Computer Architecture Fall 2006 Lecture 29: Network Connected Multiprocessors

1. Computer Architecture Fall 2006Lecture 29: Network Connected Multiprocessors Adapted from Mary Jane Irwin ( www.cse.psu.edu/~mji ) [Adapted from Computer Organization and Design, Patterson & Hennessy, © 2005]

2. Review: Bus Connected SMPs (UMAs) • Caches are used to reduce latency and to lower bus traffic • Must provide hardware for cache coherence and process synchronization • Bus traffic and bandwidth limits scalability (<~ 36 processors) Processor Processor Processor Processor Cache Cache Cache Cache Single Bus Memory I/O

3. Review: Multiprocessor Basics • Q1 – How do they share data? • Q2 – How do they coordinate? • Q3 – How scalable is the architecture? How many processors?

4. Processor Processor Processor Cache Cache Cache Memory Memory Memory Interconnection Network (IN) Network Connected Multiprocessors • Either a single address space (NUMA and ccNUMA) with implicit processor communication via loads and stores ormultiple private memories with message passing communication with sends and receives • Interconnection network supports interprocessor communication

5. Summing 100,000 Numbers on 100 Processors • Start by distributing 1000 elements of vector A to each of the local memories and summing each subset in parallel sum = 0; for (i = 0; i<1000; i = i + 1) sum = sum + Al[i]; /* sum local array subset • The processors then coordinate in adding together the sub sums (Pn is the number of the processor, send(x,y) sends value y to processor x, and receive() receives a value) half = 100; limit = 100; repeat half = (half+1)/2; /*dividing line if (Pn>= half && Pn<limit) send(Pn-half,sum); if (Pn<(limit/2)) sum = sum + receive(); limit = half; until (half == 1); /*final sum in P0’s sum

6. sum sum sum sum sum sum sum sum sum sum P0 P1 P2 P3 P4 P5 P6 P7 P8 P9 An Example with 10 Processors half = 10

7. Communication in Network Connected Multi’s • Implicit communication via loads and stores • hardware designers have to provide coherent caches and process synchronization primitive • lower communication overhead • harder to overlap computation with communication • more efficient to use an address to remote data when demanded rather than to send for it in case it might be used (such a machine has distributed shared memory (DSM)) • Explicit communication via sends and receives • simplest solution for hardware designers • higher communication overhead • easier to overlap computation with communication • easier for the programmer to optimize communication

8. Cache Coherency in NUMAs • For performance reasons we want to allow the shared data to be stored in caches • Once again have multiple copies of the same data with the same address in different processors • bus snooping won’t work, since there is no single bus on which all memory references are broadcast • Directory-base protocols • keep a directory that is a repository for the state of every block in main memory (which caches have copies, whether it is dirty, etc.) • directory entries can be distributed (sharing status of a block always in a single known location) to reduce contention • directory controller sends explicit commands over the IN to each processor that has a copy of the data

9. IN Performance Metrics • Network cost • number of switches • number of (bidirectional) links on a switch to connect to the network (plus one link to connect to the processor) • width in bits per link, length of link • Network bandwidth (NB) – represents the best case • bandwidth of each link * number of links • Bisection bandwidth (BB) – represents the worst case • divide the machine in two parts, each with half the nodes and sum the bandwidth of the links that cross the dividing line • Other IN performance issues • latency on an unloaded network to send and receive messages • throughput – maximum # of messages transmitted per unit time • # routing hops worst case, congestion control and delay

10. Bus IN • N processors, 1 switch ( ), 1 link (the bus) • Only 1 simultaneous transfer at a time • NB = link (bus) bandwidth * 1 • BB = link (bus) bandwidth * 1 Bidirectional network switch Processor node

11. Ring IN • If a link is as fast as a bus, the ring is only twice as fast as a bus in the worst case, but is N times faster in the best case • N processors, N switches, 2 links/switch, N links • N simultaneous transfers • NB = link bandwidth * N • BB = link bandwidth * 2

12. Fully Connected IN • N processors, N switches, N-1 links/switch, (N*(N-1))/2 links • N simultaneous transfers • NB = link bandwidth * (N*(N-1))/2 • BB = link bandwidth * (N/2)2

13. Crossbar (Xbar) Connected IN • N processors, N2 switches (unidirectional),2 links/switch, N2 links • N simultaneous transfers • NB = link bandwidth * N • BB = link bandwidth * N/2

14. 3-cube Hypercube (Binary N-cube) Connected IN • N processors, N switches, logN links/switch, (NlogN)/2 links • N simultaneous transfers • NB = link bandwidth * (NlogN)/2 • BB = link bandwidth * N/2 2-cube

15. 2D and 3D Mesh/Torus Connected IN • N simultaneous transfers • NB = link bandwidth * 4N or link bandwidth * 6N • BB = link bandwidth * 2 N1/2 or link bandwidth * 2 N2/3 • N processors, N switches, 2, 3, 4 (2D torus) or 6 (3D torus) links/switch, 4N/2 links or 6N/2 links

16. Fat Tree • N processors, log(N-1)*logN switches, 2 up + 4 down = 6 links/switch, N*logN links • N simultaneous transfers • NB = link bandwidth * NlogN • BB = link bandwidth * 4

17. Fat Tree • Trees are good structures. People in CS use them all the time. Suppose we wanted to make a tree network. A B C D • Any time A wants to send to C, it ties up the upper links, so that B can't send to D. • The bisection bandwidth on a tree is horrible - 1 link, at all times • The solution is to 'thicken' the upper links. • More links as the tree gets thicker increases the bisection • Rather than design a bunch of N-port switches, use pairs

18. SGI NUMAlink Fat Tree www.embedded-computing.com/articles/woodacre

19. IN Comparison • For a 64 processor system

20. Network Connected Multiprocessors

21. IBM BlueGene

22. A BlueGene/L Chip 11GB/s 32K/32K L1 440 CPU Double FPU 2KB L2 4MB L3 ECC eDRAM 128B line 8-way assoc 128 256 5.5 GB/s 16KB Multiport SRAM buffer 256 700 MHz 256 32K/32K L1 440 CPU Double FPU 2KB L2 128 256 5.5 GB/s 11GB/s Gbit ethernet DDR control 3D torus Fat tree Barrier 1 8 6 in, 6 out 1.6GHz1.4Gb/s link 3 in, 3 out 350MHz 2.8Gb/s link 4 global barriers 144b DDR 256MB 5.5GB/s

23. Networks of Workstations (NOWs) Clusters • Clusters of off-the-shelf, whole computers with multiple private address spaces • Clusters are connected using the I/O bus of the computers • lower bandwidth that multiprocessor that use the memory bus • lower speed network links • more conflicts with I/O traffic • Clusters of N processors have N copies of the OS limiting the memory available for applications • Improved system availability and expandability • easier to replace a machine without bringing down the whole system • allows rapid, incremental expandability • Economy-of-scale advantages with respect to costs

24. Commercial (NOW) Clusters

25. Summary • Flynn’s classification of processors - SISD, SIMD, MIMD • Q1 – How do processors share data? • Q2 – How do processors coordinate their activity? • Q3 – How scalable is the architecture (what is the maximum number of processors)? • Shared address multis – UMAs and NUMAs • Scalability of bus connected UMAs limited (< ~ 36 processors) • Network connected NUMAs more scalable • Interconnection Networks (INs) • fully connected, xbar • ring • mesh • n-cube, fat tree • Message passing multis • Cluster connected (NOWs) multis

26. Next Lecture and Reminders • Reminders • Final Tuesday, December 12 from 8 – 9:50 AM • Next lecture