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

[Yang Shi]

On 4/17/19 10:51 AM, Michal Hocko wrote:
> On Wed 17-04-19 10:26:05, Yang Shi wrote:
> > On 4/17/19 9:39 AM, Michal Hocko wrote:
> > > On Wed 17-04-19 09:37:39, Keith Busch wrote:
> > > > On Wed, Apr 17, 2019 at 05:39:23PM +0200, Michal Hocko wrote:
> > > > > On Wed 17-04-19 09:23:46, Keith Busch wrote:
> > > > > > On Wed, Apr 17, 2019 at 11:23:18AM +0200, Michal Hocko wrote:
> > > > > > > On Tue 16-04-19 14:22:33, Dave Hansen wrote:
> > > > > > > > Keith Busch had a set of patches to let you specify the demotion order
> > > > > > > > via sysfs for fun.  The rules we came up with were:
> > > > > > > I am not a fan of any sysfs "fun"
> > > > > > I'm hung up on the user facing interface, but there should be some way a
> > > > > > user decides if a memory node is or is not a migrate target, right?
> > > > > Why? Or to put it differently, why do we have to start with a user
> > > > > interface at this stage when we actually barely have any real usecases
> > > > > out there?
> > > > The use case is an alternative to swap, right? The user has to decide
> > > > which storage is the swap target, so operating in the same spirit.
> > > I do not follow. If you use rebalancing you can still deplete the memory
> > > and end up in a swap storage. If you want to reclaim/swap rather than
> > > rebalance then you do not enable rebalancing (by node_reclaim or similar
> > > mechanism).
> > I'm a little bit confused. Do you mean just do *not* do reclaim/swap in
> > rebalancing mode? If rebalancing is on, then node_reclaim just move the
> > pages around nodes, then kswapd or direct reclaim would take care of swap?
> Yes, that was the idea I wanted to get through. Sorry if that was not
> really clear.
>
> > If so the node reclaim on PMEM node may rebalance the pages to DRAM node?
> > Should this be allowed?
> Why it shouldn't? If there are other vacant Nodes to absorb that memory
> then why not use it?
>
> > I think both I and Keith was supposed to treat PMEM as a tier in the reclaim
> > hierarchy. The reclaim should push inactive pages down to PMEM, then swap.
> > So, PMEM is kind of a "terminal" node. So, he introduced sysfs defined
> > target node, I introduced N_CPU_MEM.
> I understand that. And I am trying to figure out whether we really have
> to tream PMEM specially here. Why is it any better than a generic NUMA
> rebalancing code that could be used for many other usecases which are
> not PMEM specific. If you present PMEM as a regular memory then also use
> it as a normal memory.

This also makes some sense. We just look at PMEM from different point of 
view. Taking into account the performance disparity may outweigh 
treating it as a normal memory in this patchset.

A ridiculous idea, may we have two modes? One for "rebalancing", the 
other for "demotion"?