On Tue, May 06, 2025 at 01:40:35PM -0400, Jeff Layton wrote: > FYI I decided to try and get some numbers with Mike's RWF_DONTCACHE > patches for nfsd [1]. Those add a module param that make all reads and > writes use RWF_DONTCACHE. > > I had one host that was running knfsd with an XFS export, and a second > that was acting as NFS client. Both machines have tons of memory, so > pagecache utilization is irrelevant for this test. Does RWF_DONTCACHE result in server side STABLE write requests from the NFS client, or are they still unstable and require a post-write completion COMMIT operation from the client to trigger server side writeback before the client can discard the page cache? > I tested sequential writes using the fio-seq_write.fio test, both with > and without the module param enabled. > > These numbers are from one run each, but they were pretty stable over > several runs: > > # fio /usr/share/doc/fio/examples/fio-seq-write.fio $ cat /usr/share/doc/fio/examples/fio-seq-write.fio cat: /usr/share/doc/fio/examples/fio-seq-write.fio: No such file or directory $ What are the fio control parameters of the IO you are doing? (e.g. is this single threaded IO, does it use the psync, libaio or iouring engine, etc) > wsize=1M: > > Normal: WRITE: bw=1034MiB/s (1084MB/s), 1034MiB/s-1034MiB/s (1084MB/s-1084MB/s), io=910GiB (977GB), run=901326-901326msec > DONTCACHE: WRITE: bw=649MiB/s (681MB/s), 649MiB/s-649MiB/s (681MB/s-681MB/s), io=571GiB (613GB), run=900001-900001msec > > DONTCACHE with a 1M wsize vs. recent (v6.14-ish) knfsd was about 30% > slower. Memory consumption was down, but these boxes have oodles of > memory, so I didn't notice much change there. So what is the IO pattern that the NFSD is sending to the underlying XFS filesystem? Is it sending 1M RWF_DONTCACHE buffered IOs to XFS as well (i.e. one buffered write IO per NFS client write request), or is DONTCACHE only being used on the NFS client side? > I wonder if we need some heuristic that makes generic_write_sync() only > kick off writeback immediately if the whole folio is dirty so we have > more time to gather writes before kicking off writeback? You're doing aligned 1MB IOs - there should be no partially dirty large folios in either the client or the server page caches. That said, this is part of the reason I asked about both whether the client side write is STABLE and whether RWF_DONTCACHE on the server side. i.e. using either of those will trigger writeback on the serer side immediately; in the case of the former it will also complete before returning to the client and not require a subsequent COMMIT RPC to wait for server side IO completion... -Dave. -- Dave Chinner david@xxxxxxxxxxxxx
