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Based on kernel version 3.16. Page generated on 2014-08-06 21:40 EST.

1	
2	Overview
3	========
4	
5	This readme tries to provide some background on the hows and whys of RDS,
6	and will hopefully help you find your way around the code.
7	
8	In addition, please see this email about RDS origins:
9	http://oss.oracle.com/pipermail/rds-devel/2007-November/000228.html
10	
11	RDS Architecture
12	================
13	
14	RDS provides reliable, ordered datagram delivery by using a single
15	reliable connection between any two nodes in the cluster. This allows
16	applications to use a single socket to talk to any other process in the
17	cluster - so in a cluster with N processes you need N sockets, in contrast
18	to N*N if you use a connection-oriented socket transport like TCP.
19	
20	RDS is not Infiniband-specific; it was designed to support different
21	transports.  The current implementation used to support RDS over TCP as well
22	as IB. Work is in progress to support RDS over iWARP, and using DCE to
23	guarantee no dropped packets on Ethernet, it may be possible to use RDS over
24	UDP in the future.
25	
26	The high-level semantics of RDS from the application's point of view are
27	
28	 *	Addressing
29	        RDS uses IPv4 addresses and 16bit port numbers to identify
30	        the end point of a connection. All socket operations that involve
31	        passing addresses between kernel and user space generally
32	        use a struct sockaddr_in.
33	
34	        The fact that IPv4 addresses are used does not mean the underlying
35	        transport has to be IP-based. In fact, RDS over IB uses a
36	        reliable IB connection; the IP address is used exclusively to
37	        locate the remote node's GID (by ARPing for the given IP).
38	
39	        The port space is entirely independent of UDP, TCP or any other
40	        protocol.
41	
42	 *	Socket interface
43	        RDS sockets work *mostly* as you would expect from a BSD
44	        socket. The next section will cover the details. At any rate,
45	        all I/O is performed through the standard BSD socket API.
46	        Some additions like zerocopy support are implemented through
47	        control messages, while other extensions use the getsockopt/
48	        setsockopt calls.
49	
50	        Sockets must be bound before you can send or receive data.
51	        This is needed because binding also selects a transport and
52	        attaches it to the socket. Once bound, the transport assignment
53	        does not change. RDS will tolerate IPs moving around (eg in
54	        a active-active HA scenario), but only as long as the address
55	        doesn't move to a different transport.
56	
57	 *	sysctls
58	        RDS supports a number of sysctls in /proc/sys/net/rds
59	
60	
61	Socket Interface
62	================
63	
64	  AF_RDS, PF_RDS, SOL_RDS
65	        These constants haven't been assigned yet, because RDS isn't in
66	        mainline yet. Currently, the kernel module assigns some constant
67	        and publishes it to user space through two sysctl files
68	                /proc/sys/net/rds/pf_rds
69	                /proc/sys/net/rds/sol_rds
70	
71	  fd = socket(PF_RDS, SOCK_SEQPACKET, 0);
72	        This creates a new, unbound RDS socket.
73	
74	  setsockopt(SOL_SOCKET): send and receive buffer size
75	        RDS honors the send and receive buffer size socket options.
76	        You are not allowed to queue more than SO_SNDSIZE bytes to
77	        a socket. A message is queued when sendmsg is called, and
78	        it leaves the queue when the remote system acknowledges
79	        its arrival.
80	
81	        The SO_RCVSIZE option controls the maximum receive queue length.
82	        This is a soft limit rather than a hard limit - RDS will
83	        continue to accept and queue incoming messages, even if that
84	        takes the queue length over the limit. However, it will also
85	        mark the port as "congested" and send a congestion update to
86	        the source node. The source node is supposed to throttle any
87	        processes sending to this congested port.
88	
89	  bind(fd, &sockaddr_in, ...)
90	        This binds the socket to a local IP address and port, and a
91	        transport.
92	
93	  sendmsg(fd, ...)
94	        Sends a message to the indicated recipient. The kernel will
95	        transparently establish the underlying reliable connection
96	        if it isn't up yet.
97	
98	        An attempt to send a message that exceeds SO_SNDSIZE will
99	        return with -EMSGSIZE
100	
101	        An attempt to send a message that would take the total number
102	        of queued bytes over the SO_SNDSIZE threshold will return
103	        EAGAIN.
104	
105	        An attempt to send a message to a destination that is marked
106	        as "congested" will return ENOBUFS.
107	
108	  recvmsg(fd, ...)
109	        Receives a message that was queued to this socket. The sockets
110	        recv queue accounting is adjusted, and if the queue length
111	        drops below SO_SNDSIZE, the port is marked uncongested, and
112	        a congestion update is sent to all peers.
113	
114	        Applications can ask the RDS kernel module to receive
115	        notifications via control messages (for instance, there is a
116	        notification when a congestion update arrived, or when a RDMA
117	        operation completes). These notifications are received through
118	        the msg.msg_control buffer of struct msghdr. The format of the
119	        messages is described in manpages.
120	
121	  poll(fd)
122	        RDS supports the poll interface to allow the application
123	        to implement async I/O.
124	
125	        POLLIN handling is pretty straightforward. When there's an
126	        incoming message queued to the socket, or a pending notification,
127	        we signal POLLIN.
128	
129	        POLLOUT is a little harder. Since you can essentially send
130	        to any destination, RDS will always signal POLLOUT as long as
131	        there's room on the send queue (ie the number of bytes queued
132	        is less than the sendbuf size).
133	
134	        However, the kernel will refuse to accept messages to
135	        a destination marked congested - in this case you will loop
136	        forever if you rely on poll to tell you what to do.
137	        This isn't a trivial problem, but applications can deal with
138	        this - by using congestion notifications, and by checking for
139	        ENOBUFS errors returned by sendmsg.
140	
141	  setsockopt(SOL_RDS, RDS_CANCEL_SENT_TO, &sockaddr_in)
142	        This allows the application to discard all messages queued to a
143	        specific destination on this particular socket.
144	
145	        This allows the application to cancel outstanding messages if
146	        it detects a timeout. For instance, if it tried to send a message,
147	        and the remote host is unreachable, RDS will keep trying forever.
148	        The application may decide it's not worth it, and cancel the
149	        operation. In this case, it would use RDS_CANCEL_SENT_TO to
150	        nuke any pending messages.
151	
152	
153	RDMA for RDS
154	============
155	
156	  see rds-rdma(7) manpage (available in rds-tools)
157	
158	
159	Congestion Notifications
160	========================
161	
162	  see rds(7) manpage
163	
164	
165	RDS Protocol
166	============
167	
168	  Message header
169	
170	    The message header is a 'struct rds_header' (see rds.h):
171	    Fields:
172	      h_sequence:
173	          per-packet sequence number
174	      h_ack:
175	          piggybacked acknowledgment of last packet received
176	      h_len:
177	          length of data, not including header
178	      h_sport:
179	          source port
180	      h_dport:
181	          destination port
182	      h_flags:
183	          CONG_BITMAP - this is a congestion update bitmap
184	          ACK_REQUIRED - receiver must ack this packet
185	          RETRANSMITTED - packet has previously been sent
186	      h_credit:
187	          indicate to other end of connection that
188	          it has more credits available (i.e. there is
189	          more send room)
190	      h_padding[4]:
191	          unused, for future use
192	      h_csum:
193	          header checksum
194	      h_exthdr:
195	          optional data can be passed here. This is currently used for
196	          passing RDMA-related information.
197	
198	  ACK and retransmit handling
199	
200	      One might think that with reliable IB connections you wouldn't need
201	      to ack messages that have been received.  The problem is that IB
202	      hardware generates an ack message before it has DMAed the message
203	      into memory.  This creates a potential message loss if the HCA is
204	      disabled for any reason between when it sends the ack and before
205	      the message is DMAed and processed.  This is only a potential issue
206	      if another HCA is available for fail-over.
207	
208	      Sending an ack immediately would allow the sender to free the sent
209	      message from their send queue quickly, but could cause excessive
210	      traffic to be used for acks. RDS piggybacks acks on sent data
211	      packets.  Ack-only packets are reduced by only allowing one to be
212	      in flight at a time, and by the sender only asking for acks when
213	      its send buffers start to fill up. All retransmissions are also
214	      acked.
215	
216	  Flow Control
217	
218	      RDS's IB transport uses a credit-based mechanism to verify that
219	      there is space in the peer's receive buffers for more data. This
220	      eliminates the need for hardware retries on the connection.
221	
222	  Congestion
223	
224	      Messages waiting in the receive queue on the receiving socket
225	      are accounted against the sockets SO_RCVBUF option value.  Only
226	      the payload bytes in the message are accounted for.  If the
227	      number of bytes queued equals or exceeds rcvbuf then the socket
228	      is congested.  All sends attempted to this socket's address
229	      should return block or return -EWOULDBLOCK.
230	
231	      Applications are expected to be reasonably tuned such that this
232	      situation very rarely occurs.  An application encountering this
233	      "back-pressure" is considered a bug.
234	
235	      This is implemented by having each node maintain bitmaps which
236	      indicate which ports on bound addresses are congested.  As the
237	      bitmap changes it is sent through all the connections which
238	      terminate in the local address of the bitmap which changed.
239	
240	      The bitmaps are allocated as connections are brought up.  This
241	      avoids allocation in the interrupt handling path which queues
242	      sages on sockets.  The dense bitmaps let transports send the
243	      entire bitmap on any bitmap change reasonably efficiently.  This
244	      is much easier to implement than some finer-grained
245	      communication of per-port congestion.  The sender does a very
246	      inexpensive bit test to test if the port it's about to send to
247	      is congested or not.
248	
249	
250	RDS Transport Layer
251	==================
252	
253	  As mentioned above, RDS is not IB-specific. Its code is divided
254	  into a general RDS layer and a transport layer.
255	
256	  The general layer handles the socket API, congestion handling,
257	  loopback, stats, usermem pinning, and the connection state machine.
258	
259	  The transport layer handles the details of the transport. The IB
260	  transport, for example, handles all the queue pairs, work requests,
261	  CM event handlers, and other Infiniband details.
262	
263	
264	RDS Kernel Structures
265	=====================
266	
267	  struct rds_message
268	    aka possibly "rds_outgoing", the generic RDS layer copies data to
269	    be sent and sets header fields as needed, based on the socket API.
270	    This is then queued for the individual connection and sent by the
271	    connection's transport.
272	  struct rds_incoming
273	    a generic struct referring to incoming data that can be handed from
274	    the transport to the general code and queued by the general code
275	    while the socket is awoken. It is then passed back to the transport
276	    code to handle the actual copy-to-user.
277	  struct rds_socket
278	    per-socket information
279	  struct rds_connection
280	    per-connection information
281	  struct rds_transport
282	    pointers to transport-specific functions
283	  struct rds_statistics
284	    non-transport-specific statistics
285	  struct rds_cong_map
286	    wraps the raw congestion bitmap, contains rbnode, waitq, etc.
287	
288	Connection management
289	=====================
290	
291	  Connections may be in UP, DOWN, CONNECTING, DISCONNECTING, and
292	  ERROR states.
293	
294	  The first time an attempt is made by an RDS socket to send data to
295	  a node, a connection is allocated and connected. That connection is
296	  then maintained forever -- if there are transport errors, the
297	  connection will be dropped and re-established.
298	
299	  Dropping a connection while packets are queued will cause queued or
300	  partially-sent datagrams to be retransmitted when the connection is
301	  re-established.
302	
303	
304	The send path
305	=============
306	
307	  rds_sendmsg()
308	    struct rds_message built from incoming data
309	    CMSGs parsed (e.g. RDMA ops)
310	    transport connection alloced and connected if not already
311	    rds_message placed on send queue
312	    send worker awoken
313	  rds_send_worker()
314	    calls rds_send_xmit() until queue is empty
315	  rds_send_xmit()
316	    transmits congestion map if one is pending
317	    may set ACK_REQUIRED
318	    calls transport to send either non-RDMA or RDMA message
319	    (RDMA ops never retransmitted)
320	  rds_ib_xmit()
321	    allocs work requests from send ring
322	    adds any new send credits available to peer (h_credits)
323	    maps the rds_message's sg list
324	    piggybacks ack
325	    populates work requests
326	    post send to connection's queue pair
327	
328	The recv path
329	=============
330	
331	  rds_ib_recv_cq_comp_handler()
332	    looks at write completions
333	    unmaps recv buffer from device
334	    no errors, call rds_ib_process_recv()
335	    refill recv ring
336	  rds_ib_process_recv()
337	    validate header checksum
338	    copy header to rds_ib_incoming struct if start of a new datagram
339	    add to ibinc's fraglist
340	    if competed datagram:
341	      update cong map if datagram was cong update
342	      call rds_recv_incoming() otherwise
343	      note if ack is required
344	  rds_recv_incoming()
345	    drop duplicate packets
346	    respond to pings
347	    find the sock associated with this datagram
348	    add to sock queue
349	    wake up sock
350	    do some congestion calculations
351	  rds_recvmsg
352	    copy data into user iovec
353	    handle CMSGs
354	    return to application
355	
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