Interconnect Networks Basics

Generic parallel/distributed system architecture On-chip interconnects (manycore processor) Off-chip interconnects (clusters of servers)

Interconnection network performance Latency: how much time does it take between the time when a send of 1 byte is issued and the time when the receive of the data is completed? – Signal propogation delay + router queuing delay Bandwidth: how much time to send a large amount of data (e.g. 1MB)? Examples: – Ethernet: Bandwidth 100Mbps, 1Gbps, 10Gbps, 100Gbps Latency: 25us -100us (user level, single hop, try ping between linprog’s) – InfiniBand Bandwidth: 20Gbps, 40Gbps, 54Gbps, 80Gbps, …… Latency: 1-3us (user level, single hop)

Interconnection network performance Latency and Bandwidth – Different levels User level: the performance that users feel Systems level, device level Which level will have the highest bandwidth? – Example: 1Gbps Ethernet, 800Mbps at system level, 650Mbps at the user level. 1Gbps Ethernet, which level? 0.115ms ping latency, which level? – Some measurement trap: single pair.vs. multiple pair.

Network components Network interface (card) Communication between a node and the network Link Bundle of wires and fibers that carry signals Switches Connects a fixed number of input channels to a fixed number of output channels. In this community, switches may also have the router functions.

Switch The cross-bar can realize a communication from any input port to any output port. The simplest form is a dedicated computer with memory (e.g. linux router).

Most expensive form: Cross-bar functionality – all permutations can be realized simultaneously inputinput output A 4x4 cross-bar Permutation: (1, 2, 3, 4) -> (3, 1, 2, 4) A communication pattern where each source happens once, each destination happens once. The input registers send control signals to the control, routing, scheduling module indicating the pattern; the control module computes and sets the dots (1,2, 3, 4)-> (3, 1, 2, 4) 1234 (1,2,3,4)->(4,3,2,2) Only (1,2,3,4)->(4,3,2,-)

Switch example: 24-port 1Gbps Ethernet switch 24 input ports and 24 output ports – each Ethernet jacket has one input port and one output port. All 24 machines can send and receive simultaneously. switch Ethernet card machine

Alternatives to cross-bars A question: why buffers when we can always do permutation? An N x N cross bar has O(N^2) cross points (on/off switches). – Not scalable, expensive An alternative for low end switches: bus and memory – When bus and memory is fast enough, moving data between input and output ports are like memory copy in a typical computer.

Bus and memory alternative to crossbar Realizing (1, 2, 3, 4) -> (4, 3, 2, 1) – Read from input port 1 to memory A – Read from input port 2 to memory B – Read from input port 3 to memory C – Read from input port 4 to memory D – Run forwarding logic (find out the output ports) – Write A to output port 4 – Write B to output port 3 – Write C to output port 2 – Write D to output port 1

Bus and memory alternative to crossbar A typical northbridge bandwidth is a few GBps. Let us assume the bandwidth is 4GBps, how many ports can the northbridge support in 100Mbps Ethernet swithes?

Another alternative: multistage interconnection network Realize all permutations without controlling O(N^2) cross-points. – Clos networks, Benes networks Each of the dot is a 2x2 switch, controlled by two states. 0 1 How to realize 0000->0000, 0001->0001, 0010->1011?

Switch All approximate crossbars – High end ones are equivalent to or close to crossbars: all permutations can happens simultaneously. – Low end ones will have limited total bandwidth (aggregate bandwidth). Example: High end and low end 24 port 1Gbps switch connecting 24 computers. – With one pair of Source/destination, the throughput will be about 800Mbps for both (no difference). – When 24 pairs send/receive at the same time High end one will get 24*800Mbps Low end one will get a total of X Mbps, X < 24*800Mbps (X can sometimes be about 5*800Mbps) – Different pairs may also have different throughput depending on the scheduling algorithm.

Network level components Topology (what) – Physical interconnection structure of the network graph. – Physically limits the performance of the networks. Routing algorithm (which) – Restricts the set of paths that messages can follow. Switching strategy (how) – How data in a message traverses a route (passing routers) Flow control mechanism (when) – When a message or portions of it traverse a route – What happens when traffic collides

Topology How the components are connected. Important properties Diameter: maximum distance between any two nodes in the network (hop count, or # of links). Nodal degree: how many links connect to each node. Bisection bandwidth: The smallest bandwidth between half of the nodes to another half of the nodes. A good topology: small diameter, small nodal degree, large bisection bandwidth.

Topology Regular topologies – Nodes are connected with some kind of patterns. The graph has a structure. – Nodes are identified by coordinates. – Routing can usually pre-determined by the coordinates of the nodes. Irregular topologies – Nodes are connected arbitrarily. The graph does not have a structure, e.g. internet More extensible in comparison to regular topology. – Usually use variations of shortest path routing.

Example regular topology: complete binary tree Nodal degree = ? Diameter = ? Bisection bandwidth = ?

Example regular topology: ring topology Nodal degree = ? Diameter = ? Bisection bandwidth = ? 01234

Routing: deciding which path to take from a source to a destination 0 to 1: 0->1 or 0->4->3->2->1 Which path to use? This is a routing issue. Routing objective: – Minimize resources used Shortest path routing – The load on all links are as balanced as possible (load balancing). ??? 01234

Classification of routing schemes 0 to 1: 0->1 or 0->4->3->2->1 Deterministic.vs. adaptive – Deterministic – always the same route – Adaptive – choose load depending on traffic condition? Minimal routing: always use shortest path Source routing: the source node supplies the path Destination routing: routing based on destination ID 01234

Switching Communication data units: – Message – Packet – Flit How a packet passes a switch. Circuit switching – circuit setup, all data pass through Packet switching: the whole packet stored in a switch, and then forwarded to the next hop

Flow-control Used between hops to make sure that when data is sent, there is available buffer for the data. Built into switching mechanism sometimes.