On Sat, Apr 27, 2024 at 7:16 AM Chris Li <chrisl@xxxxxxxxxx> wrote: > > Hi Ying, > > On Tue, Apr 23, 2024 at 7:26 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: > > > > Hi, Matthew, > > > > Matthew Wilcox <willy@xxxxxxxxxxxxx> writes: > > > > > On Mon, Apr 22, 2024 at 03:54:58PM +0800, Huang, Ying wrote: > > >> Is it possible to add "start_offset" support in xarray, so "index" > > >> will subtract "start_offset" before looking up / inserting? > > > > > > We kind of have that with XA_FLAGS_ZERO_BUSY which is used for > > > XA_FLAGS_ALLOC1. But that's just one bit for the entry at 0. We could > > > generalise it, but then we'd have to store that somewhere and there's > > > no obvious good place to store it that wouldn't enlarge struct xarray, > > > which I'd be reluctant to do. > > > > > >> Is it possible to use multiple range locks to protect one xarray to > > >> improve the lock scalability? This is why we have multiple "struct > > >> address_space" for one swap device. And, we may have same lock > > >> contention issue for large files too. > > > > > > It's something I've considered. The issue is search marks. If we delete > > > an entry, we may have to walk all the way up the xarray clearing bits as > > > we go and I'd rather not grab a lock at each level. There's a convenient > > > 4 byte hole between nr_values and parent where we could put it. > > > > > > Oh, another issue is that we use i_pages.xa_lock to synchronise > > > address_space.nrpages, so I'm not sure that a per-node lock will help. > > > > Thanks for looking at this. > > > > > But I'm conscious that there are workloads which show contention on > > > xa_lock as their limiting factor, so I'm open to ideas to improve all > > > these things. > > > > I have no idea so far because my very limited knowledge about xarray. > > For the swap file usage, I have been considering an idea to remove the > index part of the xarray from swap cache. Swap cache is different from > file cache in a few aspects. > For one if we want to have a folio equivalent of "large swap entry". > Then the natural alignment of those swap offset on does not make > sense. Ideally we should be able to write the folio to un-aligned swap > file locations. > Hi Chris, This sound interesting, I have a few questions though... Are you suggesting we handle swap on file and swap on device differently? Swap on file is much less frequently used than swap on device I think. And what do you mean "index part of the xarray"? If we need a cache, xarray still seems one of the best choices to hold the content. > The other aspect for swap files is that, we already have different > data structures organized around swap offset, swap_map and > swap_cgroup. If we group the swap related data structure together. We > can add a pointer to a union of folio or a shadow swap entry. We can > use atomic updates on the swap struct member or breakdown the access > lock by ranges just like swap cluster does. > > I want to discuss those ideas in the upcoming LSF/MM meet up as well. Looking forward to it! > > Chris
