• Chuck Lever's avatar
    xprtrdma: Use gathered Send for large inline messages · 655fec69
    Chuck Lever authored
    An RPC Call message that is sent inline but that has a data payload
    (ie, one or more items in rq_snd_buf's page list) must be "pulled
    up:"
    
    - call_allocate has to reserve enough RPC Call buffer space to
    accommodate the data payload
    
    - call_transmit has to memcopy the rq_snd_buf's page list and tail
    into its head iovec before it is sent
    
    As the inline threshold is increased beyond its current 1KB default,
    however, this means data payloads of more than a few KB are copied
    by the host CPU. For example, if the inline threshold is increased
    just to 4KB, then NFS WRITE requests up to 4KB would involve a
    memcpy of the NFS WRITE's payload data into the RPC Call buffer.
    This is an undesirable amount of participation by the host CPU.
    
    The inline threshold may be much larger than 4KB in the future,
    after negotiation with a peer server.
    
    Instead of copying the components of rq_snd_buf into its head iovec,
    construct a gather list of these components, and send them all in
    place. The same approach is already used in the Linux server's
    RPC-over-RDMA reply path.
    
    This mechanism also eliminates the need for rpcrdma_tail_pullup,
    which is used to manage the XDR pad and trailing inline content when
    a Read list is present.
    
    This requires that the pages in rq_snd_buf's page list be DMA-mapped
    during marshaling, and unmapped when a data-bearing RPC is
    completed. This is slightly less efficient for very small I/O
    payloads, but significantly more efficient as data payload size and
    inline threshold increase past a kilobyte.
    Signed-off-by: default avatarChuck Lever <chuck.lever@oracle.com>
    Signed-off-by: default avatarAnna Schumaker <Anna.Schumaker@Netapp.com>
    655fec69
xprt_rdma.h 18.1 KB