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

Chuck Lever
2025-06-17T17:31:35Z

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!

> (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.

--
Chuck Lever