Re: [v2 RFC PATCH 0/9] Another Approach to Use PMEM as NUMA Node

From: Yang Shi
Date: Mon Apr 15 2019 - 20:09:18 EST

On 4/12/19 1:47 AM, Michal Hocko wrote:
On Thu 11-04-19 11:56:50, Yang Shi wrote:
Basically, the approach is aimed to spread data from DRAM (closest to local
CPU) down further to PMEM and disk (typically assume the lower tier storage
is slower, larger and cheaper than the upper tier) by their hotness. The
patchset tries to achieve this goal by doing memory promotion/demotion via
NUMA balancing and memory reclaim as what the below diagram shows:

DRAM <--> PMEM <--> Disk
^ ^

When DRAM has memory pressure, demote pages to PMEM via page reclaim path.
Then NUMA balancing will promote pages to DRAM as long as the page is referenced
again. The memory pressure on PMEM node would push the inactive pages of PMEM
to disk via swap.

The promotion/demotion happens only between "primary" nodes (the nodes have
both CPU and memory) and PMEM nodes. No promotion/demotion between PMEM nodes
and promotion from DRAM to PMEM and demotion from PMEM to DRAM.

The HMAT is effectively going to enforce "cpu-less" nodes for any memory range
that has differentiated performance from the conventional memory pool, or
differentiated performance for a specific initiator, per Dan Williams. So,
assuming PMEM nodes are cpuless nodes sounds reasonable.

However, cpuless nodes might be not PMEM nodes. But, actually, memory
promotion/demotion doesn't care what kind of memory will be the target nodes,
it could be DRAM, PMEM or something else, as long as they are the second tier
memory (slower, larger and cheaper than regular DRAM), otherwise it sounds
pointless to do such demotion.

Defined "N_CPU_MEM" nodemask for the nodes which have both CPU and memory in
order to distinguish with cpuless nodes (memory only, i.e. PMEM nodes) and
memoryless nodes (some architectures, i.e. Power, may have memoryless nodes).
Typically, memory allocation would happen on such nodes by default unless
cpuless nodes are specified explicitly, cpuless nodes would be just fallback
nodes, so they are also as known as "primary" nodes in this patchset. With
two tier memory system (i.e. DRAM + PMEM), this sounds good enough to
demonstrate the promotion/demotion approach for now, and this looks more
architecture-independent. But it may be better to construct such node mask
by reading hardware information (i.e. HMAT), particularly for more complex
memory hierarchy.
I still believe you are overcomplicating this without a strong reason.
Why cannot we start simple and build from there? In other words I do not
think we really need anything like N_CPU_MEM at all.

In this patchset N_CPU_MEM is used to tell us what nodes are cpuless nodes. They would be the preferred demotion target. Of course, we could rely on firmware to just demote to the next best node, but it may be a "preferred" node, if so I don't see too much benefit achieved by demotion. Am I missing anything?

I would expect that the very first attempt wouldn't do much more than
migrate to-be-reclaimed pages (without an explicit binding) with a

Do you mean respect mempolicy or cpuset when doing demotion? I was wondering this, but I didn't do so in the current implementation since it may need walk the rmap to retrieve the mempolicy in the reclaim path. Is there any easier way to do so?

very optimistic allocation strategy (effectivelly GFP_NOWAIT) and if

Yes, this has been done in this patchset.

that fails then simply give up. All that hooked essentially to the
node_reclaim path with a new node_reclaim mode so that the behavior
would be opt-in. This should be the most simplistic way to start AFAICS
and something people can play with without risking regressions.

I agree it is safer to start with node reclaim. Once it is stable enough and we are confident enough, it can be extended to global reclaim.

Once we see how that behaves in the real world and what kind of corner
case user are able to trigger then we can build on top. E.g. do we want
to migrate from cpuless nodes as well? I am not really sure TBH. On one
hand why not if other nodes are free to hold that memory? Swap out is
more expensive. Anyway this is kind of decision which would rather be
shaped on an existing experience rather than ad-hoc decistion right now.

I do agree.

I would also not touch the numa balancing logic at this stage and rather
see how the current implementation behaves.

I agree we would prefer start from something simpler and see how it works.

The "twice access" optimization is aimed to reduce the PMEM bandwidth burden since the bandwidth of PMEM is scarce resource. I did compare "twice access" to "no twice access", it does save a lot bandwidth for some once-off access pattern. For example, when running stress test with mmtest's usemem-stress-numa-compact. The kernel would promote ~600,000 pages with "twice access" in 4 hours, but it would promote ~80,000,000 pages without "twice access".