On Mon, Jan 29, 2024 at 12:19:02PM -0500, Mike Snitzer wrote: > While I'm sure this legacy application would love to not have to > change its code at all, I think we can all agree that we need to just > focus on how best to advise applications that have mixed workloads > accomplish efficient mmap+read of both sequential and random. > > To that end, I heard Dave clearly suggest 2 things: > > 1) update MADV/FADV_SEQUENTIAL to set file->f_ra.ra_pages to > bdi->io_pages, not bdi->ra_pages * 2 > > 2) Have the application first issue MADV_SEQUENTIAL to convey that for > the following MADV_WILLNEED is for sequential file load (so it is > desirable to use larger ra_pages) > > This overrides the default of bdi->io_pages and _should_ provide the > required per-file duality of control for readahead, correct? I just discovered MADV_POPULATE_READ - see my reply to Ming up-thread about that. The applicaiton should use that instead of MADV_WILLNEED because it gives cache population guarantees that WILLNEED doesn't. Then we can look at optimising the performance of MADV_POPULATE_READ (if needed) as there is constrained scope we can optimise within in ways that we cannot do with WILLNEED. -Dave. -- Dave Chinner david@xxxxxxxxxxxxx
