On Tue 26-03-19 19:58:56, Yang Shi wrote: > > > On 3/26/19 11:37 AM, Michal Hocko wrote: > > On Tue 26-03-19 11:33:17, Yang Shi wrote: > > > > > > On 3/26/19 6:58 AM, Michal Hocko wrote: > > > > On Sat 23-03-19 12:44:25, Yang Shi wrote: > > > > > With Dave Hansen's patches merged into Linus's tree > > > > > > > > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c221c0b0308fd01d9fb33a16f64d2fd95f8830a4 > > > > > > > > > > PMEM could be hot plugged as NUMA node now. But, how to use PMEM as NUMA node > > > > > effectively and efficiently is still a question. > > > > > > > > > > There have been a couple of proposals posted on the mailing list [1] [2]. > > > > > > > > > > The patchset is aimed to try a different approach from this proposal [1] > > > > > to use PMEM as NUMA nodes. > > > > > > > > > > The approach is designed to follow the below principles: > > > > > > > > > > 1. Use PMEM as normal NUMA node, no special gfp flag, zone, zonelist, etc. > > > > > > > > > > 2. DRAM first/by default. No surprise to existing applications and default > > > > > running. PMEM will not be allocated unless its node is specified explicitly > > > > > by NUMA policy. Some applications may be not very sensitive to memory latency, > > > > > so they could be placed on PMEM nodes then have hot pages promote to DRAM > > > > > gradually. > > > > Why are you pushing yourself into the corner right at the beginning? If > > > > the PMEM is exported as a regular NUMA node then the only difference > > > > should be performance characteristics (module durability which shouldn't > > > > play any role in this particular case, right?). Applications which are > > > > already sensitive to memory access should better use proper binding already. > > > > Some NUMA topologies might have quite a large interconnect penalties > > > > already. So this doesn't sound like an argument to me, TBH. > > > The major rationale behind this is we assume the most applications should be > > > sensitive to memory access, particularly for meeting the SLA. The > > > applications run on the machine may be agnostic to us, they may be sensitive > > > or non-sensitive. But, assuming they are sensitive to memory access sounds > > > safer from SLA point of view. Then the "cold" pages could be demoted to PMEM > > > nodes by kernel's memory reclaim or other tools without impairing the SLA. > > > > > > If the applications are not sensitive to memory access, they could be bound > > > to PMEM or allowed to use PMEM (nice to have allocation on DRAM) explicitly, > > > then the "hot" pages could be promoted to DRAM. > > Again, how is this different from NUMA in general? > > It is still NUMA, users still can see all the NUMA nodes. No, Linux NUMA implementation makes all numa nodes available by default and provides an API to opt-in for more fine tuning. What you are suggesting goes against that semantic and I am asking why. How is pmem NUMA node any different from any any other distant node in principle? -- Michal Hocko SUSE Labs
