>On Fri, Mar 31, 2023 at 08:42:20PM +0900, Kyungsan Kim wrote: >> Given our experiences/design and industry's viewpoints/inquiries, >> I will prepare a few slides in the session to explain >> 1. Usecase - user/kernespace memory tiering for near/far placement, memory virtualization between hypervisor/baremetal OS >> 2. Issue - movability(movable/unmovable), allocation(explicit/implicit), migration(intented/unintended) >> 3. HW - topology(direct, switch, fabric), feature(pluggability,error-handling,etc) > >I think you'll find everybody else in the room understands these issues >rather better than you do. This is hardly the first time that we've >talked about CXL, and CXL is not the first time that people have >proposed disaggregated memory, nor heterogenous latency/bandwidth >systems. All the previous attempts have failed, and I expect this >one to fail too. Maybe there's something novel that means this time >it really will work, so any slides you do should focus on that. > >A more profitable discussion might be: > >1. Should we have the page allocator return pages from CXL or should > CXL memory be allocated another way? I think yes. Using CXL DRAM as System RAM interface would be the primary use case in real-world application in regards to compatibility. So, on the System RAM interface, we think it should be managed by Linux MM subsystem. (Node - Zonelist - buddy page allocator) >2. Should there be a way for userspace to indicate that it prefers CXL > memory when it calls mmap(), or should it always be at the discretion > of the kernel? I think yes. Both implcit and explict ways are meaningful for users on a different purpose. The dynamic performance variation of CXL DRAM is likely bigger than other memory types due to the topology expansion and link negotiation. I think it strengthens the needs. >3. Do we continue with the current ZONE_DEVICE model, or do we come up > with something new? In fact, ZONE_DEVICE was the our first candidate for CXL DRAM. But because ZONE_DEVICE is not managed by buddy, we thought it does not fit to provide System RAM interface.
