Re: nfs client and io_uring zero copy receive

[Trond Myklebust <trondmy@kernel.org>]

On Tue, 2025-07-22 at 21:10 +0300, Anton Gavriliuk wrote:
> Hi
> 
> I am trying to exceed 20 GB/s doing sequential read from a single
> file
> on the nfs client.
> 
> perf top shows excessive memcpy usage:
> 
> Samples: 237K of event 'cycles:P', 4000 Hz, Event count (approx.):
> 120872739112 lost: 0/0 drop: 0/0
> Overhead  Shared Object                      Symbol
>   20,54%  [kernel]                           [k] memcpy
>    6,52%  [nfs]                              [k] nfs_generic_pg_test
>    5,12%  [nfs]                              [k] nfs_page_group_lock
>    4,92%  [kernel]                           [k] _copy_to_iter
>    4,79%  [kernel]                           [k] gro_list_prepare
>    2,77%  [nfs]                              [k] nfs_clear_request
>    2,10%  [nfs]                              [k]
> __nfs_pageio_add_request
>    2,07%  [kernel]                           [k] check_heap_object
>    2,00%  [kernel]                           [k] __slab_free
> 
> Can nfs client be adopted to use zero copy ?, for example by using
> io_uring zero copy rx.
> 

The client has no idea in which order the server will return replies to
the RPC calls it sends. So no, it can't queue up those reply buffers in
advance.

The only way you can avoid memory copies here is to use RDMA to allow
the server to write its replies directly into the correct client read
buffers.

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trondmy@kernel.org, trond.myklebust@hammerspace.com