On Mon, 2017-05-01 at 13:41 -0700, John Hubbard wrote: > On 04/19/2017 12:52 AM, Balbir Singh wrote: > > This is a request for comments on the discussed approaches > > for coherent memory at mm-summit (some of the details are at > > https://lwn.net/Articles/717601/). The latest posted patch > > series is at https://lwn.net/Articles/713035/. I am reposting > > this as RFC, Michal Hocko suggested using HMM for CDM, but > > we believe there are stronger reasons to use the NUMA approach. > > The earlier patches for Coherent Device memory were implemented > > and designed by Anshuman Khandual. > > > > Hi Balbir, > > Although I think everyone agrees that in the [very] long term, these > hardware-coherent nodes probably want to be NUMA nodes, in order to decide what to > code up over the next few years, we need to get a clear idea of what has to be done > for each possible approach. > > Here, the CDM discussion is falling just a bit short, because it does not yet > include the whole story of what we would need to do. Earlier threads pointed this > out: the idea started as a large patchset RFC, but then, "for ease of review", it > got turned into a smaller RFC, which loses too much context. Hi, John I thought I explained the context, but I'll try again. I see the whole solution as a composite of the following primitives: 1. Enable hotplug of CDM nodes 2. Isolation of CDM memory 3. Migration to/from CDM memory 4. Performance enhancements for migration The RFC here is for (2) above. (3) is handled by HMM and (4) is being discussed in the community. I think the larger goals are same as HMM, except that we don't need unaddressable memory, since the memory is cache coherent. > > So, I'd suggest putting together something more complete, so that it can be fairly > compared against the HMM-for-hardware-coherent-nodes approach. > Since I intend to reuse bits of HMM, I am not sure if I want to repost those patches as a part of my RFC. I hope my answers make sense, the goal is to reuse as much of what is available. From a user perspective 1. We see no new interface being added in either case, the programming model would differ though 2. We expect the programming model to be abstracted behind a user space framework, potentially like CUDA or CXL > > > Jerome posted HMM-CDM at https://lwn.net/Articles/713035/. > > The patches do a great deal to enable CDM with HMM, but we > > still believe that HMM with CDM is not a natural way to > > represent coherent device memory and the mm will need > > to be audited and enhanced for it to even work. > > That is also true for the CDM approach. Specifically, in order for this to be of any > use to device drivers, we'll need the following: > Since Reza answered these questions, I'll skip them in this email Thanks for the review! Balbir Singh -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
