Re: [PATCH RFC 0/2] nfsd: issue POSIX_FADV_DONTNEED after READ/WRITE/COMMIT

[Chuck Lever <chuck.lever@oracle.com>]

On 7/3/25 7:16 PM, NeilBrown wrote:
> On Fri, 04 Jul 2025, Jeff Layton wrote:
>> Chuck and I were discussing RWF_DONTCACHE and he suggested that this
>> might be an alternate approach. My main gripe with DONTCACHE was that it
>> kicks off writeback after every WRITE operation. With NFS, we generally
>> get a COMMIT operation at some point. Allowing us to batch up writes
>> until that point has traditionally been considered better for
>> performance.
> 
> I wonder if that traditional consideration is justified, give your
> subsequent results.  The addition of COMMIT in v3 allowed us to both:
>  - delay kicking off writes
>  - not wait for writes to complete
> 
> I think the second was always primary.  Maybe we didn't consider the
> value of the first enough.
> Obviously the client caches writes and delays the start of writeback.
> Adding another delay on the serve side does not seem to have a clear
> justification.  Maybe we *should* kick-off writeback immediately.  There
> would still be opportunity for subsequent WRITE requests to be merged
> into the writeback queue.

Dave Chinner had the same thought a while back. So I've experimented
with starting writes as part of nfsd_write(). Kicking off writes,
even without waiting, is actually pretty costly, and it resulted in
worse performance.

Now that .pc_release is called /after/ the WRITE response has been sent,
though, that might be a place where kicking off writeback could be done
without charging that latency to the client.


-- 
Chuck Lever