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

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Date:   Tue, 26 Mar 2019 19:37:31 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     mgorman@...hsingularity.net, riel@...riel.com, hannes@...xchg.org,
        akpm@...ux-foundation.org, dave.hansen@...el.com,
        keith.busch@...el.com, dan.j.williams@...el.com,
        fengguang.wu@...el.com, fan.du@...el.com, ying.huang@...el.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/10] Another Approach to Use PMEM as NUMA Node

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?
-- 
Michal Hocko
SUSE Labs