![santosh.shilimkar@oracle.com](/assets/img/avatar_default.png)
RDS iWarp support code has become stale and non testable. As indicated earlier, am dropping the support for it. If new iWarp user(s) shows up in future, we can adapat the RDS IB transprt for the special RDMA READ sink case. iWarp needs an MR for the RDMA READ sink. Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org> Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com> Signed-off-by: David S. Miller <davem@davemloft.net>
354 lines
13 KiB
Plaintext
354 lines
13 KiB
Plaintext
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Overview
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========
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This readme tries to provide some background on the hows and whys of RDS,
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and will hopefully help you find your way around the code.
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In addition, please see this email about RDS origins:
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http://oss.oracle.com/pipermail/rds-devel/2007-November/000228.html
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RDS Architecture
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================
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RDS provides reliable, ordered datagram delivery by using a single
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reliable connection between any two nodes in the cluster. This allows
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applications to use a single socket to talk to any other process in the
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cluster - so in a cluster with N processes you need N sockets, in contrast
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to N*N if you use a connection-oriented socket transport like TCP.
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RDS is not Infiniband-specific; it was designed to support different
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transports. The current implementation used to support RDS over TCP as well
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as IB.
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The high-level semantics of RDS from the application's point of view are
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* Addressing
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RDS uses IPv4 addresses and 16bit port numbers to identify
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the end point of a connection. All socket operations that involve
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passing addresses between kernel and user space generally
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use a struct sockaddr_in.
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The fact that IPv4 addresses are used does not mean the underlying
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transport has to be IP-based. In fact, RDS over IB uses a
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reliable IB connection; the IP address is used exclusively to
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locate the remote node's GID (by ARPing for the given IP).
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The port space is entirely independent of UDP, TCP or any other
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protocol.
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* Socket interface
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RDS sockets work *mostly* as you would expect from a BSD
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socket. The next section will cover the details. At any rate,
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all I/O is performed through the standard BSD socket API.
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Some additions like zerocopy support are implemented through
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control messages, while other extensions use the getsockopt/
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setsockopt calls.
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Sockets must be bound before you can send or receive data.
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This is needed because binding also selects a transport and
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attaches it to the socket. Once bound, the transport assignment
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does not change. RDS will tolerate IPs moving around (eg in
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a active-active HA scenario), but only as long as the address
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doesn't move to a different transport.
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* sysctls
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RDS supports a number of sysctls in /proc/sys/net/rds
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Socket Interface
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================
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AF_RDS, PF_RDS, SOL_RDS
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AF_RDS and PF_RDS are the domain type to be used with socket(2)
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to create RDS sockets. SOL_RDS is the socket-level to be used
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with setsockopt(2) and getsockopt(2) for RDS specific socket
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options.
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fd = socket(PF_RDS, SOCK_SEQPACKET, 0);
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This creates a new, unbound RDS socket.
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setsockopt(SOL_SOCKET): send and receive buffer size
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RDS honors the send and receive buffer size socket options.
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You are not allowed to queue more than SO_SNDSIZE bytes to
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a socket. A message is queued when sendmsg is called, and
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it leaves the queue when the remote system acknowledges
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its arrival.
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The SO_RCVSIZE option controls the maximum receive queue length.
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This is a soft limit rather than a hard limit - RDS will
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continue to accept and queue incoming messages, even if that
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takes the queue length over the limit. However, it will also
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mark the port as "congested" and send a congestion update to
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the source node. The source node is supposed to throttle any
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processes sending to this congested port.
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bind(fd, &sockaddr_in, ...)
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This binds the socket to a local IP address and port, and a
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transport.
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sendmsg(fd, ...)
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Sends a message to the indicated recipient. The kernel will
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transparently establish the underlying reliable connection
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if it isn't up yet.
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An attempt to send a message that exceeds SO_SNDSIZE will
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return with -EMSGSIZE
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An attempt to send a message that would take the total number
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of queued bytes over the SO_SNDSIZE threshold will return
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EAGAIN.
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An attempt to send a message to a destination that is marked
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as "congested" will return ENOBUFS.
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recvmsg(fd, ...)
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Receives a message that was queued to this socket. The sockets
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recv queue accounting is adjusted, and if the queue length
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drops below SO_SNDSIZE, the port is marked uncongested, and
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a congestion update is sent to all peers.
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Applications can ask the RDS kernel module to receive
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notifications via control messages (for instance, there is a
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notification when a congestion update arrived, or when a RDMA
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operation completes). These notifications are received through
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the msg.msg_control buffer of struct msghdr. The format of the
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messages is described in manpages.
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poll(fd)
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RDS supports the poll interface to allow the application
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to implement async I/O.
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POLLIN handling is pretty straightforward. When there's an
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incoming message queued to the socket, or a pending notification,
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we signal POLLIN.
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POLLOUT is a little harder. Since you can essentially send
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to any destination, RDS will always signal POLLOUT as long as
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there's room on the send queue (ie the number of bytes queued
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is less than the sendbuf size).
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However, the kernel will refuse to accept messages to
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a destination marked congested - in this case you will loop
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forever if you rely on poll to tell you what to do.
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This isn't a trivial problem, but applications can deal with
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this - by using congestion notifications, and by checking for
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ENOBUFS errors returned by sendmsg.
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setsockopt(SOL_RDS, RDS_CANCEL_SENT_TO, &sockaddr_in)
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This allows the application to discard all messages queued to a
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specific destination on this particular socket.
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This allows the application to cancel outstanding messages if
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it detects a timeout. For instance, if it tried to send a message,
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and the remote host is unreachable, RDS will keep trying forever.
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The application may decide it's not worth it, and cancel the
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operation. In this case, it would use RDS_CANCEL_SENT_TO to
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nuke any pending messages.
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RDMA for RDS
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============
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see rds-rdma(7) manpage (available in rds-tools)
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Congestion Notifications
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========================
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see rds(7) manpage
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RDS Protocol
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============
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Message header
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The message header is a 'struct rds_header' (see rds.h):
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Fields:
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h_sequence:
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per-packet sequence number
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h_ack:
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piggybacked acknowledgment of last packet received
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h_len:
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length of data, not including header
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h_sport:
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source port
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h_dport:
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destination port
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h_flags:
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CONG_BITMAP - this is a congestion update bitmap
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ACK_REQUIRED - receiver must ack this packet
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RETRANSMITTED - packet has previously been sent
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h_credit:
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indicate to other end of connection that
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it has more credits available (i.e. there is
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more send room)
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h_padding[4]:
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unused, for future use
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h_csum:
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header checksum
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h_exthdr:
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optional data can be passed here. This is currently used for
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passing RDMA-related information.
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ACK and retransmit handling
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One might think that with reliable IB connections you wouldn't need
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to ack messages that have been received. The problem is that IB
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hardware generates an ack message before it has DMAed the message
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into memory. This creates a potential message loss if the HCA is
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disabled for any reason between when it sends the ack and before
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the message is DMAed and processed. This is only a potential issue
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if another HCA is available for fail-over.
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Sending an ack immediately would allow the sender to free the sent
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message from their send queue quickly, but could cause excessive
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traffic to be used for acks. RDS piggybacks acks on sent data
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packets. Ack-only packets are reduced by only allowing one to be
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in flight at a time, and by the sender only asking for acks when
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its send buffers start to fill up. All retransmissions are also
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acked.
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Flow Control
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RDS's IB transport uses a credit-based mechanism to verify that
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there is space in the peer's receive buffers for more data. This
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eliminates the need for hardware retries on the connection.
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Congestion
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Messages waiting in the receive queue on the receiving socket
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are accounted against the sockets SO_RCVBUF option value. Only
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the payload bytes in the message are accounted for. If the
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number of bytes queued equals or exceeds rcvbuf then the socket
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is congested. All sends attempted to this socket's address
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should return block or return -EWOULDBLOCK.
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Applications are expected to be reasonably tuned such that this
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situation very rarely occurs. An application encountering this
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"back-pressure" is considered a bug.
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This is implemented by having each node maintain bitmaps which
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indicate which ports on bound addresses are congested. As the
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bitmap changes it is sent through all the connections which
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terminate in the local address of the bitmap which changed.
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The bitmaps are allocated as connections are brought up. This
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avoids allocation in the interrupt handling path which queues
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sages on sockets. The dense bitmaps let transports send the
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entire bitmap on any bitmap change reasonably efficiently. This
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is much easier to implement than some finer-grained
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communication of per-port congestion. The sender does a very
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inexpensive bit test to test if the port it's about to send to
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is congested or not.
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RDS Transport Layer
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==================
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As mentioned above, RDS is not IB-specific. Its code is divided
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into a general RDS layer and a transport layer.
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The general layer handles the socket API, congestion handling,
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loopback, stats, usermem pinning, and the connection state machine.
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The transport layer handles the details of the transport. The IB
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transport, for example, handles all the queue pairs, work requests,
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CM event handlers, and other Infiniband details.
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RDS Kernel Structures
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=====================
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struct rds_message
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aka possibly "rds_outgoing", the generic RDS layer copies data to
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be sent and sets header fields as needed, based on the socket API.
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This is then queued for the individual connection and sent by the
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connection's transport.
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struct rds_incoming
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a generic struct referring to incoming data that can be handed from
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the transport to the general code and queued by the general code
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while the socket is awoken. It is then passed back to the transport
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code to handle the actual copy-to-user.
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struct rds_socket
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per-socket information
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struct rds_connection
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per-connection information
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struct rds_transport
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pointers to transport-specific functions
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struct rds_statistics
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non-transport-specific statistics
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struct rds_cong_map
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wraps the raw congestion bitmap, contains rbnode, waitq, etc.
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Connection management
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=====================
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Connections may be in UP, DOWN, CONNECTING, DISCONNECTING, and
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ERROR states.
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The first time an attempt is made by an RDS socket to send data to
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a node, a connection is allocated and connected. That connection is
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then maintained forever -- if there are transport errors, the
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connection will be dropped and re-established.
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Dropping a connection while packets are queued will cause queued or
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partially-sent datagrams to be retransmitted when the connection is
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re-established.
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The send path
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=============
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rds_sendmsg()
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struct rds_message built from incoming data
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CMSGs parsed (e.g. RDMA ops)
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transport connection alloced and connected if not already
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rds_message placed on send queue
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send worker awoken
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rds_send_worker()
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calls rds_send_xmit() until queue is empty
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rds_send_xmit()
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transmits congestion map if one is pending
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may set ACK_REQUIRED
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calls transport to send either non-RDMA or RDMA message
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(RDMA ops never retransmitted)
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rds_ib_xmit()
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allocs work requests from send ring
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adds any new send credits available to peer (h_credits)
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maps the rds_message's sg list
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piggybacks ack
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populates work requests
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post send to connection's queue pair
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The recv path
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=============
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rds_ib_recv_cq_comp_handler()
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looks at write completions
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unmaps recv buffer from device
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no errors, call rds_ib_process_recv()
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refill recv ring
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rds_ib_process_recv()
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validate header checksum
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copy header to rds_ib_incoming struct if start of a new datagram
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add to ibinc's fraglist
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if competed datagram:
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update cong map if datagram was cong update
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call rds_recv_incoming() otherwise
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note if ack is required
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rds_recv_incoming()
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drop duplicate packets
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respond to pings
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find the sock associated with this datagram
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add to sock queue
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wake up sock
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do some congestion calculations
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rds_recvmsg
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copy data into user iovec
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handle CMSGs
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return to application
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