rds.rst (17019B)
1.. SPDX-License-Identifier: GPL-2.0 2 3=== 4RDS 5=== 6 7Overview 8======== 9 10This readme tries to provide some background on the hows and whys of RDS, 11and will hopefully help you find your way around the code. 12 13In addition, please see this email about RDS origins: 14http://oss.oracle.com/pipermail/rds-devel/2007-November/000228.html 15 16RDS Architecture 17================ 18 19RDS provides reliable, ordered datagram delivery by using a single 20reliable connection between any two nodes in the cluster. This allows 21applications to use a single socket to talk to any other process in the 22cluster - so in a cluster with N processes you need N sockets, in contrast 23to N*N if you use a connection-oriented socket transport like TCP. 24 25RDS is not Infiniband-specific; it was designed to support different 26transports. The current implementation used to support RDS over TCP as well 27as IB. 28 29The high-level semantics of RDS from the application's point of view are 30 31 * Addressing 32 33 RDS uses IPv4 addresses and 16bit port numbers to identify 34 the end point of a connection. All socket operations that involve 35 passing addresses between kernel and user space generally 36 use a struct sockaddr_in. 37 38 The fact that IPv4 addresses are used does not mean the underlying 39 transport has to be IP-based. In fact, RDS over IB uses a 40 reliable IB connection; the IP address is used exclusively to 41 locate the remote node's GID (by ARPing for the given IP). 42 43 The port space is entirely independent of UDP, TCP or any other 44 protocol. 45 46 * Socket interface 47 48 RDS sockets work *mostly* as you would expect from a BSD 49 socket. The next section will cover the details. At any rate, 50 all I/O is performed through the standard BSD socket API. 51 Some additions like zerocopy support are implemented through 52 control messages, while other extensions use the getsockopt/ 53 setsockopt calls. 54 55 Sockets must be bound before you can send or receive data. 56 This is needed because binding also selects a transport and 57 attaches it to the socket. Once bound, the transport assignment 58 does not change. RDS will tolerate IPs moving around (eg in 59 a active-active HA scenario), but only as long as the address 60 doesn't move to a different transport. 61 62 * sysctls 63 64 RDS supports a number of sysctls in /proc/sys/net/rds 65 66 67Socket Interface 68================ 69 70 AF_RDS, PF_RDS, SOL_RDS 71 AF_RDS and PF_RDS are the domain type to be used with socket(2) 72 to create RDS sockets. SOL_RDS is the socket-level to be used 73 with setsockopt(2) and getsockopt(2) for RDS specific socket 74 options. 75 76 fd = socket(PF_RDS, SOCK_SEQPACKET, 0); 77 This creates a new, unbound RDS socket. 78 79 setsockopt(SOL_SOCKET): send and receive buffer size 80 RDS honors the send and receive buffer size socket options. 81 You are not allowed to queue more than SO_SNDSIZE bytes to 82 a socket. A message is queued when sendmsg is called, and 83 it leaves the queue when the remote system acknowledges 84 its arrival. 85 86 The SO_RCVSIZE option controls the maximum receive queue length. 87 This is a soft limit rather than a hard limit - RDS will 88 continue to accept and queue incoming messages, even if that 89 takes the queue length over the limit. However, it will also 90 mark the port as "congested" and send a congestion update to 91 the source node. The source node is supposed to throttle any 92 processes sending to this congested port. 93 94 bind(fd, &sockaddr_in, ...) 95 This binds the socket to a local IP address and port, and a 96 transport, if one has not already been selected via the 97 SO_RDS_TRANSPORT socket option 98 99 sendmsg(fd, ...) 100 Sends a message to the indicated recipient. The kernel will 101 transparently establish the underlying reliable connection 102 if it isn't up yet. 103 104 An attempt to send a message that exceeds SO_SNDSIZE will 105 return with -EMSGSIZE 106 107 An attempt to send a message that would take the total number 108 of queued bytes over the SO_SNDSIZE threshold will return 109 EAGAIN. 110 111 An attempt to send a message to a destination that is marked 112 as "congested" will return ENOBUFS. 113 114 recvmsg(fd, ...) 115 Receives a message that was queued to this socket. The sockets 116 recv queue accounting is adjusted, and if the queue length 117 drops below SO_SNDSIZE, the port is marked uncongested, and 118 a congestion update is sent to all peers. 119 120 Applications can ask the RDS kernel module to receive 121 notifications via control messages (for instance, there is a 122 notification when a congestion update arrived, or when a RDMA 123 operation completes). These notifications are received through 124 the msg.msg_control buffer of struct msghdr. The format of the 125 messages is described in manpages. 126 127 poll(fd) 128 RDS supports the poll interface to allow the application 129 to implement async I/O. 130 131 POLLIN handling is pretty straightforward. When there's an 132 incoming message queued to the socket, or a pending notification, 133 we signal POLLIN. 134 135 POLLOUT is a little harder. Since you can essentially send 136 to any destination, RDS will always signal POLLOUT as long as 137 there's room on the send queue (ie the number of bytes queued 138 is less than the sendbuf size). 139 140 However, the kernel will refuse to accept messages to 141 a destination marked congested - in this case you will loop 142 forever if you rely on poll to tell you what to do. 143 This isn't a trivial problem, but applications can deal with 144 this - by using congestion notifications, and by checking for 145 ENOBUFS errors returned by sendmsg. 146 147 setsockopt(SOL_RDS, RDS_CANCEL_SENT_TO, &sockaddr_in) 148 This allows the application to discard all messages queued to a 149 specific destination on this particular socket. 150 151 This allows the application to cancel outstanding messages if 152 it detects a timeout. For instance, if it tried to send a message, 153 and the remote host is unreachable, RDS will keep trying forever. 154 The application may decide it's not worth it, and cancel the 155 operation. In this case, it would use RDS_CANCEL_SENT_TO to 156 nuke any pending messages. 157 158 ``setsockopt(fd, SOL_RDS, SO_RDS_TRANSPORT, (int *)&transport ..), getsockopt(fd, SOL_RDS, SO_RDS_TRANSPORT, (int *)&transport ..)`` 159 Set or read an integer defining the underlying 160 encapsulating transport to be used for RDS packets on the 161 socket. When setting the option, integer argument may be 162 one of RDS_TRANS_TCP or RDS_TRANS_IB. When retrieving the 163 value, RDS_TRANS_NONE will be returned on an unbound socket. 164 This socket option may only be set exactly once on the socket, 165 prior to binding it via the bind(2) system call. Attempts to 166 set SO_RDS_TRANSPORT on a socket for which the transport has 167 been previously attached explicitly (by SO_RDS_TRANSPORT) or 168 implicitly (via bind(2)) will return an error of EOPNOTSUPP. 169 An attempt to set SO_RDS_TRANSPORT to RDS_TRANS_NONE will 170 always return EINVAL. 171 172RDMA for RDS 173============ 174 175 see rds-rdma(7) manpage (available in rds-tools) 176 177 178Congestion Notifications 179======================== 180 181 see rds(7) manpage 182 183 184RDS Protocol 185============ 186 187 Message header 188 189 The message header is a 'struct rds_header' (see rds.h): 190 191 Fields: 192 193 h_sequence: 194 per-packet sequence number 195 h_ack: 196 piggybacked acknowledgment of last packet received 197 h_len: 198 length of data, not including header 199 h_sport: 200 source port 201 h_dport: 202 destination port 203 h_flags: 204 Can be: 205 206 ============= ================================== 207 CONG_BITMAP this is a congestion update bitmap 208 ACK_REQUIRED receiver must ack this packet 209 RETRANSMITTED packet has previously been sent 210 ============= ================================== 211 212 h_credit: 213 indicate to other end of connection that 214 it has more credits available (i.e. there is 215 more send room) 216 h_padding[4]: 217 unused, for future use 218 h_csum: 219 header checksum 220 h_exthdr: 221 optional data can be passed here. This is currently used for 222 passing RDMA-related information. 223 224 ACK and retransmit handling 225 226 One might think that with reliable IB connections you wouldn't need 227 to ack messages that have been received. The problem is that IB 228 hardware generates an ack message before it has DMAed the message 229 into memory. This creates a potential message loss if the HCA is 230 disabled for any reason between when it sends the ack and before 231 the message is DMAed and processed. This is only a potential issue 232 if another HCA is available for fail-over. 233 234 Sending an ack immediately would allow the sender to free the sent 235 message from their send queue quickly, but could cause excessive 236 traffic to be used for acks. RDS piggybacks acks on sent data 237 packets. Ack-only packets are reduced by only allowing one to be 238 in flight at a time, and by the sender only asking for acks when 239 its send buffers start to fill up. All retransmissions are also 240 acked. 241 242 Flow Control 243 244 RDS's IB transport uses a credit-based mechanism to verify that 245 there is space in the peer's receive buffers for more data. This 246 eliminates the need for hardware retries on the connection. 247 248 Congestion 249 250 Messages waiting in the receive queue on the receiving socket 251 are accounted against the sockets SO_RCVBUF option value. Only 252 the payload bytes in the message are accounted for. If the 253 number of bytes queued equals or exceeds rcvbuf then the socket 254 is congested. All sends attempted to this socket's address 255 should return block or return -EWOULDBLOCK. 256 257 Applications are expected to be reasonably tuned such that this 258 situation very rarely occurs. An application encountering this 259 "back-pressure" is considered a bug. 260 261 This is implemented by having each node maintain bitmaps which 262 indicate which ports on bound addresses are congested. As the 263 bitmap changes it is sent through all the connections which 264 terminate in the local address of the bitmap which changed. 265 266 The bitmaps are allocated as connections are brought up. This 267 avoids allocation in the interrupt handling path which queues 268 sages on sockets. The dense bitmaps let transports send the 269 entire bitmap on any bitmap change reasonably efficiently. This 270 is much easier to implement than some finer-grained 271 communication of per-port congestion. The sender does a very 272 inexpensive bit test to test if the port it's about to send to 273 is congested or not. 274 275 276RDS Transport Layer 277=================== 278 279 As mentioned above, RDS is not IB-specific. Its code is divided 280 into a general RDS layer and a transport layer. 281 282 The general layer handles the socket API, congestion handling, 283 loopback, stats, usermem pinning, and the connection state machine. 284 285 The transport layer handles the details of the transport. The IB 286 transport, for example, handles all the queue pairs, work requests, 287 CM event handlers, and other Infiniband details. 288 289 290RDS Kernel Structures 291===================== 292 293 struct rds_message 294 aka possibly "rds_outgoing", the generic RDS layer copies data to 295 be sent and sets header fields as needed, based on the socket API. 296 This is then queued for the individual connection and sent by the 297 connection's transport. 298 299 struct rds_incoming 300 a generic struct referring to incoming data that can be handed from 301 the transport to the general code and queued by the general code 302 while the socket is awoken. It is then passed back to the transport 303 code to handle the actual copy-to-user. 304 305 struct rds_socket 306 per-socket information 307 308 struct rds_connection 309 per-connection information 310 311 struct rds_transport 312 pointers to transport-specific functions 313 314 struct rds_statistics 315 non-transport-specific statistics 316 317 struct rds_cong_map 318 wraps the raw congestion bitmap, contains rbnode, waitq, etc. 319 320Connection management 321===================== 322 323 Connections may be in UP, DOWN, CONNECTING, DISCONNECTING, and 324 ERROR states. 325 326 The first time an attempt is made by an RDS socket to send data to 327 a node, a connection is allocated and connected. That connection is 328 then maintained forever -- if there are transport errors, the 329 connection will be dropped and re-established. 330 331 Dropping a connection while packets are queued will cause queued or 332 partially-sent datagrams to be retransmitted when the connection is 333 re-established. 334 335 336The send path 337============= 338 339 rds_sendmsg() 340 - struct rds_message built from incoming data 341 - CMSGs parsed (e.g. RDMA ops) 342 - transport connection alloced and connected if not already 343 - rds_message placed on send queue 344 - send worker awoken 345 346 rds_send_worker() 347 - calls rds_send_xmit() until queue is empty 348 349 rds_send_xmit() 350 - transmits congestion map if one is pending 351 - may set ACK_REQUIRED 352 - calls transport to send either non-RDMA or RDMA message 353 (RDMA ops never retransmitted) 354 355 rds_ib_xmit() 356 - allocs work requests from send ring 357 - adds any new send credits available to peer (h_credits) 358 - maps the rds_message's sg list 359 - piggybacks ack 360 - populates work requests 361 - post send to connection's queue pair 362 363The recv path 364============= 365 366 rds_ib_recv_cq_comp_handler() 367 - looks at write completions 368 - unmaps recv buffer from device 369 - no errors, call rds_ib_process_recv() 370 - refill recv ring 371 372 rds_ib_process_recv() 373 - validate header checksum 374 - copy header to rds_ib_incoming struct if start of a new datagram 375 - add to ibinc's fraglist 376 - if competed datagram: 377 - update cong map if datagram was cong update 378 - call rds_recv_incoming() otherwise 379 - note if ack is required 380 381 rds_recv_incoming() 382 - drop duplicate packets 383 - respond to pings 384 - find the sock associated with this datagram 385 - add to sock queue 386 - wake up sock 387 - do some congestion calculations 388 rds_recvmsg 389 - copy data into user iovec 390 - handle CMSGs 391 - return to application 392 393Multipath RDS (mprds) 394===================== 395 Mprds is multipathed-RDS, primarily intended for RDS-over-TCP 396 (though the concept can be extended to other transports). The classical 397 implementation of RDS-over-TCP is implemented by demultiplexing multiple 398 PF_RDS sockets between any 2 endpoints (where endpoint == [IP address, 399 port]) over a single TCP socket between the 2 IP addresses involved. This 400 has the limitation that it ends up funneling multiple RDS flows over a 401 single TCP flow, thus it is 402 (a) upper-bounded to the single-flow bandwidth, 403 (b) suffers from head-of-line blocking for all the RDS sockets. 404 405 Better throughput (for a fixed small packet size, MTU) can be achieved 406 by having multiple TCP/IP flows per rds/tcp connection, i.e., multipathed 407 RDS (mprds). Each such TCP/IP flow constitutes a path for the rds/tcp 408 connection. RDS sockets will be attached to a path based on some hash 409 (e.g., of local address and RDS port number) and packets for that RDS 410 socket will be sent over the attached path using TCP to segment/reassemble 411 RDS datagrams on that path. 412 413 Multipathed RDS is implemented by splitting the struct rds_connection into 414 a common (to all paths) part, and a per-path struct rds_conn_path. All 415 I/O workqs and reconnect threads are driven from the rds_conn_path. 416 Transports such as TCP that are multipath capable may then set up a 417 TCP socket per rds_conn_path, and this is managed by the transport via 418 the transport privatee cp_transport_data pointer. 419 420 Transports announce themselves as multipath capable by setting the 421 t_mp_capable bit during registration with the rds core module. When the 422 transport is multipath-capable, rds_sendmsg() hashes outgoing traffic 423 across multiple paths. The outgoing hash is computed based on the 424 local address and port that the PF_RDS socket is bound to. 425 426 Additionally, even if the transport is MP capable, we may be 427 peering with some node that does not support mprds, or supports 428 a different number of paths. As a result, the peering nodes need 429 to agree on the number of paths to be used for the connection. 430 This is done by sending out a control packet exchange before the 431 first data packet. The control packet exchange must have completed 432 prior to outgoing hash completion in rds_sendmsg() when the transport 433 is mutlipath capable. 434 435 The control packet is an RDS ping packet (i.e., packet to rds dest 436 port 0) with the ping packet having a rds extension header option of 437 type RDS_EXTHDR_NPATHS, length 2 bytes, and the value is the 438 number of paths supported by the sender. The "probe" ping packet will 439 get sent from some reserved port, RDS_FLAG_PROBE_PORT (in <linux/rds.h>) 440 The receiver of a ping from RDS_FLAG_PROBE_PORT will thus immediately 441 be able to compute the min(sender_paths, rcvr_paths). The pong 442 sent in response to a probe-ping should contain the rcvr's npaths 443 when the rcvr is mprds-capable. 444 445 If the rcvr is not mprds-capable, the exthdr in the ping will be 446 ignored. In this case the pong will not have any exthdrs, so the sender 447 of the probe-ping can default to single-path mprds. 448