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

[Yang Shi]

On 3/27/19 10:34 AM, Dan Williams wrote:
> On Wed, Mar 27, 2019 at 2:01 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > 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?
> Agree. It's just another NUMA node and shouldn't be special cased.
> Userspace policy can choose to avoid it, but typical node distance
> preference should otherwise let the kernel fall back to it as
> additional memory pressure relief for "near" memory.

In ideal case, yes, I agree. However, in real life world the performance 
is a concern. It is well-known that PMEM (not considering NVDIMM-F or 
HBM) has higher latency and lower bandwidth. We observed much higher 
latency on PMEM than DRAM with multi threads.

In real production environment we don't know what kind of applications 
would end up on PMEM (DRAM may be full, allocation fall back to PMEM) 
then have unexpected performance degradation. I understand to have 
mempolicy to choose to avoid it. But, there might be hundreds or 
thousands of applications running on the machine, it sounds not that 
feasible to me to have each single application set mempolicy to avoid it.

So, I think we still need a default allocation node mask. The default 
value may include all nodes or just DRAM nodes. But, they should be able 
to be override by user globally, not only per process basis.

Due to the performance disparity, currently our usecases treat PMEM as 
second tier memory for demoting cold page or binding to not memory 
access sensitive applications (this is the reason for inventing a new 
mempolicy) although it is a NUMA node.

Thanks,
Yang