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Advanced Computer Architecture CSE 8383

Advanced Computer Architecture CSE 8383. April 4, 2006 Session 21. Contents. Message Passing Systems (Chapters 5 & 7) Communication Patterns Client/Server Systems Clusters. Message Passing Mechanisms. Message Format Message  arbitrary number of fixed length packets

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Advanced Computer Architecture CSE 8383

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  1. Advanced Computer ArchitectureCSE 8383 April 4, 2006 Session 21

  2. Contents • Message Passing Systems (Chapters 5 & 7) • Communication Patterns • Client/Server Systems • Clusters

  3. Message Passing Mechanisms • Message Format • Message  arbitrary number of fixed length packets • Packet  basic unit containing destination address. Sequence number is needed • A packet can further be divided into flits (flow control digits) • Routing and sequence occupy header flit

  4. Message, Packets, Flits Message Packet Destination Sequence Data flit

  5. Store and Forward Routing • Packets are the basic units of information flow • Each node uses a packet buffer • A packet is transferred from S to D through a sequence of intermediate nodes • Channel and buffer must be available

  6. Wormhole Routing • Flits are the basic units of information flow • Each node uses a flit buffer • Flits are transferred from S to D through a sequence of intermediate routers in order (Pipeline) • Can be visualized as a railroad train • Flits from different packets cannot be mixed up

  7. Latency Analysis • L  packet length (in bits) • W  Channel bandwidth (bits/sec) • D  Distance (number of hops) • F  flit length (in bits)

  8. Store and Forward Latency D

  9. WH Latency D

  10. Latency Analysis • L  packet length (in bits) • W  Channel bandwidth (bits/sec) • D  Distance (number of hops) • F  flit length (in bits) • TSF = D * L/W • TWH = L/W + D* F/W  L/Wif L>>F (independent of D)

  11. Communication Patterns • Point to Point  1 - 1 • Multicast  1 - n • Broadcast  1 - all • Conference  n - n

  12. Routing potential problems Deadlock: • When 2 messages, each is holding the resources required by the other in order to move, both messages will be blocked (cyclic dependency for resources) • Straightforward solution (but inefficient) is rerouting • Another solution is avoidance of occurrence of deadlock using a strict monotonic order of network resources • Channel dependency graph (CDG) is a technique for developing a deadlock-free routing algorithm.

  13. 0 1 3 2 c1 c1 c2 c3 c4 c5 c4 c6 c2 c8 c7 c8 c5 c6 c7 c3 A 4-node network and its CDGs (a) A 4-node network (b) Channel dependency graph (CDG) c2 c3 c4 c1 c5 c6 c7 c8 (c) CDG for a deadlock-free version of the network

  14. Livelock: • A message goes around the network and never reaches its destination • It results from using adaptive routing algorithms with dynamic injection, where nodes inject their messages in the network at arbitrary times • Policies to avoid livelock are based on assigning a priority to a message injected to the network: • Messages are routed according to their priorities • Once a message is injected, only a finite number of messages will be injected with higher or equal priority.

  15. Starvation: • A node suffers from starvation if it has a message to inject into the network but is never allowed to do so. • The simplest policy to avoid starvation is to allow each node to have an injection queue that competes with the queues of the incoming links to the same node. • The main disadvantage is that a node with a high message injection rate can slow down all the other nodes in the network.

  16. Routing Efficiency • Two Parameters • Channel Traffic (number of channels used to deliver the message involved) • Communication Latency (distance)

  17. Multicast on a mesh (5 unicasts) Traffic ? Latency ?

  18. Multicast on a mesh (multicast pattern 1) Traffic ? Latency ?

  19. Multicast on a mesh (multicast pattern 2) Traffic ? Latency ?

  20. Broadcast (tree structure) 3 2 3 4 2 1 2 3 1 1 2

  21. Message Passing in PVM (Revisit) Sending Task Receiving Task User application Library User application Library 5 1 4 8 6 2 3 7 Daemon Daemon

  22. Client Client Server Server Threads Interconnection Network Client/Server Systems

  23. Server 1 Server 2 Server 3 Server n Slaves (Workers) Interconnection Network Client Master (Supervisor) A Client Server Framework for Parallel Applications

  24. Programming Environment and Tools Middleware Interconnection Network OS OS OS M M M I/O I/O I/O C C C P P P Clusters

  25. Interconnection Network Data Rate Switching Routing Ethernet 10 Mbit/sec Packet Table-based Fast Ethernet 100 Mbit/sec Packet Table-based Gigabit Ethernet 1 Gbit/sec Packet Table-based Myrinet 1.28 Gbit/sec wormhole Source-path Quadrics 7.2 Gbyte/sec wormhole Source-path Interconnection Networks in Clusters

  26. Port 4 Port 0 5 0 6 Port 5 Port 1 Port 6 Port 2 Port 7 Port 3 id 6 Dest-id Port 4 Port 0 Port 5 Port 1 Port 6 Port 2 Port 7 Port 3 Routing table Source-Path versus Table Based

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