On 5/6/25 1:40 PM, 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. > > 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 > > 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. > > Chris suggested that the write sizes were too small in this test, so I > grabbed Chuck's patches to increase the max RPC payload size [2] to 4M, > and patched the client to allow a wsize that big: > > wsize=4M: > > Normal: WRITE: bw=1053MiB/s (1104MB/s), 1053MiB/s-1053MiB/s (1104MB/s-1104MB/s), io=930GiB (999GB), run=904526-904526msec > DONTCACHE: WRITE: bw=1191MiB/s (1249MB/s), 1191MiB/s-1191MiB/s (1249MB/s-1249MB/s), io=1050GiB (1127GB), run=902781-902781msec > > Not much change with normal buffered I/O here, but DONTCACHE is faster > with a 4M wsize. My suspicion (unconfirmed) is that the dropbehind flag > ends up causing partially-written large folios in the pagecache to get > written back too early, and that slows down later writes to the same > folios. My feeling is that at this point, the NFSD read and write paths are not currently tuned for large folios -- they break every I/O into single pages. > 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? Mike has suggested that NFSD should limit the use RWF_UNCACHED to WRITE requests with large payloads (for some arbitrary definition of "large"). > This might also be a good reason to think about a larger rsize/wsize > limit in the client. > > I'd like to also test reads with this flag, but I'm currently getting > back that EOPNOTSUPP error when I try to test them. That's expected for that patch series. But I have to ask: what problem do you expect RWF_UNCACHED to solve? > [1]: https://lore.kernel.org/linux-nfs/20250220171205.12092-1- > snitzer@xxxxxxxxxx/ > [2]: https://lore.kernel.org/linux-nfs/20250428193702.5186-15- > cel@xxxxxxxxxx/ -- Chuck Lever
