Re: [PATCH 1/6] NFSD: add the ability to enable use of RWF_DONTCACHE for all IO

Mike Snitzer
2025-06-19T20:19:12Z

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@infradead.org/

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: