On Tue, Jun 17, 2025 at 01:31:23PM -0400, Chuck Lever wrote: > On 6/17/25 1:22 PM, Mike Snitzer wrote: > > On Mon, Jun 16, 2025 at 12:10:38PM -0400, Mike Snitzer wrote: > >> On Mon, Jun 16, 2025 at 09:32:16AM -0400, Chuck Lever wrote: > >>> On 6/12/25 12:00 PM, Mike Snitzer wrote: > >>>> On Thu, Jun 12, 2025 at 09:21:35AM -0400, Chuck Lever wrote: > >>>>> On 6/11/25 3:18 PM, Mike Snitzer wrote: > >>>>>> On Wed, Jun 11, 2025 at 10:31:20AM -0400, Chuck Lever wrote: > >>>>>>> On 6/10/25 4:57 PM, Mike Snitzer wrote: > >>>>>>>> Add 'enable-dontcache' to NFSD's debugfs interface so that: Any data > >>>>>>>> read or written by NFSD will either not be cached (thanks to O_DIRECT) > >>>>>>>> or will be removed from the page cache upon completion (DONTCACHE). > >>>>>>> > >>>>>>> I thought we were going to do two switches: One for reads and one for > >>>>>>> writes? I could be misremembering. > >>>>>> > >>>>>> We did discuss the possibility of doing that. Still can-do if that's > >>>>>> what you'd prefer. > >>>>> > >>>>> For our experimental interface, I think having read and write enablement > >>>>> as separate settings is wise, so please do that. > >>>>> > >>>>> One quibble, though: The name "enable_dontcache" might be directly > >>>>> meaningful to you, but I think others might find "enable_dont" to be > >>>>> oxymoronic. And, it ties the setting to a specific kernel technology: > >>>>> RWF_DONTCACHE. > >>>>> > >>>>> So: Can we call these settings "io_cache_read" and "io_cache_write" ? > >>>>> > >>>>> They could each carry multiple settings: > >>>>> > >>>>> 0: Use page cache > >>>>> 1: Use RWF_DONTCACHE > >>>>> 2: Use O_DIRECT > >>>>> > >>>>> You can choose to implement any or all of the above three mechanisms. > >>>> > >>>> I like it, will do for v2. But will have O_DIRECT=1 and RWF_DONTCACHE=2. > >>> > >>> For io_cache_read, either settings 1 and 2 need to set > >>> disable_splice_read, or the io_cache_read setting has to be considered > >>> by nfsd_read_splice_ok() when deciding to use nfsd_iter_read() or > >>> splice read. > >> > >> Yes, I understand. > >> > >>> However, it would be slightly nicer if we could decide whether splice > >>> read can be removed /before/ this series is merged. Can you get NFSD > >>> tested with IOR with disable_splice_read both enabled and disabled (no > >>> direct I/O)? Then we can compare the results to ensure that there is no > >>> negative performance impact for removing the splice read code. > >> > >> I can ask if we have a small window of opportunity to get this tested, > >> will let you know if so. > >> > > > > I was able to enlist the help of Keith (cc'd) to get some runs in to > > compare splice_read vs vectored read. A picture is worth 1000 words: > > https://original.art/NFSD_splice_vs_buffered_read_IOR_EASY.jpg > > > > Left side is with splice_read running IOR_EASY with 48, 64, 96 PPN > > (Processes Per Node on each client) respectively. Then the same > > IOR_EASY workload progression for buffered IO on the right side. > > > > 6x servers with 1TB memory and 48 cpus, each configured with 32 NFSD > > threads, with CPU pinning and 4M Read Ahead. 6x clients running IOR_EASY. > > > > This was Keith's take on splice_read's benefits: > > - Is overall faster than buffered at any PPN. > > - Is able to scale higher with PPN (whereas buffered is flat). > > - Safe to say splice_read allows NFSD to do more IO then standard > > buffered. > > I thank you and Keith for the data! You're welcome. > > (These results came _after_ I did the patch to remove all the > > splice_read related code from NFSD and SUNRPC.. while cathartic, alas > > it seems it isn't meant to be at this point. I'll let you do the > > honors in the future if/when you deem splice_read worthy of removal.) > > If we were to make all NFS READ operations use O_DIRECT, then of course > NFSD's splice read should be removed at that point. Yes, that makes sense. I still need to try Christoph's idea (hope to do so over next 24hrs): https://lore.kernel.org/linux-nfs/aEu3o9imaQQF9vyg@xxxxxxxxxxxxx/ But for now, here is my latest NFSD O_DIRECT/DONTCACHE work, think of the top 6 commits as a preview of what'll be v2 of this series: https://git.kernel.org/pub/scm/linux/kernel/git/snitzer/linux.git/log/?h=kernel-6.12.24/nfsd-testing
