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Multiprocessors and multicomputers
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CSE539: Advanced Computer Architecture
Chapter 7
Multiprocessors and Multicomputers Book: “Advanced Computer Architecture – Parallelism, Scalability, Programmability”, Hwang & Jotwani
Sumit Mittu
Assistant Professor, CSE/IT
Lovely Professional University
In this chapter…
• Multiprocessor System Interconnects
• Cache Coherence and Synchronization Mechanisms
• Three Generations of Multi-computers
• Message Routing Schemes
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 2
MULTIPROCESSOR SYSTEM INTERCONNECTS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 3
MULTIPROCESSOR SYSTEM INTERCONNECTS
• Network Characteristics o Topology
• Dynamic Networks
o Timing control protocol
• Synchronous (with global clock)
• Asynchronous (with handshake or interlocking mechanism)
o Switching method
• Circuit switching
• Packet switching
o Control Strategy
• Centralized (global controller to receive requests from all devices and grant network access)
• Distributed (requests handled by local devices independently)
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 4
MULTIPROCESSOR SYSTEM INTERCONNECTS
• Hierarchical Bus System o Local Bus (board level)
• Memory bus, data bus
o Backplane Bus (backplane level)
• VME bus (IEEE 1014-1987), Multibus II (IEEE 1296-1987), Futurebus+ (IEEE 896.1-1991)
o I/O Bus (I/O level)
o E.g. Encore Multimax multprocessor’s nanobus
• 20 slots
• 32-bit address path
• 64-bit data path
• Clock rate: 12.5 MHz
• Total Memory bandwidth: 100 Megabytes per second
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 5
MULTIPROCESSOR SYSTEM INTERCONNECTS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 6
MULTIPROCESSOR SYSTEM INTERCONNECTS
• Hierarchical Buses and Caches o Cache Levels
• First level caches
• Second level caches
o Buses
• (Intra) Cluster Bus
• Inter-cluster bus
o Cache coherence
• Snoopy cache protocol for coherence among first level caches of same cluster
• Intra-cluster cache coherence controlled among second level caches and results passed to
first level caches
o Use of Bridges between multiprocessor clusters
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 7
MULTIPROCESSOR SYSTEM INTERCONNECTS
• Hierarchical Buses and Caches
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 8
MULTIPROCESSOR SYSTEM INTERCONNECTS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 9
MULTIPROCESSOR SYSTEM INTERCONNECTS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 10
MULTIPROCESSOR SYSTEM INTERCONNECTS
• Crossbar Switch Design o Based on number of network stages
• Single stage (or recirculating) networks
• Multistage networks
o Blocking networks
o Non-blocking (re-arranging) networks
• Crossbar networks
o n x m and n2 Cross-point switch design
o Crossbar benefits and limitations
• Multiport Memory Design o Multiport Memory
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 11
MULTIPROCESSOR SYSTEM INTERCONNECTS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 12
MULTIPROCESSOR SYSTEM INTERCONNECTS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 13
CACHE COHERENCE MECHANISMS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 14
• Cache Coherence Problem o Inconsistent copies of same memory block in different caches
o Sources of inconsistency:
• Sharing of writable data
• Process migration
• I/O activity
• Protocol Approaches o Snoopy Bus Protocol
o Directory Based Protocol
• Write Policies o (Write-back, Write-through) x (Write-invalidate, Write-update)
CACHE COHERENCE MECHANISMS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 15
CACHE COHERENCE MECHANISMS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 16
CACHE COHERENCE MECHANISMS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 17
CACHE COHERENCE MECHANISMS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 18
CACHE COHERENCE MECHANISMS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 19
• Snoopy Bus Protocols o Write-through caches
• Write invalidate coherence protocol for write-through caches
• Write-update coherence protocol for write-through caches
• Data item states:
o VALID
o INVALID
• Possible operations:
o Read by same processor R(i) Read by different processor R( j )
o Write by same processor W(i) Write by different processor W( j )
o Replace by same processor Z(i) Replace by different processor Z( j )
CACHE COHERENCE MECHANISMS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 20
CACHE COHERENCE MECHANISMS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 21
• Snoopy Bus Protocols o Write-through caches – write invalidate scheme
Current
State Operation
New
State
Valid
R(i) Valid
W(i) Valid
Z(i) Invalid
R(j) Valid
W(j) Invalid
Z(j) Valid
Current
State Operation
New
State
Invalid
R(i) Valid
W(i) Valid
Z(i) Invalid
R(j) Invalid
W(j) Invalid
Z(j) Invalid
CACHE COHERENCE MECHANISMS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 22
• Snoopy Bus Protocols o Write-back caches
• Ownership protocol: Write invalidate coherence protocol for write-through caches
• Data item states:
o RO : Read Only (Valid state)
o RW : Read Write (Valid state)
o INV : Invalid state
• Possible operations:
o Read by same processor R(i) Read by different processor R( j )
o Write by same processor W(i) Write by different processor W( j )
o Replace by same processor Z(i) Replace by different processor Z( j )
CACHE COHERENCE MECHANISMS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 23
• Snoopy Bus Protocols o Write-back caches – write invalidate (ownership protocol) scheme
Current
State Operation
New
State
RO
(Valid)
R(i) RO
W(i) RW
Z(i) INV
R(j) RO
W(j) INV
Z(j) RO
Current
State Operation
New
State
RW
(Valid)
R(i) RW
W(i) RW
Z(i) INV
R(j) RO
W(j) INV
Z(j) RW
Current
State Operation
New
State
INV
(Invalid)
R(i) RO
W(i) RW
Z(i) INV
R(j) INV
W(j) INV
Z(j) INV
CACHE COHERENCE MECHANISMS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 24
CACHE COHERENCE MECHANISMS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 25
• Snoopy Bus Protocols o Write-once Protocol
• First write using write-through policy
• Subsequent writes using write-back policy
• In both cases, data item copy in remote caches is invalidated
• Data item states:
o Valid :cache block consistent with main memory copy
o Reserved : data has been written exactly once and is consistent with main memory
copy
o Dirty : data is written more than once but is not consistent with main memory copy
o Invalid :block not found in cache or is inconsistent with main memory copy
CACHE COHERENCE MECHANISMS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 26
• Snoopy Bus Protocols o Write-once Protocol
• Cache events and actions:
o Read-miss
o Read-hit
o Write-miss
o Write-hit
o Block replacement
CACHE COHERENCE MECHANISMS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 27
• Multilevel Cache Coherence
CACHE COHERENCE MECHANISMS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 28
• Protocol Performance issues o Snoopy Cache Protocol Performance determinants:
• Workload Patterns
• Implementation Efficiency
o Goals/Motivation behind using snooping mechanism
• Reduce bus traffic
• Reduce effective memory access time
o Data Pollution Point
• Miss ratio decreases as block size increases, up to a data pollution point (that is, as blocks become larger, the probability of finding a desired data item in the cache increases).
• The miss ratio starts to increasing as the block size increases to data pollution point.
o Ping-Pong effect on data shared between multiple caches
• If two processes update a data item alternately, data will continually migrate between two caches with high miss-rate
THREE GENERATIONS OF MULTICOMPUTERS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 29
• Multicomputer v/s Multiprocessor
• Design Choices for Multi-computers o Processors
• Low cost commodity (off-the-shelf) processors
o Memory Structure
• Distributed memory organization
• Local memory with each processor
o Interconnection Schemes
• Message passing, point-to-point , direct networks with send/receive semantics with/without uniform message communication speed
o Control Strategy
• Asynchronous MIMD, MPMD and SPMD operations
THREE GENERATIONS OF MULTICOMPUTERS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 30
THREE GENERATIONS OF MULTICOMPUTERS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 31
• The Past, Present and Future Development o First Generation
• Example Systems: Caltech’s Cosmic Cube, Intel iPSC/1, Ametek S/14, nCube/10
o Second Generation
• Example Systems: iPSC/2, i860, Delta, nCube/2, Supernode 1000, Ametek Series 2010
o Third Generation
• Example Systems: Caltech’s Mosaic C, J-Machine, Intel Paragon
o First and second generation multi-computers are regarded as medium-grain systems
o Third generation multi-computers were regarded as fine-grain systems.
o Fine-grain and shared memory approach can, in theory, combine the relative merits of
multiprocessors and multi-computers in a heterogeneous processing environment.
THREE GENERATIONS OF MULTICOMPUTERS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 32
1st Generation 2nd Generation 3rd Generation
Typical
Node
Attributes
MIPS 1 10 100
MFLOPS (scalar) 0.1 2 40
MFLOPS (vector) 10 40 200
Memory Size (in MB) 0.5 4 32
Typical
System
Attributes
Number of Nodes (N) 64 256 1024
MIPS 64 2560 100 K
MFLOPS (scalar) 6.4 512 40 K
MFLOPS (vector) 640 10 K 200 K
Memory Size (in MB) 32 1 K 32 K
Communi-
cation
Latency
Local Neighbour
(in microseconds) 2000 5 0.5
Non-local node
(in microseconds) 6000 5 0.5
THREE GENERATIONS OF MULTICOMPUTERS
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 33
MESSAGE PASSING SCHEMES
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 34
• Message Routing Schemes
• Message Formats o Messages
o Packets
o Flits (Control Flow Digits)
• Data Only Flits
• Sequence Number
• Routing Information
• Store-and-forward routing
• Wormhole routing
MESSAGE PASSING SCHEMES
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 35
MESSAGE PASSING SCHEMES
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 36
• Asynchronous Pipelining
MESSAGE PASSING SCHEMES
Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University 37
• Latency Analysis o L: Packet length (in bits)
o W: Channel Bandwidth (in bits per second)
o D: Distance (number of nodes traversed minus 1)
o F: Flit length (in bits)
o Communication Latency in Store-and-forward Routing
• TSF = L (D + 1) / W
o Communication Latency in Wormhole Routing
• TWH = L / W + F D / W