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The RPC client uses the rq_xtime field in each RPC request to determine the round-trip time of the request. Currently, the rq_xtime field is initialized by each transport just before it starts enqueing a request to be sent. However, transports do not handle initializing this value consistently; sometimes they don't initialize it at all. To make the measurement of request round-trip time consistent for all RPC client transport capabilities, pull rq_xtime initialization into the RPC client's generic transport logic. Now all transports will get a standardized RTT measure automatically, from: xprt_transmit() to xprt_complete_rqst() This makes round-trip time calculation more accurate for the TCP transport. The socket ->sendmsg() method can return "-EAGAIN" if the socket's output buffer is full, so the TCP transport's ->send_request() method may call the ->sendmsg() method repeatedly until it gets all of the request's bytes queued in the socket's buffer. Currently, the TCP transport sets the rq_xtime field every time through that loop so the final value is the timestamp just before the *last* call to the underlying socket's ->sendmsg() method. After this patch, the rq_xtime field contains a timestamp that reflects the time just before the *first* call to ->sendmsg(). This is consequential under heavy workloads because large requests often take multiple ->sendmsg() calls to get all the bytes of a request queued. The TCP transport causes the request to sleep until the remote end of the socket has received enough bytes to clear space in the socket's local output buffer. This delay can be quite significant. The method introduced by this patch is a more accurate measure of RTT for stream transports, since the server can cause enough back pressure to delay (ie increase the latency of) requests from the client. Additionally, this patch corrects the behavior of the RDMA transport, which entirely neglected to initialize the rq_xtime field. RPC performance metrics for RDMA transports now display correct RPC request round trip times. Signed-off-by: Chuck Lever <chuck.lever@oracle.com> Acked-by: Tom Talpey <thomas.talpey@netapp.com> Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com> |
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