Download Lecture 3: DSP - University of California, Berkeley

Lecture 18:
Introduction to Multiprocessors
Prepared and presented by:
Kurt Keutzer
with thanks for materials from
Kunle Olukotun, Stanford;
David Patterson, UC Berkeley
1
Why Multiprocessors?
Needs
 Relentless demand for higher performance
» Servers
» Networks

Commercial desire for product differentiation
Opportunities
 Silicon capability
 Ubiquitous computers
2
Exploiting (Program) Parallelism
Levels of Parallelism
Process
Thread
Loop
Instruction
1
10
100
1K
10K
100K
1M
Grain Size (instructions)
3
Exploiting (Program) Parallelism -2
Levels of Parallelism
Process
Thread
Loop
Instruction
Bit
1
10
100
1K
10K
100K
1M
Grain Size (instructions)
4
Need for Parallel Computing

Diminishing returns from ILP

Performance
» Limited ILP in programs
» ILP increasingly expensive
to exploit
Peak performance increases
linearly with more processors
» Amhdahl’s law applies
Adding processors is
inexpensive
» But most people add
memory also
2P+M
Performance

Die Area
2P+2M
P+M
Die Area
5
What to do with a billion transistors ?
1 clk

Technology changes the cost
and performance of computer
elements in a non-uniform
manner
» logic and arithmetic is
becoming plentiful and
cheap
» wires are becoming slow and
scarce

This changes the tradeoffs
between alternative
architectures
» superscalar doesn’t scale
well
– global control and data

So what will the architectures
of the future be?
1998
2001
2004
64 x the area
4x the speed
slower wires
2007
3 (10, 16, 20?) clks
6
Elements of a multiprocessing system







General purpose/special purpose
Granularity - capability of a basic module
Topology - interconnection/communication geometry
Nature of coupling - loose to tight
Control-data mechanisms
Task allocation and routing methodology
Reconfigurable
» Computation
» Interconnect



Programmer’s model/Language support/ models of computation
Implementation - IC, Board, Multiboard, Networked
Performance measures and objectives
[After E. V. Krishnamurty - Chapter 5
7
Use, Granularity
General purpose
 attempting to improve general purpose computation (e.g. Spec
benchmarks) by means of multiprocessing
Special purpose
 attempting to improve a specific application or class of
applications by means of multiprocessing
Granularity - scope and capability of a processing element (PE)
 Nand gate
 ALU with registers
 Execution unit with local memory
 RISC R1000 processor
8
Topology
Topology - method of interconnection of processors
 Bus
 Full-crossbar switch
 Mesh
 N-cube
 Torus
 Perfect shuffle, m-shuffle
 Cube-connected components
 Fat-trees
9
Coupling
Relationship of communication among processors
 Shared clock (Pipelined)
 Shared registers (VLIW)
 Shared memory (SMM)
 Shared network
10
Control/Data
Way in which data and control are organized
Control - how the instruction stream is managed (e.g. sequential
instruction fetch)
Data - how the data is accessed (e.g. numbered memory
addresses)
 Multithreaded control flow - explicit constructs: fork, join, wait,
control program flow - central controller
 Dataflow model - instructions execute as soon as operands are
ready, program structures flow of data, decentralized control
11
Task allocation and routing
Way in which tasks are scheduled and managed
Static - allocation of tasks onto processing elements predetermined before runtime
Dynamic - hardware/software support allocation of tasks to
processors at runtime

12
Reconfiguration
Computational
 restructuring of computational elements
» reconfigurable - reconfiguration at compile time
» dynamically reconfigurable- restructuring of computational
elements at runtime
Interconnection scheme
 switching network - software controlled
 reconfigurable fabric
13
Programmer’s model
How is parallelism expressed by the user?
Expressive power
 Process-level parallelism
» Shared-memory
» Message-passing
Operator-level parallelism
 Bit-level parallelism
Formal guarantees
 Deadlock-free
 Livelock free
Support for other real-time notions
 Exception handling

14
Parallel Programming Models

Message Passing
» Fork thread
– Typically one per node
» Explicit communication
– Send messages
– send(tid, tag, message)
– receive(tid, tag, message)
» Synchronization
– Block on messages
(implicit sync)
– Barriers

Shared Memory (address space)
» Fork thread
– Typically one per node
» Implicit communication
– Using shared address
space
– Loads and stores
» Synchronization
– Atomic memory operators
– barriers
15
Message Passing Multicomputers

Computers (nodes) connected by a network
» Fast network interface
– Send, receive, barrier
» Nodes not different than regular PC or workstation

Cluster conventional workstations or PCs with fast network
» cluster computing
» Berkley NOW
» IBM SP2
Node
P
P
P
M
M
M
Network
16
Shared-Memory Multiprocessors

Several processors share one
address space
» conceptually a shared
memory
» often implemented just like a
multicomputer
– address space
distributed over private
memories

Communication is implicit
» read and write accesses to
shared memory locations

Synchronization
» via shared memory locations
– spin waiting for non-zero
» barriers
P
P
P
Network
M
Conceptual Model
P
P
P
M
M
M
Network
Actual Implementation
17
Cache Coherence - A Quick Overview


With caches, action is required
to prevent access to stale data
P1
P2
PN
» Processor 1 may read old
data from its cache instead
of new data in memory or
» Processor 3 may read old
data from memory rather
than new data in Processor
2’s cache
$
$
$
Solutions
» no caching of shared data
– Cray T3D, T3E, IBM RP3,
BBN Butterfly
» cache coherence protocol
– keep track of copies
– notify (update or
invalidate) on writes
Network
M
A:3
P1: Rd(A)
P2:
P3:
Rd(A)
Wr(A,5)
Rd(A)
18
Implementation issues
Underlying hardware implementation
 Bit-slice
 Board assembly
 Integration in an integrated-circuit
Exploitation of new technologies
 DRAM integration on IC
 Low-swing chip-level interconnect
19
Performance objectives
Objectives
 Speed
 Power
 Cost
 Ease of programming/time to market/ time to money
 In-field flexibility
Methods of measurement
 Modeling
 Emulation
 Simulation
» Transaction
» Instruction-set
» Hardware
20
Flynn’s Taxonomy of Multiprocessing
Single-instruction single-datastream (SISD) machines
Single-instruction multiple-datastream (SIMD) machines
Multiple-instruction single-datastream (MISD) machines
Multiple-instruction multiple-datastream (MIMD) machines
Examples?
21
Examples
Single-instruction single-datastream (SISD) machines
» Non-pipelined Uniprocessors
Single-instruction multiple-datastream (SIMD) machines
» Vector processors (VIRAM)
Multiple-instruction single-datastream (MISD) machines
» Network processors (Intel IXP1200
Multiple-instruction multiple-datastream (MIMD) machines
» Network of workstations (NOW)
22
Predominant Approaches
Pipelining ubiquitious
Much academic research focused on performance improvements
of ``dusty decks’’
 Illiac 4 - Speed-up of Fortran
 SUIF, Flash - Speed-up of C
Niche market in high-performance computing
 Cray
Commercial support for high-end servers
 Shared-memory multiprocessors for server market
Commercial exploitation of silicon capability
 General purpose: Super-scalar, VLIW
 Special purpose: VLIW for DSP, Media processors, Network
processors
Reconfigurable computing
23
C62x Pipeline Operation
Pipeline Phases
Fetch
Decode
Execute
PG PS PW PR DP DC E1 E2 E3 E4 E5




Single-Cycle Throughput
Operate in Lock Step
Fetch
»
»
»
»
PG
PS
PW
PR
Decode
»
»

DP
DC
Instruction Dispatch
Instruction Decode
Execute
»
E1 - E5
Execute 1 through Execute 5
E3
E2
E1
DC
DP
PR
PW
E5
E4
E3
E2
E1
DC
DP
Program Address Generate
Program Address Send
Program Access Ready Wait
Program Fetch Packet Receive
Execute Packet 1 PG PS PW PR DP DC
Execute Packet 2 PG PS PW PR DP
Execute Packet 3 PG PS PW PR
Execute Packet 4 PG PS PW
Execute Packet 5 PG PS
Execute Packet 6 PG
Execute Packet 7
E1
DC
DP
PR
PW
PS
PG
E2
E1
DC
DP
PR
PW
PS
E4
E3
E2
E1
DC
DP
PR
E5
E4
E3
E2
E1
DC
E5
E4
E3
E2
E1
E5
E4 E5
E3 E4 E5
E2 E3 E4 E5
24
Superscalar: PowerPC 604 and Pentium Pro

Both In-order Issue, Out-of-order execution, In-order
Commit
25
IA-64 aka EPIC aka VLIW


Compiler schedules instructions
Encodes dependencies
explicitly
» saves having the hardware
repeatedly rediscover them

Instruction Cache
Instruction Issue
Support speculation
» speculative load
» branch prediction

Really need to make
communication explicit too
» still has global registers and
global instruction issue
Register File
26
Phillips Trimedia Processor
27
TMS320C6201 Revision 2
Program Cache / Program Memory
32-bit address, 256-Bit data512K Bits RAM
Pwr
Dwn
C6201 CPU Megamodule
Program Fetch
Control
Registers
Instruction Dispatch
Host
Port
Interface
4-DMA
Instruction Decode
Data Path 1
Data Path 2
A Register File
Control
Logic
B Register File
Test
Emulation
Ext.
Memory
Interface
L1
S1
M1
D1
D2 M2
S2
L2
Interrupts
2 Timers
Data Memory
32-Bit address, 8-, 16-, 32-Bit data
512K Bits RAM
2 Multichannel
buffered
serial ports
(T1/E1)
28
TMS320C6701 DSP
Block Diagram
Program Cache/Program Memory
32-bit address, 256-Bit data
512K Bits RAM
Power ’C67x Floating-Point CPU Core
Down
Program Fetch
Host
Port
Interface
Control
Registers
Instruction Dispatch
4
Channel
DMA
Instruction Decode
Data Path 1
Data Path 2
A Register File
Control
Logic
B Register File
Test
Emulation
L1
S1
M1
D1
External
Memory
Interface
D2 M2
S2
L2
Interrupts
2 Timers
Data Memory
32-Bit address
8-, 16-, 32-Bit data
512K Bits RAM
2 Multichannel
buffered
serial ports
(T1/E1)
29
TMS320C67x CPU Core
’C67x Floating-Point CPU Core
Program Fetch
Instruction Dispatch
Control
Registers
Instruction Decode
Data Path 1
Data Path 2
A Register File
B Register File
Control
Logic
Test
Emulation
L1 S1 M1 D1
Arithmetic
Logic
Unit
Auxiliary
Logic
Unit
D2 M2 S2 L2
Multiplier
Unit
Interrupts
Floating-Point
Capabilities
30
Single-Chip Multiprocessors
CMP

Build a multiprocessor on a
single chip
» linear increase in peak
performance
» advantage of fast interaction
between processors

Fine grain threads
» make communication and
synchronization very fast (1
cycle)
» break the problem into
smaller pieces

P
P
P
P
$
$
$
$
$
M
Memory bandwidth
» Makes more effective use of
limited memory bandwidth

Programming model
» Need parallel programs
31
Intel IXP1200 Network Processor
SDRAM
Ctrl

»
»
»
MicroEng
PCI
Interface
ICache
SA
Core
MicroEng
MicroEng
Hash
Engine
IX Bus
Interface
MicroEng
DCache
Mini
DCache
MicroEng
6 micro-engines

IX Bus Interface
»
»


less critical tasks
Hash engine
»

packet I/O
connect IXPs
– scalable
StrongARM
»
Scratch
Pad
SRAM
RISC engines
4 contexts/eng
24 threads total
level 2 lookups
PCI interface
MicroEng
SRAM
Ctrl
32
IXP1200 MicroEngine
from SRAM
32 SRAM
Read XFER
Registers
to SRAM
32 SRAM
Write XFER
Registers
64 GPRs
(A-Bank)
ALU
32 SDRAM
Read XFER
Registers
64 GPRs
(B-Bank)
from SDRAM





to SDRAM
32-bit RISC instruction set
Multithreading support for 4 threads
»

32 SDRAM
Read XFER
Registers
Maximum switching overhead of 1 cycle
128 32-bit GPRs in two banks of 64
Programmable 1KB instruction store (not shown in diagram)
128 32-bit transfer registers
Command bus arbiter and FIFO (not shown in diagram)
33
IXP1200 Instruction Set

Powerful ALU instructions:
» can manipulate word and part of word quite effectively

Swap-thread on memory reference
» Hides memory latency
» sram[read, r0, base1, offset, 1], ctx_swap

Can use an “intelligent” DMA-like controller to copy packets to/from
memory
» sdram[t_fifo_wr, --, pkt_bffr, offset, 8]

Exposed branch behavior
» can fill variable branch slots
» can select a static prediction on a per-branch basis
ARM
mov r1, r0, lsl #16
mov r1, r1, r0, asr #16
add r0, r1, r0, asr #16
IXP1200
ld_field_w_clr[temp, 1100, accum]
alu_shf[accum, temp, +, accum, <<16]
34
UCB: Processor with DRAM (PIM)
IRAM, VIRAM

Put the processor and the main
memory on a single chip
» much lower memory latency
» much higher memory
bandwidth

P
V
M
But
» need to build systems with
more than one chip
64Mb SDRAM Chip
Internal - 128 512K subarrays
4 bits per subarray each 10ns
51.2 Gb/s
External - 8 bits at 10ns, 800Mb/s
1 Integer processor ~ 100KBytes DRAM
1 FP processor ~ 500KBytes DRAM
1 Vector Unit ~ 1 MByte DRAM
35
IRAM Vision Statement
Microprocessor & DRAM
on a single chip:
» on-chip memory latency
5-10X, bandwidth 50-100X
» improve energy efficiency
2X-4X (no off-chip bus)
» serial I/O 5-10X v. buses
» smaller board area/volume
» adjustable memory size/width
I/O I/O
Bus
Proc
$ $
L2$
Bus
L
o f
ga
i b
c
D R A M
I/O
I/O
Proc
Bus
D R A M
D
Rf
Aa
Mb
36
Potential Multimedia Architecture

“New” model: VSIW=Very Short Instruction Word!
»
»
»
»
»
Compact: Describe N operations with 1 short instruct.
Predictable (real-time) performance vs. statistical performance (cache)
Multimedia ready: choose N*64b, 2N*32b, 4N*16b
Easy to get high performance
Compiler technology already developed, for sale!
– Don’t have to write all programs in assembly language
37
Revive Vector (= VSIW) Architecture!







Cost: ≈ $1M each?
Low latency, high BW
memory system?
Code density?
Compilers?
Performance?
Power/Energy?
Limited to scientific
applications?







Single-chip CMOS MPU/IRAM
IRAM
Much smaller than VLIW
For sale, mature (>20 years)
Easy scale speed with technology
Parallel to save energy, keep perf
Multimedia apps vectorizable too: N*64b,
2N*32b, 4N*16b
38
V-IRAM1: 0.18 µm, Fast Logic, 200 MHz
1.6 GFLOPS(64b)/6.4 GOPS(16b)/16MB
4 x 64
or
8 x 32
or
16 x 16
+
x
2-way
Superscalar
Processor
I/O
Vector
Instruction
Queue
I/O
÷
Load/Store
Vector Registers
16K I cache 16K D cache
4 x 64
4 x 64
Serial
I/O
Memory Crossbar Switch
M
I/O
M
4…x 64
I/O
M
M
M
M
M
M
M
…
M
4…x 64
M
x 64
… 4…
…
M
M
M
M
M
M
M
M
M
4…x 64
M
M
M
M
M
…
M
4…
x 64
…
M
M
M
M
…
39
Tentative VIRAM-1 Floorplan
0.18 µm DRAM
16-32 MB in 16 banks x
Memory (128 Mbits / 16 MBytes) 256b

0.18 µm,
5 Metal Logic

≈ 200 MHz MIPS IV,
C
16K I$, 16K D$
RingP

≈ 4 200 MHz
based 4 Vector Pipes/Lanes U
I/O FP/int. vector units
Switch
+$

die:
≈ 20x20 mm

xtors: ≈ 130-250M

power: ≈2 Watts

Memory (128 Mbits / 16 MBytes)
40
Tentative VIRAM-”0.25” Floorplan

Memory
(32 Mb /
4 MB)
C
P
1 VU
U
+$
Memory
(32 Mb /
4 MB)






Demonstrate
scalability via
2nd layout
(automatic from
1st)
8 MB in 2 banks
x 256b, 32
subbanks
≈ 200 MHz
CPU,
8K I$, 8K D$
1 ≈ 200 MHz
FP/int. vector
units
die:
≈ 5 x 20
mm
xtors: ≈ 70M
power: ≈0.5
Watts
Kernel
Comp.
iDCT
Clr.Conv.
Convol.
FP Matrix
V-1
6.40
3.10
2.95
3.16
3.19
GOPS
V-0.25
1.6
0.8
0.8
0.8
0.8
41
Stanford: Hydra Design
Centralized Bus Arbitration Mechanisms
CPU 0
L1 Ins t.
Cac he
CP2
CPU 1
L1 Data Cache &
Spe cula tion Bits
L1 Ins t.
Cac he
CP2
L1 Data Cache &
Spe cula tion Bits
CPU 1 Me mory Controller
CPU 0 Me mory Controller
CP2
CPU 2
L1 Ins t.
Cac he
L1 Data Cache &
Spe cula tion Bits
CPU 2 Me mory Controller
CPU 3
L1 Ins t.
Cac he
CP2
L1 Data Cache &
Spe cula tion Bits
CPU 3 Me mory Controller
Write-through Bus (64b)
Read/Replace Bus (256b)
Speculation Write Buffers
#0
#1
#2
#3
On-chip L2 Cache




retire
Rambus Memory Interface
I/O Bus Interface
DRAM Main Memory
I/O Devices
Single-chip multiprocessor
Four processors
Separate primary caches
Write-through data caches
to maintain coherence



Shared 2nd-level cache
Separate read and write
busses
Data Speculation Support
42
Mescal Architecture
Scott Weber
University of California at Berkeley
43
Outline






Architecture rationale and motivation
Architecture goals
Architecture template
Processing elements
Multiprocessor architecture
Communication architecture
44
Architectural Rationale and Motivation


Configurable processors have shown orders of magnitude performance
improvements
Tensilica has shown ~2x to ~50x performance improvements
» Specialized functional units
» Memory configurations

Tensilica matches the architecture with software development tools
Memory
RegFile
FU
ICache
Set memory parameters
Add DCT and Huffman
blocks for a JPEG app
Configuration
Memory
RegFile
FU FU DCT HUF FU
ICache
45
Architectural Rationale and Motivation

In order to continue this performance improvement trend
» Architectural features which exploit more concurrency are required
» Heterogeneous configurations need to be made possible
» Software development tools support new configuration options
Memory
RegFile
FU FU DCTHUF FU
...begins to
look like a
VLIW...
PE
Memory PE
PE
PE
RegFile
PE
PE
FU FU DCTHUF FU
PE
ICache
PE
PE
PE
PE
PE
PE
PE
PE
PE
...concurrent processes
are required in order
to continue performance
improvement trend...
PE
PE
ICache
...generic mesh PE
may not suit the
application’s
PE
topology...
PE
PE
PE
PE
PE
...configurable VLIW
PEs and network
topology...
PE
46
Architecture Goals

Provide template for the exploration of a range of architectures

Retarget compiler and simulator to the architecture

Enable compiler to exploit the architecture

Concurrency
» Multiple instructions per processing element
» Multiple threads per and across processing elements
» Multiple processes per and across processing elements

Support for efficient computation
» Special-purpose functional units, intelligent memory, processing elements

Support for efficient communication
» Configurable network topology
» Combined shared memory and message passing
47
Architecture Template

Prototyping template for array of processing elements
» Configure processing element for efficient computation
» Configure memory elements for efficient retiming
» Configure the network topology for efficient communication
Memory
RegFile
...configure
PE...
Memory
RegFile
FU FU FU FU FU
FU FU DCTHUF FU
ICache
ICache
...configure
memory
elements...
Memory
Memory
RegFile
RegFile
FU FU
DCTHUF FU
ICache
...configure PEs
and network to
match the
application...
48
Range of Architectures
Memory System



Register File



FU
Instruction Cache
Scalar Configuration
EPIC Configuration
EPIC with special FUs
Mesh of HPL-PD PEs
Customized PEs, network
Supports a family of
architectures
» Plan to extend the family
with the micro-architectural
features presented
49
Range of Architectures

PE
PE
Memory
System
PE


Register File
PE
PE


PE

FU
PE
FU
PE
FU
FU
PE
Instruction Cache
FU
Scalar Configuration
EPIC Configuration
EPIC with special FUs
Mesh of HPL-PD PEs
Customized PEs, network
Supports a family of
architectures
» Plan to extend the family
with the micro-architectural
features presented
50
Range of Architectures
Memory System



Register File



FU
FU DES DCT FFT
Instruction Cache
Scalar Configuration
EPIC Configuration
EPIC with special FUs
Mesh of HPL-PD PEs
Customized PEs, network
Supports a family of
architectures
» Plan to extend the family
with the micro-architectural
features presented
51
Range of Architectures
PE
Memory
PE System
PE



PE
Register File
PE

PE


FU
PE
FU DES DCT FFT
PE
Instruction Cache
PE
Scalar Configuration
EPIC Configuration
EPIC with special FUs
Mesh of HPL-PD PEs
Customized PEs, network
Supports a family of
architectures
» Plan to extend the family
with the micro-architectural
features presented
52
Range of Architectures
PE
PE
PE




PE
PE


PE
PE
PE
Scalar Configuration
EPIC Configuration
EPIC with special FUs
Mesh of HPL-PD PEs
Customized PEs, network
Supports a family of
architectures
» Plan to extend the family
with the micro-architectural
features presented
53
Range of Architectures (Future)
SDRAM
Ctrl

MicroEng
PCI
Interface
ICache
SA
Core
MicroEng
MicroEng
Hash
Engine


IX Bus
Interface
MicroEng
DCache
Mini
DCache
MicroEng
Template support for
such an architecture
Prototype architecture
Software development
tools generated
» Generate compiler
» Generate simulator
Scratch
Pad
SRAM
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IXP1200 Network Processor (Intel)
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The RAW Architecture
Slides prepared by Manish Vachhrajani
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Outline

RAW architecture
» Overview
» Features
» Benefits and Disadvantages

Compiling for RAW
»
»
»
»
Overview
Structure of the compiler
Basic block compilation
Other techniques
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RAW Machine Overview


Scalable architecture without
global interconnect
Constructed from Replicated
Tiles
» Each tile has a mP and a
switch
» Interconnect via a static
and dynamic network
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RAW Tiles

Simple 5 stage pipelined mP
w/ local PC(MIMD)
» Can contain configurable
logic


Per Tile IMEM and DMEM,
unlike other modern
architectures
mP contains ins. to send and
recv. data
IMEM
DMEM
PC
REGS
CL
PC
SMEM
Switch
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RAW Tiles(cont.)

Tiles have local switches
» Implemented with a stripped down mP
» Static Network
– Fast, easy to implement
– Need to know data transfers, source and destintation at
compile time
» Dynamic Network
– Much slower and more complex
– Allows for messages whose route is not known at compile
time
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Configurable Hardware in RAW




Each tile Contains its own configurable hardware
Each tile has several ALUs and logic gates that can operate at
bit/byte/word levels
Configurable interconnect to wire componenets together
Coarser than FPGA based implementations
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Benefits of RAW

Scalable
» Each tile is simple and replicated
» No global wiring, so it will scale even if wire delay doesn’t
» Short wires and simple tiles allow higher clock rates


Can target many forms of Parallelism
Ease of design
» Replication reduces design overhead
» Tiles are relatively simple designs
» simplicity makes verification easier
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Disadvantages of RAW

Complex Compilation
» Full space-time compilation
» Distributed memory system
» Need sophisticated memory analysis to resolve “static
references”

Software Complexity
» Low-level code is complex and difficult to examine and write
by hand

Code Size?
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Traditional Operations on RAW

How does one exploit the Raw architecture across function
calls, especially in libraries?
» Can we easily maintain portability with different tile counts?

Memory Protection and OS Services
» Context switch overhead
» Load on dynamic network for memory protection and virtual
memory?
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Compiling for RAW machines



Determine available parallelism
Determine placement of memory items
Discover memory constraints
» Dependencies between parallel threads
» Disambiguate memory references to allow for static access to
data elements
» Trade-off memory dependence and Parallelism
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Compiling for RAW(cont.)

Generate route instructions for switches
» static network only

Generate message handlers for dynamic events
» Speculative execution
» Unpredictable memory references

Optimal partitioning algorithm is NP complete
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Structure of RAWCC
Source Language
Traditional Dataflow
Optimizations
Build CFG



Partition data to increase
static accesses
Partition instructions to allow
parallel execution
allocate data to tiles to
minimize communication
overhead
MAPS System
Space-time scheduler
RAW executable
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The MAPS System



Manages memory to generate static promotions of data
structures
For loop accesses to arrays uses modulo unrolling
For data structures, uses SPAN analysis package to identify
potential references and partition memory
» structures can be split across processing elements.
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Space-Time Scheduler

For Basic Blocks
»
»
»
»

Maps instructions to processors
Maps scalar data to processors
Generates communication instructions
Schedules computation and communication
For overall CFG, performs control localization
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Basic Block Orchestrator

Initial Code
Transformation
Instruction
Partitioner


Global Data
Partitioner
Data & Ins.
Placer
Comm Code
Generator
Event
Scheduler
All values are copied to the tiles
that work on the data from the
home tile
Within a Block, all access are
local
At the end of a block, values
are copied to home tiles
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Initial Code Transformation

Convert Block to static single assignment form
» removes false dependencies
» Analagous to register renaming

Live on entry, and live on exit variables marked with dummy
instructions
» Allows for overlap of “stitch” code with useful work
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Instruction Partitioner


Partitions stream into multiple streams, one for each tile
Clustering
» Partition instructions to minimize runtime considering only
communication

Merging
» Reduces cluster count to match tile count
» Uses a heuristic based algorithm to achieve good balance and
low communication overhead
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Global Data Partitioner

Partitions global data for assignment to home locations
» Local data is copied at the start of a basic block


Summarize instruction stream’s data access pattern with affinity
Maps instructions and data to virtual processors
» Map instructions, optimally place data based on affinity
» Remap instructions with data placement knowledge
» Repeat until local minima is reached

Only real data are mapped, not dummies formed in ICT
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Data and Instruction Placer

Places data items onto physical tiles
» driven by static data items

Places instructions onto tiles
» Uses data information to determine cost


Takes into account actual model of communications network
Uses a swap based greedy allocation
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Event Scheduler



Schedules routing instructions as well as computation
instructions in a basic block
Schedules instructions using a greedy list based scheduler
Switch schedule is ensured to be deadlock free
» Allows tolerance of dynamic events
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Control Flow

Control Localization
» Certain branches are enveloped in macro instructions, and
the surrounding blocks merged
» Allows branch to occur only on one tile

Global Branching
» Done through target broadcast and local branching
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Performance




RAW achieves anywhere from 1.5 to 9 times speedup
depending on application and tile count
Applications tested were particularly well suited to RAW
Heavily dependent integer programs may do poorly(encryption,
etc.))
Depends on its ability to statically schedule and localize memory
accesses
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Future Work

Use multisequential execution to run multiple applications
simultaneously
» Allow static communication between threads known at
compile time
» Minimize dynamic overhead otherwise


Target ILP across branches more agressively
Explore configurability vs. parallelism in RAW
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Reconfigurable processors

Adapt the processor to the
application
»
»

Builds on FPGA technology
»
»
»

special function units
special wiring between function
units
FPGAs are inefficient
– a multiplier built from an
FPGA is about 100x larger
and 10x slower than a
custom multiplier.
Need to raise the granularity
– configure ALUs, or whole
processors
Memory and communication are
usually the bottleneck
– not addressed by
configuring a lot of ALUs
Programming model
»
»
Difficult to program
Verilog
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SCORE
Stream Computation Organized for
Reconfigurable Execution
Eylon Caspi
Michael Chu
André DeHon
Randy Huang
Joseph Yeh
John Wawrzynek
Nicholas Weaver
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Opportunity
High-throughput, regular operations
 can be mapped spatially onto FPGA-like
(programmable, spatial compute substrate)
 achieving higher performance
» (throughput per unit area)

than conventional, programmable devices
» (e.g. processors)
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Problem







Only have raw devices
Solutions non-portable
Solutions not scale to new hardware
Device resources exposed to developer
Little or no abstraction of implementations
Composition of subcomponents hard/ad hoc
No unifying computational model or run-time environment
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Introduce: SCORE

Compute Model
»
»
»
»
»
virtualizes RC hardware resources
supports automatic scaling
supports dynamic program requirements efficiently
provides compositional semantics
defines runtime environment for programs
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Viewpoint

SCORE (or something like it) is a necessary condition to enable
automatic exploitation of new RC hardware as it becomes
available.

Automatic exploitation is essential to making RC a long-term
viable computing solution.
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Outline



Opportunity
Problem
Review
» related work
» enabling hardware

Model
» execution
» programmer


Preliminary Results
Challenges and Questions ahead
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…borrows heavily from...







RC, RTR
P+FPGA
Dataflow
Streaming Dataflow
Multiprocessors
Operating System
(see working paper)



Tried to steal all the good ideas
:-)
build a coherent model
exploit strengths of RC
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Enabling Hardware

High-speed, computational arrays
» [250MHz, HSRA, FPGA’99]

Large, on-chip memories
» [2Mbit, VLSI Symp. ‘99]
» [allow microsecond reconfiguration]

Processor and FPGA hybrids
» [GARP, NAPA, Triscend, etc.]
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BRASS Architecture
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Array Model
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Platform Vision

Hardware capacity scales up with each generation
» Faster devices
» More computation
» More memory

With SCORE, old programs should run on new
hardware
» and exploit the additional capacity automatically
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Example: SCORE Execution
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Spatial Implementation
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Serial Implementation
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Summary: Elements of a
multiprocessing system







General purpose/special purpose
Granularity - capability of a basic module
Topology - interconnection/communication geometry
Nature of coupling - loose to tight
Control-data mechanisms
Task allocation and routing methodology
Reconfigurable
» Computation
» Interconnect



Programmer’s model/Language support/ models of computation
Implementation - IC, Board, Multiboard, Networked
Performance measures and objectives
[After E. V. Krishnamurty - Chapter 5
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Conclusions
Portions of multi/parallel processing have become successful
» Pipelining ubiquitious
» Superscalar ubiquitious
» VLIW successful in DSP, Multimedia - GPP?
Silicon capability re-invigorating multiprocessor research
» GPP - Flash, Hydra, RAW
» SPP - Intel IXP 1200, IRAM/VIRAM, Mescal
Reconfigurable computing has found a niche in wireless
communications
Problem of programming models, languages, computational
models etc. for multiprocessors still largely unsolved
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