Communication has significant impact on application performance. Interconnection networks therefore have a vital role in cluster systems.

Transcription

1 Cluster Networks Introduction Communication has significant impact on application performance. Interconnection networks therefore have a vital role in cluster systems. As usual, the driver is performance An increase in compute power typically demands proportional increases in lower latency / higher bandwidth communication services. 1

2 Cluster Networks Issues with cluster interconnections are similar to those with normal networks: Latency & Bandwidth Topology type (bus, ring, torus, hypercube etc). Routing Direct connections (point-to-point) or indirect connections. NIC (Network Interface Card) capabilities. Physical medium (Twisted pair, fibre optic) Balance between performance and cost 2

3 Interconnection Topologies In standard LANs we have two general structures: Shared network (bus) All messages are broadcast each processor listens to every message. Requires complex access control (e.g. CSMA/CD). CSMA/CD: Carriers Sense Multiple Access with Collision Detection Collisions can occur: requires back-off policies and retransmissions. Suitable when the offered load is low - inappropriate for high performance applications. Very little reason to use this form of network today. Switched network Permits point-to-point communications between sender & receiver. Fast internal transport provides high aggregate bandwidth. Multiple messages are sent simultaneously. 3

4 Metrics to evaluate network topology Useful metrics for switched network topology: Scalability : the network s switch scalability with nodes. Degree: number of links to / from a node measure aggregate bandwidth Diameter: the shortest path between the furthest nodes. measure latency Bisection width: the minimum number of links that must be cut in order to divide the topology into two independent networks of the same size (+/- one node). Essentially a measure of bottleneck bandwidth - if higher, the network will perform better under load. 4

5 Interconnection Topologies Crossbar switch: Low latency and high throughput. Switch scalability is poor - O(N 2 ) Lots of wiring 5

6 Interconnection Topologies Linear Arrays and Rings Consider networks with switch scaling costs better than O(N 2 ). In one dimension, we have simple linear arrays. O(N) switches. These can wrap around to make a ring or 1D torus. latency is high. 2D/3D Cartesian applications will perform poorly with this network. 6

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8 Interconnection Topologies 2D Meshes Can wrap-around as a 2D torus. Switch scaling: O(N) Average degree: 4 Diameter: O(2n 1/2 ) Bisection width: O(n 1/2 ) 7

9 Interconnection Topologies Hypercubes: K dimension, Switches N= 2 K. Diameter: O(K). Good bisectional width (O(2 K-1 )). 8

10 Interconnection Topologies Binary Tree: Scaling: n = 2 d processor nodes (where d = depth) 2 d+1-1 switches Degree: 3 Diameter: O(2d) Bisection width: O(1) 9

11 Interconnection Topologies Fat trees: Similar in diameter to a binary tree. Bisection width (which equates to bottleneck) is greatly improved due to additional dimensions. 10

12 Interconnection Topologies Summary of topologies: Topology Degree Diameter Bisection 1D Array 2 N-1 1 1D Ring 2 N/2 2 2D Mesh 4 2N 1/2 N 1/2 2D Torus 4 N 1/2 2N 1/2 Hypercube n=log 2 (N) n N/2 11

13 Switching Operational modes: Store-and-forward: Each switch receives an entire packet before it forwards it onto the next switch - useful in a non-dedicated environment (I.e. a LAN). usually, there is a finite buffer size so it is possible that packets will be dropped under heavy load. Also impose a larger in-switch latency. Can detect errors in the packets Worm hole routing (Also called cut-through switching): Packet is divided into small flits (flow control digits). Switch examines the first flit (header) which contains the destination address, sets up a circuit and forwards the flit immediately. Subsequent flits of the message are forwarded as they arrive (near wirespeed). Reduces latency and buffer overhead. Messaging occurs at a speed close to the processors being directly connected. Less error detection 12

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17 Cluster Network Products Cluster interconnects include, among others: Gigabit Ethernet Myrinet Quadrics InfiniBand 1

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19 Interconnects in Top500 list 11/2009 2

20 Interconnects in Top500 list 11/2008 3

21 Cluster Network Technologies Gigabit Ethernet: The technology has matured and now offers very good performance at a very low cost. Latency performance is moderate - many Ethernet switches are designed for general LANs (store & forward) where latency reduction is not necessary the primary incentive (the latency is order of ms). Zero-copy OS-bypass message passing can be supported with programmable NIC and direct memory access. 4

22 Cluster Network Technologies Myrinet: using fibre optic cable Uses a fat-tree structure Low latency (7-10 µsec) with a peak bandwidth of 4G bps. Provides zero-copy message passing and can offload packet processing to the NIC. Uses cut-through/worm-hole switching to reduce latency. More expensive than Ethernet (a) Twisted pair cable in Ethernet (b) Fibre optic cable 5

23 Zero copy protocol 6

24 Cluster Network Technologies Quadrics: product of a strategic partnership between Quadrics & Compaq (used in ASCI/Q). Uses a fat quad-tree topology Very low latency of 2-5 µsec; bandwidth is about 2Gbps 7

25 Cluster Network Technologies InfiniBand: by Intel. Basic link speed of 2.5Gb/s. Cut-through/worm-hole switches are used. Latency is about 200 nanoseconds. 8

26 BlueGene/L No. 1 in Top500 list from Source: IBM 10

27 BlueGene/L networking BlueGene system employs various network types. Central is the torus interconnection network: 3D torus with wrap-around. Each node connects to six neighbours (bidirectional). Routing achieved in hardware. each link with 1.4 Gbit/s. 1.4 x 6 x 2= 16.8 Gbit/s aggregate bandwidth 11

28 BlueGene/L Other three networks: Binary combining tree Used for collective/global operations - reductions, sums, products, barriers etc. Low latency (2μS) Gigabit Ethernet I/O network Support file I/O An I/O node is responsible for performing I/O operations for 128 processors Diagnostic & control network Booting nodes, monitoring processors. Each chip has the above four network interfaces (torus, tree, i/o, diagnostics) Note specialised networks are used for different purposes - quite different from many other HPC cluster architectures. 12

29 BlueGene/L Message Passing: The BlueGene focussed a good deal of energy developing an efficient MPI implementation to reduce latency in the software stack. Using the MPICH code-base as a start-point: MPI library was enhanced with respect to machine architecture. For example, using the combining tree for reductions & broadcasts. Reading paper: Filtering Failure Logs for a BlueGene/L Prototype 13

30 ASCI Q The Q supercomputing system at Los Alamos National Laboratory (LANL) Product of Advanced Simulation and Computing (ASCI) program Used for simulation and computational modelling No. 2 in 2002 in Top500 supercomputer list 14

31 ASCI Q Classical cluster architecture SMPs (AlphaServer ES45s from HP) are put in one segment Each with four EV Ghz CPUs with 16-MB cache the whole system has 3 segments The three segments can operate independently or as a single system Aggregate 60 TeraFLOPS capability. 33 Terabytes of memory 664 TB of global storage Interconnection using Quadrics switch interconnect (QSNet) High bandwidth (250MB/s) and Low latency (5us) network. Top500 list: 15

32 Earth Simulator Built by NEC, located in the Earth Simulator Centre in Japan Used for running global climate models to evaluate the effects of global warming No.1 from

33 Earth Simulator 640 nodes, each with 8 vector processors and 16GB memory Two nodes are installed in one cabinet In total: 5120 processors (NEC SX-5) 10 TeraByte memory 700 TeraByte of disk storage and 1.6 PetaByte of Tape storage Computing capacity: 36 TFlop/s Networking: Crossbar interconnection (very expensive) Bandwidth: 16GB/s between any two nodes Latency: 5us Dual level parallelism: OpenMP in-node, MPI out of node Physical installation: Machine resides on 3th floor; Cables on 2nd ; Power generation & cooling on 1st and ground floor. 17