On Wed 22-02-17 09:59:15, Jerome Glisse wrote: > On Wed, Feb 22, 2017 at 10:29:21AM +0100, Michal Hocko wrote: > > On Tue 21-02-17 18:39:17, Anshuman Khandual wrote: > > > On 02/17/2017 07:02 PM, Mel Gorman wrote: > > [...] > > > [...] > > > These are the reasons which prohibit the use of HMM for coherent > > > addressable device memory purpose. > > > > > [...] > > > (3) Application cannot directly allocate into device memory from user > > > space using existing memory related system calls like mmap() and mbind() > > > as the device memory hides away in ZONE_DEVICE. > > > > Why cannot the application simply use mmap on the device file? > > This has been said before but we want to share the address space this do > imply that you can not rely on special allocator. For instance you can > have an application that use a library and the library use the GPU but > the application is un-aware and those any data provided by the application > to the library will come from generic malloc (mmap anonymous or from > regular file). > > Currently what happens is that the library reallocate memory through > special allocator and copy thing. Not only does this waste memory (the > new memory is often regular memory too) but you also have to paid the > cost of copying GB of data. > > Last bullet to this, is complex data structure (list, tree, ...) having > to go through special allocator means you have re-build the whole structure > with the duplicated memory. > > > Allowing to directly use memory allocated from malloc (mmap anonymous > private or from a regular file) avoid the copy operation and the complex > duplication of data structure. Moving the dataset to the GPU is then a > simple memory migration from kernel point of view. > > This is share address space without special allocator is mandatory in new > or future standard such as OpenCL, Cuda, C++, OpenMP, ... some other OS > already have this and the industry want it. So the questions is do we > want to support any of this, do we care about GPGPU ? > > > I believe we want to support all this new standard but maybe i am the > only one. > > In HMM case i have the extra painfull fact that the device memory is > not accessible by the CPU. For CDM on contrary, CPU can access in a > cache coherent way the device memory and all operation behave as regular > memory (thing like atomic operation for instance). > > > I hope this clearly explain why we can no longer rely on dedicated/ > specialized memory allocator. Yes this clarifies this point. Thanks for the information which would be really helpful in the initial description. Maybe I've just missed it, though. -- Michal Hocko SUSE Labs -- 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>
