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