On Thu 10-01-19 11:25:56, Jerome Glisse wrote: > On Fri, Dec 28, 2018 at 08:52:24PM +0100, Michal Hocko wrote: > > [Ccing Mel and Andrea] > > > > On Fri 28-12-18 21:31:11, Wu Fengguang wrote: > > > > > > I haven't looked at the implementation yet but if you are proposing a > > > > > > special cased zone lists then this is something CDM (Coherent Device > > > > > > Memory) was trying to do two years ago and there was quite some > > > > > > skepticism in the approach. > > > > > > > > > > It looks we are pretty different than CDM. :) > > > > > We creating new NUMA nodes rather than CDM's new ZONE. > > > > > The zonelists modification is just to make PMEM nodes more separated. > > > > > > > > Yes, this is exactly what CDM was after. Have a zone which is not > > > > reachable without explicit request AFAIR. So no, I do not think you are > > > > too different, you just use a different terminology ;) > > > > > > Got it. OK.. The fall back zonelists patch does need more thoughts. > > > > > > In long term POV, Linux should be prepared for multi-level memory. > > > Then there will arise the need to "allocate from this level memory". > > > So it looks good to have separated zonelists for each level of memory. > > > > Well, I do not have a good answer for you here. We do not have good > > experiences with those systems, I am afraid. NUMA is with us for more > > than a decade yet our APIs are coarse to say the least and broken at so > > many times as well. Starting a new API just based on PMEM sounds like a > > ticket to another disaster to me. > > > > I would like to see solid arguments why the current model of numa nodes > > with fallback in distances order cannot be used for those new > > technologies in the beginning and develop something better based on our > > experiences that we gain on the way. > > I see several issues with distance. First it does fully abstract the > underlying topology and this might be problematic, for instance if > you memory with different characteristic in same node like persistent > memory connected to some CPU then it might be faster for that CPU to > access that persistent memory has it has dedicated link to it than to > access some other remote memory for which the CPU might have to share > the link with other CPUs or devices. > > Second distance is no longer easy to compute when you are not trying > to answer what is the fastest memory for CPU-N but rather asking what > is the fastest memory for CPU-N and device-M ie when you are trying to > find the best memory for a group of CPUs/devices. The answer can > changes drasticly depending on members of the groups. While you might be right, I would _really_ appreciate to start with a simpler model and go to a more complex one based on realy HW and real experiences than start with an overly complicated and over engineered approach from scratch. > Some advance programmer already do graph matching ie they match the > graph of their program dataset/computation with the topology graph > of the computer they run on to determine what is best placement both > for threads and memory. And those can still use our mempolicy API to describe their needs. If existing API is not sufficient then let's talk about which pieces are missing. > > I would be especially interested about a possibility of the memory > > migration idea during a memory pressure and relying on numa balancing to > > resort the locality on demand rather than hiding certain NUMA nodes or > > zones from the allocator and expose them only to the userspace. > > For device memory we have more things to think of like: > - memory not accessible by CPU > - non cache coherent memory (yet still useful in some case if > application explicitly ask for it) > - device driver want to keep full control over memory as older > application like graphic for GPU, do need contiguous physical > memory and other tight control over physical memory placement Again, I believe that HMM is to target those non-coherent or non-accessible memory and I do not think it is helpful to put them into the mix here. > So if we are talking about something to replace NUMA i would really > like for that to be inclusive of device memory (which can itself be > a hierarchy of different memory with different characteristics). I think we should build on the existing NUMA infrastructure we have. Developing something completely new is not going to happen anytime soon and I am not convinced the result would be that much better either. -- Michal Hocko SUSE Labs
