Re: [RFC PATCH 0/10] Another Approach to Use PMEM as NUMA Node

Michal Hocko
2019-03-27T09:01:04Z

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