On Sun, Mar 26, 2023 at 6:24 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: > > Chris Li <chrisl@xxxxxxxxxx> writes: > > > On Fri, Mar 24, 2023 at 12:28:31AM -0700, Yosry Ahmed wrote: > >> > In fact, I just suggest to use the minimal design on top of the current > >> > implementation as the first step. Then, you can improve it step by > >> > step. > >> > > >> > The first step could be the minimal effort to implement indirection > >> > layer and moving swapped pages between swap implementations. Based on > >> > that, you can build other optimizations, such as pulling swap counting > >> > to the swap core. For each step, we can evaluate the gain and cost with > >> > data. > >> > >> Right, I understand that, but to implement the indirection layer on > >> top of the current implementation, then we will need to support using > >> zswap without a backing swap device. In order to do this without > > > > Agree with Ying on the minimal approach here as well. > > > > There are two ways to approach this. > > > > 1) Forget zswap, make a minimal implementation to move the page between > > two swapfile device. It can be swapfile back to two loop back files. > > > > Any indirect layer you design will need to convert this usage case > > any way. > > > > 2) Make zswap work without a swapfile. > > You can implement the zswap on a fake ghosts swap file. > > > > If you keep the zswap as frontswap, just make zswap can work without > > a real swapfile. > > > > Make that as your first minimal step. Then it does not need to touch > > the swap count changes. > > > > I view make that step is independent of moving pages between swap device. > > > > That patch exists and I consider it has value to some users. > > This sounds like an even smaller approach as the first step. Further > improvement can be built on top of it. I am not sure how this would be a step towards the abstraction goal we have been discussing. We have been discussing starting out with a minimal indirection layer, in the shape of an xarray that maps a swap ID to a swap entry, and that can be disabled with a config option. For such a design to work, we have to implement swap entry management & swap counting in zswap, right? Am I missing something? > > Best Regards, > Huang, Ying > > >> > Anyway, I don't think you can just implement all your final solution in > >> > one step. And, I think the minimal design suggested could be a starting > >> > point. > >> > >> I agree that's a great point, I am just afraid that we will avoid > >> implementing that full final solution and instead do a lot of work > >> inside zswap to make up for the difference (e.g. swap entry > >> management, swap counting). Also, that work in zswap may end up being > >> unacceptable due to the maintenance burden and/or complexity. > > > > If you do either 1) or 2), you can keep these two paths separate. > > > > Even if you want to move the page between zswap and swapfile. > > > > Idea 3) > > You don't have to change the swap count code, you can do a > > minimal change moves the page between zswap and another block > > device. That way you can get two differenet swap entry with > > existing code. > > > > Chris >
