On Tue, Jan 28, 2025 at 11:29:07AM +0100, Vitaly Wool wrote: > Hi Yosry, > > On Tue, Jan 28, 2025 at 12:58 AM Yosry Ahmed <yosry.ahmed@xxxxxxxxx> wrote: > > > The zbud compressed pages allocator is rarely used, most users use > > zsmalloc. zbud consumes much more memory (only stores 1 or 2 compressed > > pages per physical page). The only advantage of zbud is a marginal > > performance improvement that by no means justify the memory overhead. > > > > Historically, zsmalloc had significantly worse latency than zbud and > > z3fold but offered better memory savings. This is no longer the case as > > shown by a simple recent analysis [1]. In a kernel build test on tmpfs > > in a limited cgroup, zbud 2-3% less time than zsmalloc, but at the cost > > of using ~32% more memory (1.5G vs 1.13G). The tradeoff does not make > > sense for zbud in any practical scenario. > > > > The only alleged advantage of zbud is not having the dependency on > > CONFIG_MMU, but CONFIG_SWAP already depends on CONFIG_MMU anyway, and > > zbud is only used by zswap. > > > > Following in the footsteps of [2], which deprecated z3fold, deprecated > > zbud as planned and remove it in a few cycles if no objections are > > raised from active users. > > > > Rename the user-visible config options so that users with CONFIG_ZBUD=y > > get a new prompt with explanation during make oldconfig. Also, remove > > CONFIG_ZBUD from defconfig. > > > > [1] > > https://lore.kernel.org/lkml/CAJD7tkbRF6od-2x_L8-A1QL3=2Ww13sCj4S3i4bNndqF+3+_Vg@xxxxxxxxxxxxxx/ > > [2] > > https://lore.kernel.org/lkml/20240904233343.933462-1-yosryahmed@xxxxxxxxxx/ > > > > Signed-off-by: Yosry Ahmed <yosry.ahmed@xxxxxxxxx> > > > > Let me disagree here. The footprint for zbud and the ability to work in > MMU-less setups make it self sufficient and I don't support its deprecation. > I can give a formal NAK if that's required. What's the advtange of the ability to work on MMU-less setups when CONFIG_SWAP itself depends on MMU? Please elaborate. I agree that zbud has a smaller footprint, but I don't think that's enough reason to keep it around if it's not being used, especially that its memory utilization is really low.
