Messages in this thread | | | From | Yang Shi <> | Subject | Re: [v2 RFC PATCH 0/9] Another Approach to Use PMEM as NUMA Node | Date | Thu, 18 Apr 2019 09:24:35 -0700 |
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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"?
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