On Tue, Apr 04, 2023 at 11:31:08AM +0300, Mike Rapoport wrote: > On Fri, Mar 31, 2023 at 08:45:25PM +0900, Kyungsan Kim wrote: > > Thank you Mike Rapoport for participating discussion and adding your thought. > > > > >Hi, > > > > > >On Thu, Mar 23, 2023 at 07:51:05PM +0900, Kyungsan Kim wrote: > > >> I appreciate dan for the careful advice. > > >> > > >> >Kyungsan Kim wrote: > > >> >[..] > > >> >> >In addition to CXL memory, we may have other kind of memory in the > > >> >> >system, for example, HBM (High Bandwidth Memory), memory in FPGA card, > > >> >> >memory in GPU card, etc. I guess that we need to consider them > > >> >> >together. Do we need to add one zone type for each kind of memory? > > >> >> > > >> >> We also don't think a new zone is needed for every single memory > > >> >> device. Our viewpoint is the sole ZONE_NORMAL becomes not enough to > > >> >> manage multiple volatile memory devices due to the increased device > > >> >> types. Including CXL DRAM, we think the ZONE_EXMEM can be used to > > >> >> represent extended volatile memories that have different HW > > >> >> characteristics. > > >> > > > >> >Some advice for the LSF/MM discussion, the rationale will need to be > > >> >more than "we think the ZONE_EXMEM can be used to represent extended > > >> >volatile memories that have different HW characteristics". It needs to > > >> >be along the lines of "yes, to date Linux has been able to describe DDR > > >> >with NUMA effects, PMEM with high write overhead, and HBM with improved > > >> >bandwidth not necessarily latency, all without adding a new ZONE, but a > > >> >new ZONE is absolutely required now to enable use case FOO, or address > > >> >unfixable NUMA problem BAR." Without FOO and BAR to discuss the code > > >> >maintainability concern of "fewer degress of freedom in the ZONE > > >> >dimension" starts to dominate. > > >> > > >> One problem we experienced was occured in the combination of hot-remove and kerelspace allocation usecases. > > >> ZONE_NORMAL allows kernel context allocation, but it does not allow hot-remove because kernel resides all the time. > > >> ZONE_MOVABLE allows hot-remove due to the page migration, but it only allows userspace allocation. > > >> Alternatively, we allocated a kernel context out of ZONE_MOVABLE by adding GFP_MOVABLE flag. > > >> In case, oops and system hang has occasionally occured because ZONE_MOVABLE can be swapped. > > >> We resolved the issue using ZONE_EXMEM by allowing seletively choice of the two usecases. > > >> As you well know, among heterogeneous DRAM devices, CXL DRAM is the first PCIe basis device, which allows hot-pluggability, different RAS, and extended connectivity. > > >> So, we thought it could be a graceful approach adding a new zone and separately manage the new features. > > > > > >This still does not describe what are the use cases that require having > > >kernel allocations on CXL.mem. > > > > > >I believe it's important to start with explanation *why* it is important to > > >have kernel allocations on removable devices. > > > > > > > In general, a memory system with DDR/CXL DRAM will have near/far memory. > > And, we think kernel already includes memory tiering solutions - Meta TPP, zswap, and pagecache. > > Some kernel contexts would prefer fast memory. For example, a hot data with time locality or a data for fast processing such as metadata or indexing. > > Others would enough with slow memory. For example, a zswap page which is being used while swapping. > > The point of zswap IIUC is to have small and fast swap device and > compression is required to better utilize DRAM capacity at expense of CPU > time. > > Presuming CXL memory will have larger capacity than DRAM, why not skip the > compression and use CXL as a swap device directly? I like to shy away from saying CXL memory should be used for swap. I see a swap device as storing pages in a manner that is no longer directly addressable by the cpu. Migrating pages to a CXL device is a reasonable approach and I believe we have the ability to do this in the page reclaim code. > > And even supposing there's an advantage in putting zswap on CXL memory, > why that can be done with node-based APIs but warrants a new zone? > > -- > Sincerely yours, > Mike.
