On Tue, Dec 04, 2018 at 12:11:42PM -0700, Logan Gunthorpe wrote: > > > On 2018-12-04 11:57 a.m., Jerome Glisse wrote: > >> That sounds needlessly restrictive. Let the kernel arbitrate what > >> memory an application gets, don't design a system where applications > >> are hard coded to a memory type. Applications can hint, or optionally > >> specify an override and the kernel can react accordingly. > > > > You do not want to randomly use non cache coherent memory inside your > > application :) This is not gonna go well with C++ or atomic :) Yes they > > are legitimate use case where application can decide to give up cache > > coherency temporarily for a range of virtual address. But the application > > needs to understand what it is doing and opt in to do that knowing full > > well that. The version thing allows for scenario like. You do not have > > to define a new version with every new type of memory. If your new memory > > has all the properties of v1 than you expose it as v1 and old application > > on the new platform will use your new memory type being non the wiser. > > I agree with Dan and the general idea that this version thing is really > ugly. Define some standard attributes so the application can say "I want > cache-coherent, high bandwidth memory". If there's some future > new-memory attribute, then the application needs to know about it to > request it. So version is a bad prefix, what about type, prefixing target with a type id. So that application that are looking for a certain type of memory (which has a set of define properties) can select them. Having a type file inside the directory and hopping application will read that sysfs file is a recipies for failure from my point of view. While having it in the directory name is making sure that the application has some idea of what it is doing. > > Also, in the same vein, I think it's wrong to have the API enumerate all > the different memory available in the system. The API should simply > allow userspace to say it wants memory that can be accessed by a set of > initiators with a certain set of attributes and the bind call tries to > fulfill that or fallback on system memory/hmm migration/whatever. We have existing application that use topology today to partition their workload and do load balancing. Those application leverage the fact that they are only running on a small set of known platform with known topology here i want to provide a common API so that topology can be queried in a standard by application. Yes basic application will not leverage all this information and will be happy enough with give me memory that will be fast for initiator A and B. That can easily be implemented inside userspace library which dumbs down the topology on behalf of application. I believe that proposing a new infrastructure should allow for maximum expressiveness. The HMS API in this proposal allow to express any kind of directed graph hence i do not see any limitation going forward. At the same time userspace library can easily dumbs this down for average Joe/Jane application. Cheers, Jérôme
