On Tue, Apr 04, 2023 at 05:58:05PM +0000, Adam Manzanares wrote: > On Tue, Apr 04, 2023 at 11:31:08AM +0300, Mike Rapoport wrote: > > > > 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. > The argument is "why do you need swap if memory itself is elastic", and I think there are open questions about how performant using large amounts of high-latency memory is. Think 1us-1.5us+ cross-rack attached memory. Does it make sense to use that as CPU-addressible and migrate it on first use? Isn't that just swap with more steps? What happens if we just use it as swap, is the performance all that different? I think there's a reasonable argument for exploring the idea at the higher ends of the latency spectrum. And the simplicity of using an existing system (swap) to implement a form of proto-tiering is rather attractive in my opinion. ~Gregory
