can demote to any node > > > in the next tier, and the priority of the target nodes is sorted for > > > each source node), we don't need per-node demotion order override from > > > the userspace. What we need is to specify what nodes should be in > > > each tier and support NUMA mempolicy in demotion. > > > > > > > I have been wondering how we would handle this. For ex: If an > > application has specified an MPOL_BIND policy and restricted the > > allocation to be from Node0 and Node1, should we demote pages allocated > > by that application > > to Node10? The other alternative for that demotion is swapping. So from > > the page point of view, we either demote to a slow memory or pageout to > > swap. But then if we demote we are also breaking the MPOL_BIND rule. > > IMHO, the MPOL_BIND policy should be respected and demotion should be > skipped in such cases. Such MPOL_BIND policies can be an important > tool for applications to override and control their memory placement > when transparent memory tiering is enabled. If the application > doesn't want swapping, there are other ways to achieve that (e.g. > mlock, disabling swap globally, setting memcg parameters, etc). > > > > The above says we would need some kind of mem policy interaction, but > > what I am not sure about is how to find the memory policy in the > > demotion path. > > This is indeed an important and challenging problem. One possible > approach is to retrieve the allowed demotion nodemask from > page_referenced() similar to vm_flags. This works for mempolicy in struct vm_area_struct, but not for that in struct task_struct. Mutiple threads in a process may have different mempolicy. Best Regards, Huang, Ying > > > > > Cross-socket demotion should not be too big a problem in practice > > > because we can optimize the code to do the demotion from the local CPU > > > node (i.e. local writes to the target node and remote read from the > > > source node). The bigger issue is cross-socket memory access onto the > > > demoted pages from the applications, which is why NUMA mempolicy is > > > important here. > > > > > > > > -aneesh
