On 02/22/2017 10:24 PM, Michal Hocko wrote: > 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. Sure, will add this into the patch description. -- 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>
