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ESE532: System-on-a-Chip Architecture

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Presentation on theme: "ESE532: System-on-a-Chip Architecture"— Presentation transcript:

1 ESE532: System-on-a-Chip Architecture
Day 19: March 29, 2017 Network-on-a-Chip (NoC) Penn ESE532 Spring DeHon

2 Today Ring 2D Mesh Networks Design Issues Buffering and deflection
Dynamic and static routing Penn ESE532 Spring DeHon

3 Message Scalable interconnect for locality
has rich design space Customize to compute and application Support real-time with static scheduled communication Penn ESE532 Spring DeHon

4 Interconnect Will need an infrastructure for programmable connections
Day 8 Interconnect Will need an infrastructure for programmable connections Rich design space to tune area-bandwidth-locality Will explore more later in course Penn ESE532 Spring DeHon

5 Interconnect Concerns
Avoid being a bottleneck Bandwidth Latency Competes for area and energy against compute and memory Penn ESE532 Spring DeHon

6 Crossbar Connect any I inputs, O outputs Area ~ I×O
For N PEs scale as N2 Penn ESE532 Spring DeHon

7 Today’s SoC Large At 1mm2 per A9, can put 100 on 1cm2 chip
120 core MIPS on Stratix V FPGA FPGA 2017 1680 core RISC-V on Xilinx Ultrascale Scaling to 100s and 1000s of processing elements (PEs) that need interconnect Penn ESE532 Spring DeHon

8 Locality Delay and energy proportional to distance
Want to keep communications short Data near compute From compute block to compute block How build network? Scalable (Area ~ N = things connected?) Supports locality Penn ESE532 Spring DeHon

9 Day 8 Mesh Penn ESE532 Spring DeHon

10 Bus to Ring Penn ESE532 Spring DeHon

11 Ring Penn ESE532 Spring DeHon

12 Preclass 1 Traffic pattern Similar bandwidth?
One has higher bandwidth? Penn ESE532 Spring DeHon

13 Bidirectional Ring Penn ESE532 Spring DeHon

14 Interleaved Layout What problem does this layout solve?
Penn ESE532 Spring DeHon

15 2D Layout Penn ESE532 Spring DeHon

16 Scaling How does area scale with N?
How does neighbor distance scale with N? Unidirectional bidirectional How does worst-case distance in ring scale with N? Penn ESE532 Spring DeHon

17 1D to 2D Penn ESE532 Spring DeHon

18 Ring Abstract Penn ESE532 Spring DeHon

19 Row and Column Rings Penn ESE532 Spring DeHon

20 Mesh as Row & Column Rings
Penn ESE532 Spring DeHon

21 Directional Mesh (Torus)
Penn ESE532 Spring DeHon

22 Mesh Datapath Penn ESE532 Spring DeHon

23 Bidirectional Mesh Penn ESE532 Spring DeHon

24 2D Mesh Scaling How does area scale with N?
How does neighbor distance scale with N? How does worst-case distance in mesh scale with N? Penn ESE532 Spring DeHon

25 Specifying Destination
Simple: add destination address Ring or Mesh wires carry: Valid bit + Address + Payload (Data) Penn ESE532 Spring DeHon

26 Mesh Routing Route in Y until reach row
Then route in X until reach column Consume from PE when arrives Penn ESE532 Spring DeHon

27 Mesh Routing Not deal with congestion
Yin Mesh Routing Xin Xout Yout=Yin.valid & row(Yin.address)!=row & Yin + Pin.valid & P Xout=Xin.valid & column(Xin.address)!=column & Xin + Yin.valid & row(Yin.address==row) Not deal with congestion Yout Penn ESE532 Spring DeHon

28 Mesh Routing Yout=Yin.valid & row(Yin.address)!=row & Yin + Pin.valid & P Xout=Xin.valid & column(Xin.address)!=column & Xin + Yin.valid & row(Yin.address==row) Complexity of route function can impact Area, cycle time, route latency Penn ESE532 Spring DeHon

29 Mesh Congestion Penn ESE532 Spring DeHon

30 Mesh Congest What happens when inputs from 2 sides want to travel out same output? (here Xin, Yin) Penn ESE532 Spring DeHon

31 Dealing with Congestion
Don’t let it happen (offline/static) Schedule to avoid Online/dynamic Store in place -- Buffer Misroute -- Deflect Penn ESE532 Spring DeHon

32 Congestion 1D For simplicity, we look at congestion in 1D case (Preclass 2) Penn ESE532 Spring DeHon

33 Preclass 2a Complete table – identify uncongested latencies
Penn ESE532 Spring DeHon

34 Preclass 2b Cycles from simulation? Penn ESE532 Spring DeHon

35 Observe Did have congestion How we make decisions matters
Ran slower than the single-link case How we make decisions matters Who gets to route, which is stalled Best, global decision can be better than local decisions Penn ESE532 Spring DeHon

36 Offline vs. Online [Kapre et al., FCCM 200]
Penn ESE532 Spring DeHon

37 Dealing with Congestion
Don’t let it happen (offline/static) Schedule to avoid Online/dynamic Store in place -- Buffer Misroute -- Deflect Penn ESE532 Spring DeHon

38 Congestion: Buffer Store inputs that must wait until path available
Typically store in FIFO buffer How big do we make the FIFO? FIFO Buffers cost space Often more than multiplexers Penn ESE532 Spring DeHon

39 Congestion: Buffer Store inputs that must wait until path available
Typically store in FIFO buffer How big do we make the FIFO? What if FIFO full? Penn ESE532 Spring DeHon

40 Congestion: Buffer Store inputs that must wait until path available
Typically store in FIFO buffer How big do we make the FIFO? What if FIFO full? Penn ESE532 Spring DeHon

41 Congestion: Deflect Misroute: (deflection routing)
Send in to an available (wrong) direction Avoid Buffer Requires balance of ins and outs Can make work on mesh How much more traffic do we create misrouting? Penn ESE532 Spring DeHon

42 Mesh Routing: Yout=Yin.valid & row(Yin.address)!=row & Yin + Pin.valid & P +row(Yin.address)==row & (column.Xin.address)!=column) & Y.in Xout=Xin.valid & column(Xin.address)!=column & Xin + Yin.valid & row(Yin.address==row) Gives Preference to X Penn ESE532 Spring DeHon

43 Alternates: random selection preference based on aging (keep track of # of times misrouted) Mesh Routing: Yout=Yin.valid & row(Yin.address)!=row & Yin + Pin.valid & P +row(Yin.address)==row & (column.Xin.address)!=column) & Y.in Xout=Xin.valid & column(Xin.address)!=column & Xin + Yin.valid & row(Yin.address==row) Penn ESE532 Spring DeHon

44 Static Schedule Store per-cycle instruction for switch
Doesn’t need address header on route Static, local memories control destination Penn ESE532 Spring DeHon

45 Alternate Static Schedule
Control injection cycle from processor so never have conflict Simple datapath logic to select available data Needs address header on routed data Penn ESE532 Spring DeHon

46 Mesh Packet Switched 32b Split-Merge FIFO Hoplite Deflection
bidrectional 1800 LUTs Hoplite Deflection undirectional 60 LUTs Big difference in area costs. Need to look at area and benefits. [Kapre+Gray, FPL 2015] Penn ESE532 Spring DeHon

47 Deflection Route What concerns might we have about deflection route?
Penn ESE532 Spring DeHon

48 Buffer vs. Deflection [Kapre+Gray, FPL 2015]
Penn ESE532 Spring DeHon

49 Take 2, they are small [Kapre+Gray, FPL 2015]
Penn ESE532 Spring DeHon

50 Tune Bandwidth Add channels to tune bandwidth Rings per row, column
Single Hoplite channel ~60 …two around 120 … still << 1800 Penn ESE532 Spring DeHon

51 Mesh Area Deflection PS/TM
[Kapre FCCM 2015] Penn ESE532 Spring DeHon

52 Static Schedule vs. Deflection
[Kapre FCCM 2015] Penn ESE532 Spring DeHon

53 Static Schedule vs. Deflection
Routing 142K message add20 benchmark Marathon statically schedule PS [Kapre FCCM 2015] Penn ESE532 Spring DeHon

54 Mesh Customization Penn ESE532 Spring DeHon

55 Tuning Down Bandwidth If need less bandwidth, cluster multiple PEs to share a router. Penn ESE532 Spring DeHon

56 Simple Bandwidth/Area Control
Width of channels Like SIMD All bits going to same destination Penn ESE532 Spring DeHon

57 Packets Simple story is, each “word” routed on mesh is: address+payload Alternately: Multiword packet with single address Share “address” across larger payload Control width of datapath separate from size of payload Additional control issues to route packet together and buffer Penn ESE532 Spring DeHon

58 Customization Bandwidth Directional/Bidirectional
Width, clustering, channels Directional/Bidirectional Online dynamic/offline static Buffer/deflect Buffer depth Route function sophistication Penn ESE532 Spring DeHon

59 Large VLIW Natural to use static network with VLIW clusters
Network routing becomes part of long instruction word Extreme one operator per mesh PE Tune bandwidth by clustering Penn ESE532 Spring DeHon

60 Big Ideas Scalable interconnect for locality
Has rich design space Customize to compute and application Support real-time with static scheduled communication Penn ESE532 Spring DeHon

61 Admin Project Design Space Milestone Next milestone out by Friday
Due Friday Next milestone out by Friday 4x, area estimate Penn ESE532 Spring DeHon

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