computer organization - penn engineeringese534/fall2016/lectures/day23_6up.pdfoptimal delay for...

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Penn ESE534 Fall 2016 -- DeHon 1

ESE534: Computer Organization

Day 23: November 21, 2016 Time Multiplexing

Tabula

•  March 1, 2010 – Announced new

architecture •  We would say

– w=1, c=8 arch. •  March, 2015

– Tabula closed doors

Penn ESE534 Fall 2016 -- DeHon 2 [src: www.tabula.com]

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Previously

•  Basic pipelining •  Saw how to reuse resources at

maximum rate to do the same thing •  Saw how to use instructions to reuse

resources in time to do different things •  Saw demand-for and options-to-support

data retiming

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Today

•  Multicontext – Review why – Cost – Packing into contexts – Retiming requirements for Multicontext – Some components

•  [concepts we saw in overview week 2-3, we can now dig deeper into details]

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How often is reuse of the same operation applicable?

•  In what cases can we exploit high-frequency, heavily pipelined operation?

•  …and when can we not?

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How often is reuse of the same operation applicable?

•  Can we exploit higher frequency offered? – High throughput, feed-forward (acyclic) – Cycles in flowgraph

•  abundant data level parallelism [C-slow] •  no data level parallelism

– Low throughput tasks •  structured (e.g. datapaths) [serialize datapath] •  unstructured

– Data dependent operations •  similar ops [local control – lecture after next] •  dis-similar ops

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Structured Datapaths

•  Datapaths: same pinst for all bits

•  Can serialize and reuse the same data elements in succeeding cycles

•  example: adder

Preclass 1

•  Recall looked at mismatches – Width, instruction depth/task length

•  Sources of inefficient mapping Wtask=4, Ltask=4

to Warch=1, C=1 architecture?

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Preclass 1

•  How transform Wtask=4, Ltask=4 (path length from throughput) to run efficiently on Warch=1, C=1 architecture?

•  Impact on efficiency?

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Throughput Yield

FPGA Model -- if throughput requirement is reduced for wide word operations, serialization allows us to reuse active area for same computation

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Throughput Yield

Same graph, rotated to show backside.

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How often is reuse of the same operation applicable?

•  Can we exploit higher frequency offered? – High throughput, feed-forward (acyclic) – Cycles in flowgraph

•  abundant data level parallelism [C-slow] •  no data level parallelism

– Low throughput tasks •  structured (e.g. datapaths) [serialize datapath] •  unstructured

– Data dependent operations •  similar ops [local control -- next time] •  dis-similar ops

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Remaining Cases

•  Benefit from multicontext as well as high clock rate

•  i.e. – cycles, no parallelism – data dependent, dissimilar operations –  low throughput, irregular (can’t afford swap?)

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Single Context/Fully Spatial •  When have:

– cycles and no data parallelism –  low throughput, unstructured tasks – dis-similar data dependent tasks

•  Active resources sit idle most of the time – Waste of resources

•  Cannot reuse resources to perform different function, only same

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Resource Reuse

•  To use resources in these cases – must direct to do different things.

•  Must be able tell resources how to behave

•  ! separate instructions (pinsts) for each behavior

Preclass 2

•  How schedule onto 3 contexts?

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Preclass 2

•  How schedule onto 4 contexts?

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Preclass 2

•  How schedule onto 6 contexts?

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Multicontext Organization/Area

•  Actxt≈20KF2 –  dense encoding

•  Abase≈200KF2

•  Actxt :Abase = 1:10

Preclass 3

•  Area: – Single context? – 3 contexts? – 4 contexts? – 6 contexts?

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Multicontext Tradeoff Curves

•  Assume Ideal packing: Nactive=Ntotal/L

Reminder: Robust point: c*Actxt=Abase Penn ESE534 Fall 2016 -- DeHon

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In Practice

Limitations from: •  Scheduling •  Retiming

Scheduling

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Scheduling Limitations

•  NA (active) – size of largest stage

•  Precedence (data dependence): can evaluate a LUT only after predecessors

have been evaluated "cannot always, completely equalize stage

requirements

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Scheduling •  Precedence limits packing freedom •  Note difference Preclass 2 vs. Ntotal/L

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Reminder (Preclass 2)

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Sequentialization

•  Adding time slots – more sequential (more latency) – add slack

•  allows better balance

L=4 →NA=2 (4 contexts)

Retiming

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Multicontext Data Retiming

•  How do we accommodate intermediate data?

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Signal Retiming

•  Single context, non-pipelined – hold value on LUT Output (wire)

•  from production through consumption

– Wastes wire and switches by occupying •  for entire critical path delay L •  not just for 1/L’th of cycle takes to cross wire

segment

– How show up in multicontext?

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Signal Retiming

•  Multicontext equivalent – need LUT to hold value for each

intermediate context

Retiming in Multicontext

•  May need to retime a value for multiple cycles to deliver to the consumer

•  If our retiming resource is a LUT output flop – We are forced to use a full LUT for signal

retiming •  …but we saw last time could retime

more economically…

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ASCII→Hex Example

Single Context: 21 LUTs @ 220KF2=4.6MF2

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ASCII→Hex Example

Three Contexts: 12 LUTs @ 260KF2=3.1MF2

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ASCII→Hex Example •  All retiming on wires (active outputs)

– saturation based on inputs to largest stage

Ideal≡Perfect scheduling spread + no retime overhead

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Alternate Retiming •  Recall from last time (Day 22)

– Net buffer •  smaller than LUT

– Output retiming •  may have to route multiple times

–  Input buffer chain •  only need LUT every depth cycles

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Depth Mismatch Efficiency

•  What happens when Arch(retime) > App(retime) ?

•  What happens when Arch(retime) < App(retime) ?

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Input Buffer Retiming

•  Can only take K unique inputs per cycle •  Configuration depth differ from context-

to-context –  Cannot schedule LUTs in slot 2 and 3 on the same

physical block, since require 6 inputs.

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ASCII→Hex Example •  All retiming on wires (active outputs)

– saturation based on inputs to largest stage

Ideal≡Perfect scheduling spread + no retime overhead

Reminder

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ASCII→Hex Example (input retime)

@ depth=4, c=6: 1.4M F2

(compare 4.6M F2 ) 3.4×

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General throughput mapping: •  If only want to achieve limited throughput •  Target produce new result every t cycles 1.  Spatially pipeline every t stages

cycle = t 2.  retime to minimize register requirements 3.  multicontext evaluation w/in a spatial stage

try to minimize resource usage 4.  Map for depth (i) and contexts (c)

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Benchmark Set •  23 MCNC circuits

– area mapped with SIS and Chortle

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Multicontext vs. Throughput

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Multicontext vs. Throughput

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General Theme

•  Ideal Benefit – e.g. Active=N/C

•  Logical Constraints – Precedence

•  Resource Limits – Sometimes

bottleneck •  Net Benefit •  Resource Balance

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Computing Device Composition

•  Retiming as key parameter (d) –  Alongside c, w, p

•  How balance for designs with c>1

Beyond Area

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Only an Area win?

•  If area were free, would we always want a fully spatial design?

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Communication Latency

•  Communication latency across chip can limit designs

•  Serial design is smaller " less latency

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Optimal Delay for Graph App.

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What Minimizes Energy

•  HW5 (different model)

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0

5000

10000

15000

20000

25000

1 2 4 8 16

32

64

128

256

512

1024

20

48

4096

81

92

1638

4

"EnergyALU"

"EnergyALU"

€

B = N

Multicontext Energy

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N=229 gates

Multicontext

Energy normalized to FPGA

Processor W=64, I=128

FPGA

[DeHon / FPGA 2014]

Compare vs. p, W=1, N=219

DeHon--FPT 2015 53

Processor

Multicontext

FPGA

Energy

Rent p

Components

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DPGA (1995)

[Tau et al., FPD 1995]

Xilinx Time-Multiplexed FPGA

•  Mid 1990s Xilinx considered Multicontext FPGA – Based on XC4K (pre-Virtex) devices – Prototype Layout in F=500nm – Required more physical interconnect than XC4K – Concerned about power (10W at 40MHz)

Penn ESE534 Fall 2016 -- DeHon 56 [Trimberger, FCCM 1997]

Xilinx Time-Multiplexed FPGA •  Two unnecessary expenses:

– Used output registers with separate outs – Based on XC4K design

•  Did not densely encode interconnect configuration •  Compare 8 bits to configure input C-Box connection

– Versus log2(8)=3 bits to control mux select – Approx. 200b pinsts vs. 64b pinsts

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Tabula •  8 context, 1.6GHz, 40nm

– 64b pinsts •  Our model w/ input retime

– 1Mλ2 base •  80Kλ2 / 64b pinst Instruction mem/context •  40Kλ2 / input-retime depth

– 1Mλ2+8×0.12Mλ2~=2Mλ2 ! 4× LUTs (ideal) •  Recall ASCIItoHex 3.4, similar for thput map

•  They claim 2.8× LUTs Penn ESE534 Fall 2016 -- DeHon

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[MPR/Tabula 3/29/2009]

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Big Ideas [MSB Ideas]

•  Several cases cannot profitably reuse same logic at device cycle rate – cycles, no data parallelism –  low throughput, unstructured – dis-similar data dependent computations

•  These cases benefit from more than one instructions/operations per active element

•  Actxt<< Aactive makes interesting – save area by sharing active among

instructions Penn ESE534 Fall 2016 -- DeHon 60

Big Ideas [MSB-1 Ideas]

•  Energy benefit for large W, p •  For c>1, retiming balance drives d>1

•  one output reg/LUT leads to retiming bottleneck

•  c=4--8, I=4--6 automatically mapped designs roughly 1/3 single context size

•  Most FPGAs typically run in realm where multicontext is smaller (maybe not less energy) – How many for intrinsic reasons? – How many for lack of register/CAD support?

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Admin

•  Wednesday 11/23 is a “Friday” – No class for us

•  Next lecture Monday – Reading on web

•  Final Exercise – Nothing due “Friday” Wednesday 11/23 – First milestone due Wed 11/30