On Thu, Apr 28, 2022 at 7:59 PM Wei Xu <weixugc@xxxxxxxxxx> wrote: > > On Thu, Apr 28, 2022 at 7:21 PM ying.huang@xxxxxxxxx > <ying.huang@xxxxxxxxx> wrote: > > > > On Fri, 2022-04-29 at 11:27 +1000, Alistair Popple wrote: > > > On Friday, 29 April 2022 3:14:29 AM AEST Yang Shi wrote: > > > > On Wed, Apr 27, 2022 at 9:11 PM Wei Xu <weixugc@xxxxxxxxxx> wrote: > > > > > > > > > > On Wed, Apr 27, 2022 at 5:56 PM ying.huang@xxxxxxxxx > > > > > <ying.huang@xxxxxxxxx> wrote: > > > > > > > > > > > > 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. > > > > > > > > > > From vm_area_struct, we can get to mm_struct and then to the owner > > > > > task_struct, which has the process mempolicy. > > > > > > > > > > It is indeed a problem when a page is shared by different threads or > > > > > different processes that have different thread default mempolicy > > > > > values. > > > > > > > > Sorry for chiming in late, this is a known issue when we were working > > > > on demotion. Yes, it is hard to handle the shared pages and multi > > > > threads since mempolicy is applied to each thread so each thread may > > > > have different mempolicy. And I don't think this case is rare. And not > > > > only mempolicy but also may cpuset settings cause the similar problem, > > > > different threads may have different cpuset settings for cgroupv1. > > > > > > > > If this is really a problem for real life workloads, we may consider > > > > tackling it for exclusively owned pages first. Thanks to David's > > > > patches, now we have dedicated flags to tell exclusively owned pages. > > > > > > One of the problems with demotion when I last looked is it does almost exactly > > > the opposite of what we want on systems like POWER9 where GPU memory is a > > > CPU-less memory node. > > > > > > On those systems users tend to use MPOL_BIND or MPOL_PREFERRED to allocate > > > memory on the GPU node. Under memory pressure demotion should migrate GPU > > > allocations to the CPU node and finally other slow memory nodes or swap. > > > > > > Currently though demotion considers the GPU node slow memory (because it is > > > CPU-less) so will demote CPU memory to GPU memory which is a limited resource. > > > And when trying to allocate GPU memory with MPOL_BIND/PREFERRED it will swap > > > everything to disk rather than demote to CPU memory (which would be preferred). > > > > > > I'm still looking at this series but as I understand it it will help somewhat > > > because we could make GPU memory the top-tier so nothing gets demoted to it. > > > > Yes. If we have a way to put GPU memory in top-tier (tier 0) and > > CPU+DRAM in tier 1. Your requirement can be satisfied. One way is to > > override the auto-generated demotion order via some user space tool. > > Another way is to change the GPU driver (I guess where the GPU memory is > > enumerated and onlined?) to change the tier of GPU memory node. > > > > > However I wouldn't want to see demotion skipped entirely when a memory policy > > > such as MPOL_BIND is specified. For example most memory on a GPU node will have > > > some kind of policy specified and IMHO it would be better to demote to another > > > node in the mempolicy nodemask rather than going straight to swap, particularly > > > as GPU memory capacity tends to be limited in comparison to CPU memory > > > capacity. > > > > > > > > Can you use MPOL_PREFERRED? Even if we enforce MPOL_BIND as much as > > possible, we will not stop demoting from GPU to DRAM with > > MPOL_PREFERRED. And in addition to demotion, allocation fallbacking can > > be used too to avoid allocation latency caused by demotion. > > I expect that MPOL_BIND can be used to either prevent demotion or > select a particular demotion node/nodemask. It all depends on the > mempolicy nodemask specified by MPOL_BIND. Preventing demotion doesn't make too much sense to me IMHO. But I tend to agree the demotion target should be selected from the nodemask. I think this could follow what numa fault does. > > > This is another example of a system with 3 tiers if PMEM is installed in > > this machine too. > > > > Best Regards, > > Huang, Ying > > > > > > > On the other hand, it can already support most interesting use cases > > > > > for demotion (e.g. selecting the demotion node, mbind to prevent > > > > > demotion) by respecting cpuset and vma mempolicies. > > > > > > > > > > > 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 > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > >
