On Thu, Nov 03, 2022 at 11:10:47AM -0700, Yosry Ahmed wrote: < snip > > > > > > I am also worry about that LRU stuff should be part of allocator > > > > > instead of higher level. > > > > > > > > I'm sorry, but that's not a reasonable objection. > > > > > > > > These patches implement a core feature of being a zswap backend, using > > > > standard LRU and locking techniques established by the other backends. > > > > > > > > I don't disagree that it would nicer if zswap had a strong abstraction > > > > for backend pages and a generalized LRU. But that is major surgery on > > > > a codebase of over 6,500 lines. It's not a reasonable ask to change > > > > all that first before implementing a basic feature that's useful now. > > > > > > With same logic, folks added the LRU logic into their allocators > > > without the effort considering moving the LRU into upper layer. > > > > > > And then trend is still going on since I have seen multiple times > > > people are trying to add more allocators. So if it's not a reasonable > > > ask to consier, we couldn't stop the trend in the end. > > > > So there is actually an ongoing effort to do that. Yosry and I have > > spent quite some time on coming up with an LRU design that's > > independent from compression policy over email and at Plumbers. > > > > My concern is more about the order of doing things: > > > > 1. The missing writeback support is a gaping hole in zsmalloc, which > > affects production systems. A generalized LRU list is a good idea, > > but it's a huge task that from a user pov really is not > > critical. Even from a kernel dev / maintainer POV, there are bigger > > fish to fry in the zswap code base and the backends than this. > > > > 2. Refactoring existing functionality is much easier than writing > > generalized code that simultaneously enables new behavior. zsmalloc > > is the most complex of our backends. To make its LRU writeback work > > we had to patch zswap's ->map ordering to accomodate it, e.g. Such > > tricky changes are easier to make and test incrementally. > > > > The generalized LRU project will hugely benefit from already having > > a proven writeback implementation in zsmalloc, because then all the > > requirements in zswap and zsmalloc will be in black and white. > > > > > > I get that your main interest is zram, and so this feature isn't of > > > > interest to you. But zram isn't the only user, nor is it the primary > > > > > > I am interest to the feature but my interest is more of general swap > > > layer to manage the LRU so that it could support any hierarchy among > > > swap devices, not only zswap. > > > > I think we're on the same page about the longer term goals. > > > > Yeah. As Johannes said, I was also recently looking into this. This > can also help solve other problems than consolidating implementations. > Currently if zswap rejects a page, it goes into swap, which is > more-or-less a violation of page LRUs since hotter pages that are more > recently reclaimed end up in swap (slow), while colder pages that were > reclaimed before are in zswap. Having a separate layer managing the > LRU of swap pages can also make sure this doesn't happen. True. > > More broadly, making zswap a separate layer from swap enables other > improvements such as using zswap regardless of the presence of a > backend swapfile and not consuming space in swapfiles if a page is in > zswap. Of course, this is a much larger surgery. If we could decouple the LRU writeback from zswap and supports compression without backing swapfile, sounds like becoming more of zram. ;-) > > I am intending to spend more time looking further into this, but other > things keep popping up :) Same with me. Thanks for looking it, Yosry!
