Re: [PATCH] NFSD: Increase the default max_block_size to 4MB

[Jeff Layton <jlayton@kernel.org>]

On Mon, 2026-04-20 at 11:38 -0400, Chuck Lever wrote:
> From: Chuck Lever <chuck.lever@oracle.com>
> 
> Commit 8a81f16de64f ("NFSD: Add a "default" block size") introduced
> NFSSVC_DEFBLKSIZE at 1MB, well below the 4MB NFSSVC_MAXBLKSIZE
> ceiling, with the stated intent that a later change would raise the
> default.
> 
> Raising the default reduces per-RPC overhead on fast networks by
> amortizing header processing and scheduling costs across larger
> payloads. The halving loop in nfsd_get_default_max_blksize()
> constrains the returned value to 1/4096 of available RAM, so the
> new 4MB default takes effect only on systems with at least 16GB of
> RAM. Smaller machines continue to receive the same computed value
> as before. Administrators can still override the computed value
> through /proc/fs/nfsd/max_block_size.
> 
> On systems where the new default takes effect,
> svc_sock_setbufsize() sizes each service socket's send and receive
> buffers as nreqs * max_mesg * 2. Quadrupling max_mesg therefore
> quadruples the per-socket buffer reservation at a fixed thread
> count, which operators tuning large thread pools should account
> for.
> 
> Note well: Your NFS client implementation must support large read
> and write size settings to benefit from this change.
> 
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>  fs/nfsd/nfsd.h | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/nfsd/nfsd.h b/fs/nfsd/nfsd.h
> index a01d70953358..daa63c1b161c 100644
> --- a/fs/nfsd/nfsd.h
> +++ b/fs/nfsd/nfsd.h
> @@ -45,11 +45,10 @@ bool nfsd_support_version(int vers);
>  
>  /*
>   * Default and maximum payload size (NFS READ or WRITE), in bytes.
> - * The default is historical, and the maximum is an implementation
> - * limit.
> + * The maximum is an implementation limit.
>   */
>  enum {
> -	NFSSVC_DEFBLKSIZE       = 1 * 1024 * 1024,
> +	NFSSVC_DEFBLKSIZE       = 4 * 1024 * 1024,
>  	NFSSVC_MAXBLKSIZE       = RPCSVC_MAXPAYLOAD,
>  };
>  

Makes sense, particularly with dynamic threading.

Reviewed-by: Jeff Layton <jlayton@kernel.org>