On Fri, Mar 1, 2024 at 1:58 PM David Laight <David.Laight@xxxxxxxxxx> wrote: > > From: Chris Li > > Sent: 29 February 2024 08:46 > > > > Very deep RB tree requires rebalance at times. That > > contributes to the zswap fault latencies. Xarray does not > > need to perform tree rebalance. Replacing RB tree to xarray > > can have some small performance gain. > > > > One small difference is that xarray insert might fail with > > ENOMEM, while RB tree insert does not allocate additional > > memory. > > What is the difference in kernel memory use? > IIRC someone pointed out (in the rosebush thread) that xarray > uses a lot of kernel memory if the items are randomly distributed. Do you have any suggestions on what script I can run to collect that information for you? If the swapfile is fully utilized, then every 1G of swapfile space will save about 4M of memory using xarray compared to RB tree. Every swap entry saves about 2 pointers (ignoring the intermedia xarray node) Every 1G of swapfile, fully populated xarray pointer uses about 2M of memory. The RB tree node (left pointer, right pointer, color) will use about 6M of memory. In the worst case, you are wasting at most 2M of memory on xarray for every 1G of swapfile. Hope that calculation helps. Keep in mind that the swap cache is already using xarray, using the same swap entry offset as index. So whatever memory waste caused by the sparse distribution of the swap entry, you are already getting that in the current swap cache. Chris
