Re: [PATCH v1 07/12] xprtrdma: Don't provide a reply chunk when expecting a short reply

Sagi Grimberg
2015-07-14T09:54:39Z

On 7/12/2015 9:38 PM, Chuck Lever wrote:
> Hi Sagi-
>
>
> On Jul 12, 2015, at 10:58 AM, Sagi Grimberg <sagig-LDSdmyG8hGV8YrgS2mwiifqBs+8SCbDb@public.gmane.org> wrote:
>
>> On 7/9/2015 11:42 PM, Chuck Lever wrote:
>>> Currently Linux always offers a reply chunk, even for small replies
>>> (unless a read or write list is needed for the RPC operation).
>>>
>>> A comment in rpcrdma_marshal_req() reads:
>>>
>>>> Currently we try to not actually use read inline.
>>>> Reply chunks have the desirable property that
>>>> they land, packed, directly in the target buffers
>>>> without headers, so they require no fixup. The
>>>> additional RDMA Write op sends the same amount
>>>> of data, streams on-the-wire and adds no overhead
>>>> on receive. Therefore, we request a reply chunk
>>>> for non-writes wherever feasible and efficient.
>>>
>>> This considers only the network bandwidth cost of sending the RPC
>>> reply. For replies which are only a few dozen bytes, this is
>>> typically not a good trade-off.
>>>
>>> If the server chooses to return the reply inline:
>>>
>>> - The client has registered and invalidated a memory region to
>>> catch the reply, which is then not used
>>>
>>> If the server chooses to use the reply chunk:
>>>
>>> - The server sends a few bytes using a heavyweight RDMA WRITE for
>>> operation. The entire RPC reply is conveyed in two RDMA
>>> operations (WRITE_ONLY, SEND) instead of one.
>>
>> Pipelined WRITE+SEND operations are hardly an overhead compared to
>> copying chunks of data.
>>
>>>
>>> Note that both the server and client have to prepare or copy the
>>> reply data anyway to construct these replies. There's no benefit to
>>> using an RDMA transfer since the host CPU has to be involved.
>>
>> I think that preparation (posting 1 or 2 WQEs) and copying
>> chunks of data of say 8K-16K might be different.
>
> Two points that are probably not clear from my patch description:
>
> 1. This patch affects only replies (usually much) smaller than the
> client’s inline threshold (1KB). Anything larger will continue
> to use RDMA transfer.
>
> 2. These replies are constructed in the RPC buffer by the server,
> and parsed in the receive buffer by the client. They are not
> simple data copies on either endpoint.
>
> Think NFS GETATTR: the server is gathering metadata from multiple
> sources, and XDR encoding it in the reply send buffer. The data
> is not copied, it is manipulated before the SEND.
>
> The client then XDR decodes the received stream and scatters the
> decoded results into multiple in-memory data structures.
>
> Because XDR encoding/decoding is involved, there really is no
> benefit to an RDMA transfer for these replies.

I see. Thanks for the clarification.

Reviewed-By: Sagi Grimberg <sagig-VPRAkNaXOzVWk0Htik3J/w@public.gmane.org>