On 1/15/25 01:50, Joanne Koong wrote: > Hi all, > > I would like to propose a discussion topic about improving large folio > writeback performance. As more filesystems adopt large folios, it > becomes increasingly important that writeback is made to be as > performant as possible. There are two areas I'd like to discuss: > > > == Granularity of dirty pages writeback == > Currently, the granularity of writeback is at the folio level. If one > byte in a folio is dirty, the entire folio will be written back. This > becomes unscalable for larger folios and significantly degrades > performance, especially for workloads that employ random writes. > > One idea is to track dirty pages at a smaller granularity using a > 64-bit bitmap stored inside the folio struct where each bit tracks a > smaller chunk of pages (eg for 2 MB folios, each bit would track 32k > pages), and only write back dirty chunks rather than the entire folio. I think this might be tricky in some cases? I.e. with 2 MB and pmd-mapped folio, it's possible to write-protect only the whole pmd, not individual 32k chunks in order to catch the first write to a chunk to mark it dirty.
