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• Moore’s Law
– The number of transistors that can be placed
inexpensively on an integrated circuit will
double approximately every 18 months.
– Self-fulfilling prophecy
• Computer architect goal
• Software developer assumption
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• Impediments to Moore’s Law
– Theoretical Limit
– What to do with all that die space?
– Design complexity
– How do you meet the expected performance
increase?
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• von Neumann model – Execute a stream of instructions (machine code) – Instructions can specify
• Arithmetic operations • Data addresses • Next instruction to execute
– Complexity • Track billions of data locations and millions of instructions • Manage with:
– Modular design – High-level programming languages
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• Parallelism
– Continue to increase performance via
parallelism.
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• From a software point-of-view, need to
solve demanding problems
– Engineering Simulations
– Scientific Applications
– Commercial Applications
• Need the performance, resource gains
afforded by parallelism
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• Engineering Simulations – Aerodynamics – Engine efficiency
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• Scientific Applications – Bioinformatics – Thermonuclear processes – Weather modeling
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• Commercial Applications – Financial transaction processing – Data mining – Web Indexing
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• Unfortunately, greatly increases coding
complexity
– Coordinating concurrent tasks
– Parallelizing algorithms
– Lack of standard environments and support
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• The challenge
– Provide the abstractions, programming
paradigms, and algorithms needed to
effectively design, implement, and maintain
applications that exploit the parallelism
provided by the underlying hardware in order
to solve modern problems.
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• Standard sequential architecture
CPU RAM
BUS
Bottlenecks
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• Use multiple
– Datapaths
– Memory units
– Processing units
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• SIMD
– Single instruction stream, multiple data
stream Processing
Unit
Control
Unit
Interco
nnect
Processing
Unit
Processing
Unit
Processing
Unit
Processing
Unit 14
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• SIMD
– Advantages
• Performs vector/matrix operations well
– EX: Intel’s MMX chip
– Disadvantages
• Too dependent on type of computation
– EX: Graphics
• Performance/resource utilization suffers if
computations aren’t “embarrasingly parallel”.
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• MIMD
– Multiple instruction stream, multiple data
stream Processing/Control
Unit
Processing/Control
Unit
Processing/Control
Unit
Processing/Control
Unit
Interco
nnect
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• MIMD – Advantages
• Can be built with off-the-shelf components • Better suited to irregular data access patterns
– Disadvantages • Requires more hardware (!sharing control unit) • Store program/OS at each processor
• Ex: Typical commodity SMP machines we see
today.
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• Task Communication
– Shared address space
• Use common memory to exchange data
• Communication and replication are implicit
– Message passing
• Use send()/receive() primitives to exchange data
• Communication and replication are explicit
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• Shared address space
– Uniform memory access (UMA)
• Access to a memory location is independent of
which processing unit makes the request.
– Non-uniform memory access (NUMA)
• Access to a memory location depends on the
location of the processing unit relative to the
memory accessed.
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• Message passing
– Each processing unit has its own private
memory
– Exchange of messages used to pass data
– APIs
• Message Passing Interface (MPI)
• Parallel Virtual Machine (PVM)
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• Algorithm
– a sequence of finite instructions, often used
for calculation and data processing.
• Parallel Algorithm
– An algorithm that which can be executed a
piece at a time on many different processing
devices, and then put back together again at
the end to get the correct result
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• Challenges
– Identifying work that can be done
concurrently.
– Mapping work to processing units.
– Distributing the work
– Managing access to shared data
– Synchronizing various stages of execution.
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• Models
– A way to structure a parallel algorithm by
selecting decomposition and mapping
techniques in a manner to minimize
interactions.
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• Models
– Data-parallel
– Task graph
– Work pool
– Master-slave
– Pipeline
– Hybrid
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• Data-parallel
– Mapping of Work
• Static
• Tasks -> Processes
– Mapping of Data
• Independent data items assigned to processes
(Data Parallelism)
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• Data-parallel – Computation
• Tasks process data, synchronize to get new data or exchange results, continue until all data processed
– Load Balancing • Uniform partitioning of data
– Synchronization • Minimal or barrier needed at end of a phase
– Examples • Ray Tracing
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• Data-parallel
P
P
P
P
P
D D
D D
D
O
O
O
O
O
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• Task graph
– Mapping of Work
• Static
• Tasks are mapped to nodes in a data dependency
task dependency graph (Task parallelism)
– Mapping of Data
• Data moves through graph (Source to Sink)
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• Task graph – Computation
• Each node processes input from previous node(s) and send output to next node(s) in the graph
– Load Balancing • Assign more processes to a given task • Eliminate graph bottlenecks
– Synchronization • Node data exchange
– Examples • Parallel Quicksort, Divide and Conquer approaches • Scientific Applications that can be expressed in workflows
(e.g. DAGs)
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• Task graph
P
P P
P
P
P
D
D
D
O
O
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• Work pool
– Mapping of Work/Data
• No desired pre-mapping
• Any task performed by any process
• Pull-model oriented
– Computation
• Processes work as data becomes available (or
requests arrive)
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• Work pool
– Load Balancing
• Dynamic mapping of tasks to processes
– Synchronization
• Adding/removing work from input queue
– Examples
• Web Server
• Bag-of-tasks
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• Work pool
P
Work Pool
P P
P P
Inp
ut q
ueu
e
Outp
ut q
ueu
e
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• Master-slave
– Modification to Worker Pool Model
• One or more Master processes generate and
assign work to worker processes\
• Push-model oriented
– Load Balancing
• A Master process can better distribute load to
worker processes
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• Pipeline
– Mapping of work
• Processes are assigned tasks that correspond to
stages in the pipeline
• Static
– Mapping of Data
• Data processed in FIFO order
– Stream parallelism
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• Pipeline – Computation
• Data is passed through a succession of processes, each of which will perform some task on it
– Load Balancing • Insure all stages of the pipeline are balanced
(contain the same amount of work)
– Synchronization • Producer/Consumer buffers between stages
– Ex: Processor pipeline, graphics pipeline
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• Pipeline
P
Input q
ueu
e
Outp
ut q
ueu
e
P P
buffer
buffer
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• Message-Passing
• Shared Address Space
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• Message-Passing
– Most widely used for programming parallel
computers (clusters of workstations)
– Key attributes:
• Partitioned address space
• Explicit parallelization
– Process interactions
• Send and receive data
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• Message-Passing – Communications
• Sending and receiving messages • Primitives
– send(buff, size, destination) – receive(buff, size, source) – Blocking vs non-blocking – Buffered vs non-buffered
• Message Passing Interface (MPI) – Popular message passing library – ~125 functions
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• Message-Passing
Workstation
Workstation
Workstation
Workstation
Data
P4 P3 P2 P1
send(buff1, 1024, p3) receive(buff3, 1024, p1)
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• Shared Address Space – Mostly used for programming SMP machines
(multicore chips) – Key attributes
• Shared address space – Threads – Shmget/shmat UNIX operations
• Implicit parallelization
– Process/Thread communication • Memory reads/stores
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• Shared Address Space – Communication
• Read/write memory – EX: x++;
– Posix Thread API • Popular thread API • Operations
– Creation/deletion of threads – Synchronization (mutexes, semaphores) – Thread management
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• Shared Address Space
Workstation
T3 T4 T2 T1
Data SMP RAM
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• Synchronization
– Deadlock
– Livelock
– Fairness
• Efficiency
– Maximize parallelism
• Reliability
– Correctness
– Debugging
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