On Wed, Apr 23, 2025 at 11:30:21AM -0400, Chuck Lever wrote: > On 4/23/25 11:22 AM, Matthew Wilcox wrote: > > On Wed, Apr 23, 2025 at 10:38:37AM -0400, Chuck Lever wrote: > >> On 4/23/25 12:25 AM, trondmy@xxxxxxxxxx wrote: > >>> From: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx> > >>> > >>> The following patch set attempts to add support for the RWF_DONTCACHE > >>> flag in preadv2() and pwritev2() on NFS filesystems. > >> > >> Hi Trond- > >> > >> "RFC" in the subject field noted. > >> > >> The cover letter does not explain why one would want this facility, nor > >> does it quantify the performance implications. > >> > >> I can understand not wanting to cache on an NFS server, but don't you > >> want to maintain a data cache as close to applications as possible? > > > > If you look at the original work for RWF_DONTCACHE, you'll see this is > > the application providing the hint that it's doing a streaming access. > > It's only applied to folios which are created as a result of this > > access, and other accesses to these folios while the folios are in use > > clear the flag. So it's kind of like O_DIRECT access, except that it > > does go through the page cache so there's none of this funky alignment > > requirement on the userspace buffers. > > OK, was wondering whether this behavior was opt-in; sounds like it is. > Thanks for setting me straight. Yes, its certainly opt-in (requires setting a flag for each use). Jens added support in fio relatively recently, see: https://git.kernel.dk/cgit/fio/commit/?id=43c67b9f3a8808274bc1e0a3b7b70c56bb8a007f Looking ahead relative to NFSD, as you know we've discussed exposing per-export config controls to enable use of DONTCACHE. Finer controls (e.g. only large sequential IO) would be more desirable but I'm not aware of a simple means to detect such workloads with NFSD. Could it be that we'd do well to carry through large folio support in NFSD and expose a configurable threshold that if met or exceeded then DONTCACHE used? What is the status of large folio support in NFSD? Is anyone actively working on it? Thanks, Mike
