Re: [nfsv4] Fwd: New Version Notification for draft-dnoveck-nfsv4-rpcrdma-rtissues-01.txt

[David Noveck <[e-mail]>]

> I'm confused by this summarization.

:-(.  Let's see what I can do to make this clearer.

> In the text above you indicate 3 different places where "internode round
trip is involved, yet in the summary you only mention 2.

The point I was trying to make was that, although there were three
round-trips, only two contribute to the request latency.  In some
cases, there is a round trip because an ack is sent, but because neither
the client nor the server is waiting for it.

> What is the definition of an "internode round trip?"

Any situation in which a message is sent in one direction and, after that,
another message is sent in the opposite direction.

> Also its unclear to me what you mean my "in the context of a connected
operation".

maybe I should have said, "Because this reliable connected operation in
which messages are acked.

> Also you mention that there are two-responder-side interrupt latencies,
are you referring to the notification of the RDMA_READ
> and the send completion queue for sending the response?

I'm referring to the notification that the request has been received and
and the notification that the RDMA_READ has comnpleted.

Does this interrupt latency come into play in the latency of the operation?

I think the two I mentioned do.

> Once the client side gets the response it can continue, even if the
server thread is still waiting for notification of a successful send
correct?

Yes.

> Also are you missing the interrupt latency of the send on the client? In
addition to the interrupt latency of receiving the reply?

I don't think that contributes to latency.  The request processing can
continue once the request is received on the server, even if the client
has not received notification of the completion of the send.

On Mon, Sep 12, 2016 at 3:52 PM, karen deitke <[e-mail]>
wrote:

> Hi Dave,
>
> I'm struggling following this below:
>
>    o  First, the memory to be accessed remotely is registered.  This is
>       a local operation.
>
>    o  Once the registration has been done, the initial send of the
>       request can proceed.  Since this is in the context of connected
>       operation, there is an internode round trip involved.  However,
>       the next step can proceed after the initial transmission is
>       received by the responder.  As a result, only the responder-bound
>       side of the transmission contributes to overall operation latency.
>
>    o  The responder, after being notified of the receipt of the request,
>       uses RDMA READ to fetch the bulk data.  This involves an internode
>       round-trip latency.  After the fetch of the data, the responder
>       needs to be notified of the completion of the explicit RDMA
>       operation
>
>    o  The responder (after performing the requested operation) sends the
>       response.  Again, as this is in the context of connected
>       operation, there is an internode round trip involved.  However,
>       the next step can proceed after the initial transmission is
>       received by the requester.
>
>    o  The memory registered before the request was issued needs to be
>       deregistered, before the request is considered complete and the
>       sending process restarted.  When remote invalidation is not
>       available, the requester, after being notified of the receipt of
>       the response, performs a local operation to deregister the memory
>       in question.  Alternatively, the responder will use Send With
>       Invalidate and the responder's RNIC will effect the deregistration
>       before notifying the requester of the response which has been
>       received.
>
>    To summarize, if we exclude the actual server execution of the
>    request, the latency consists of two internode round-trip latencies
>    plus two-responder-side interrupt latencies plus one requester-side
>    interrupt latency plus any necessary registration/de-registration
>    overhead.  This is in contrast to a request not using explicit RDMA
>    operations in which there is a single inter-node round-trip latency
>    and one interrupt latency on the requester and the responder.
>
> I'm confused by this summarization.  In the text above you indicate 3
> different places where "internode round trip is involved, yet in the
> summary you only mention 2.  What is the definition of an "internode round
> trip?"  Also its unclear to me what you mean my "in the context of a
> connected operation".
>
> Also you mention that there are two-responder-side interupt latencies, are
> you referring to the notification of the RDMA_READ and the send completion
> queue for sending the response?  Does this interrupt latency come into play
> in the latency of the operation? Once the client side gets the response it
> can continue, even if the server thread is still waiting for notification
> of a successful send correct?
>
> Also are you missing the interrupt latency of the send on the client? In
> addition to the interrupt latency of receiving the reply?
>
> Karen
>
>
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