Aca2 10 11

CSE539: Advanced Computer Architecture
PART-IV

{Chapter 10, 11}
Software for Parallel Programming
Book: “Advanced Computer Architecture – Parallelism, Scalability, Programmability”, Hwang & Jotwani

Sumit Mittu
Assistant Professor, CSE/IT
Lovely Professional University
sumit.12735@lpu.co.in

In this chapter…
• Chapter 10
o Parallel Programming Models
o Parallel Languages and Compilers

• Chapter 11
o Parallel Programming Environments

Sumit Mittu, Assistant Professor, CSE/IT, Lovely Professional University

2

PARALLEL PROGRAMING MODELS
• Programming Model
o A collection of program abstractions providing a simplified and transparent view of the computer
hardware/software system to the programmer.

• Parallel Programming models
o Specifically designed for:
• Multiprocessor, Multicomputer and Vector/SIMD Computer
o Basic Models
• Shared-Variable Model
• Message-Passing Model
• Data-Parallel Model
• Object-Oriented Model*
• Functional and Logic Models*


3

• Resources in Programming Systems
o ACTIVE RESOURCES – processors
o PASSIVE RESOURCES – memory and I/O devices

• Processes
o A processes are the basic computational units in parallel program
o A program is a collection of processes
o Parallelism depends on implementation of IPC (inter-process communication)

• Fundamentals issues around parallel programming
o Specification, Creation, Suspension, Reactivation, Migration, Termination and
Synchronization of concurrent processes
o Process address space may be shared or restricted by limiting scope and access rights

4

Shared-Variable Model
• Shared Variable Communication
o Shared Variable and mutual exclusion
o Main Issues:
• Protected access of critical sections
• Memory Consistency
• Atomicity of Memory Operations
• Fast synchronization
• Shared data structures
• Fast data movement techniques
o Critical Section (CS)
• Code segments accessing shared variable with atomic operation
• Requirements – Mutual Exclusion, No Deadlock in waiting, Non-preemption, Eventual Entry

5

• Shared Variable Communication
o Protected Access using CS
• CS boundary
o Too large…may limit parallelism
o Too small…may add unnecessary code complexity and software overhead
o Shorten a heavy-duty CS or use conditional CSs to maintain balanced performance
• Binary and Counting Semaphores
• Monitors

• Operational Modes used in programming multiprocessor systems
o Multiprogramming
o Level of

Multiprocessing

Multitasking


Multithreading

6

• Partitioning and Replication
o Program Partitioning
o Program Replication
o Role of programmer and compiler in partitioning and replication

• Scheduling and Synchronization
o Static Scheduling
o Dynamic Scheduling

• Cache Coherence and Protection
o Multiprocessor Cache Coherence
o Sequential Consistency Model
o Strong and Weak Consistency Model

7

Message-Passing Model
• Message may be:
o Instructions, Data, Synchronization Signals or Interrupt Signals

• Communication Delays caused by message passing is more than
that caused by accessing shared variables
• Message passing Models
o Synchronous Message Passing
o Asynchronous Message Passing

• Critical issue in programing this model
o How to distribute or duplicate program codes and data sets over processing modes?
• Distributing the computations!
o Trade-offs between computation time and communication overhead to be considered

8

Message-Passing Model
• Synchronous Message Passing
o
o
o
o
o

Sender and receiver processes synchronized in time and space
No need of mutual exclusion
No buffers required
Blocking communication scheme
Uniform Communication Delays, in general

• Asynchronous Message Passing
o
o
o
o

Sender and receiver processes do not require to be synchronized in time and space
Often uses buffers in channels
Non-blocking communication scheme
Non-uniform or arbitrary communication delays


9

Data-Parallel Model
•
•
•
•
•
•
•
•
•

SIMD Lockstep operations
Parallelism explicitly handled by hardware synchronization and flow control
Synchronization done at compile time rather than at run time
Data parallel programs require the use of pre-distributed data sets
Choice of parallel data structure is an important consideration
Emphasis on local computation and data routing operations
Can be applicable to fine-grain problems
Implemented either on SIMD or SPMD
Leads to high degree of parallelism involving thousands of data operations
concurrently

10

Data-Parallel Model
• Data Parallelism
o
o
o
o

Illiac-IV processing model
Connection Machine CM-2 processing model
Synchronous SIMD programming v/s Asynchronous MIMD programming
SIMD Characterization for data parallelism
• Scalar operations and scalar data operands
• Vector operations and vector data operands
• Constant data operands
• Masking operations
• Data-routing operations


11

Data-Parallel Model
• Array Language Extensions for data-parallel processing
o Languages
• Fortran 90 array notation
• CFD for Illiac IV
• DAP Fortran for AMT Distributed Array Processor (DAP)
• C* for TMC Connection Machine
• MPF for MasPar family of MPPs
• Actus for SIMD programming
o Expected Characteristics
• Global Address Space
• Explicit data routing among PEs
• No. of PEs be the function of problem size rather than machine size

12

Data-Parallel Model
• Compiler Support for data parallel programming
o Array language expressions and their optimizing compilers must be embedded in familiar standards
such as Fortran and C with an idea to:
• Unify program execution model
• Facilitate precise control of massively parallel hardware
• Enable incremental migration to data parallel execution
o Array Sectioning
• Allows a programmer to reference a section of a region of a multidimensional array
designated by specifying a:
o Start Index
o Bound (or upper limit)
o Stride (or step-size)
• Vector-valued subscripts are often used to construct arrays from arbitrary permutations of
another array

13

PARALLEL LANGUAGES AND COMPILERS
Language Features for Parallelism
• Chang and Smith (1990) classified language features for parallel
programming into 6 classes according to functionality:
o
o
o
o
o
o

Optimization features
Availability features
Synchronization/Communication features
Parallelism Control features
Data Parallelism features
Process Management features

• In practice, real languages might have some or no features of these
incorporated in them.
o The features act as guidelines for user-friendly and efficient programming environment
o Compiler support, OS assistance, integration with existing environment is required

14

• Optimization features
o Used for Program restructuring and compilation directives in coverting sequentially coded program
into parallel forms
o Purpose: to match software parallelism with hardware parallelism in target machine
o Automated parallelizer
• Express C, Alliant FX Fortran compiler
o Semi-automated parallelizer
• DINO
o Interactive restructure support
• MIMDizer from Pacific Sierra


15

• Availability features
o Purpose:
• Enhance user-friendliness
• Make the language portable to a large class of parallel computers
• Expand applicability of software libraries
o Scalability
• in terms of no. of processors available
o Independence
• From hardware topology
o Compatibility
• With an established sequential language
o Portability
• across shared-memory and message-passing computers

16

• Synchronization/Communication features
o
o
o
o
o
o
o

Single assignment languages
Shared variables (locks) for IPC
Logically shared memory (e.g. tuple space in Linda)
Send/receive primitives for message passing
Remote procedure call
Data flow languages (e.g. ID)
Support for Barriers, mailbox, semaphores and monitors


17

• Parallelism Control features
o
o
o
o
o
o
o
o
o

Coarse, Medium or Fine grain parallelism
Explicit or Implicit parallelism
Global parallelism
Loop parallelism
Task-split parallelism
Shared task queue
Divide-and-conquer paradigm
Shared abstract data types
Task dependency specification


18

• Data Parallelism features
o Purpose:
• How data are accessed and distributed in SIMD or MIMD computers
o Runtime automatic Decomposition
• Express C
o Mapping Specification
• DINO
o Virtual Processor Support
• PISCES 2 and DINO
o Direct Access to Shared Data
• Linda
o SPMD
• DINO and Hypertasking

19

• Process Management features
o Purpose:
• Efficient creation of parallel processes
• Implement multi-threading or multi-asking
• Program Partitioning and Replication
• Dynamic Load balancing at runtime
o Dynamic Process creation
o LWP (threads)
o Replicated Workers
o Partitioned networks
o Automatic Load Balancing


20

Parallel Language Constructs
• Fortran 90 Array Notations
o Lower Bound : Upper Bound : Stride
• E1 : E2 : E3
• E1: E2
• E1: * : E3
• E1 : *
• E1
• *

• Parallel Flow Control
o
o
o
o

Doall – Endall or Forall – Endall
Doacross – Endacross or ParDo – ParEnd
Cobegin – Coend or ParBegin – ParEnd
Fork – Join


21

Optimizing Compilers for Parallelism
• Major Phases of parallelizing compiler
o Phase I : Flow Analysis
• Data Dependence
• Control Dependence
• Reuse Analysis
o Phase II : Program Optimizations
• Vectorization
• Parallelizations
• Locality
• Pipelining
o Phase III : Parallel Code Generation
• Granularity
• Degree of Parallelism
• Code Scheduling

Flow Analyis

Program
Optimizations

Parallel Code
Generation
22

Optimizing Compilers for Parallelism
• Parallelizing Compilers
o Parafrase [1984]
• David Kuck, University of Illinois
• Vectorization of paralleiszation of Fortran 77 programs
o Parafrase 2
• David Kuck, University of Illinois
• Parallelization of C or Pascal programs
o KAP vectorizer
• Kuck and associatesBased on Parafrase 2
o PFC, PFC+ and Parascope
• Allen and Kennedy (1984)
o Other compilers
• PTRAN, Alliant FX/F, Convex Vectorizing compiler, Cray CFT compiler, VAST vectorizer, etc.

23

PARALLEL PROGRAMMING ENVIRONMENTS
• Constituents of Environment for parallel programming
o
o
o
o

Hardware Platforms or Machine models
Languages Supported
OS and Software Tools
Application Packages

• Software Tools and Environments
o Parallel Languages
o Integrated Environment Components
• Editor
• Debugger
• Performance Monitors
• Program Visualizers


24

• Fig 11.1


25

• Types of Integrated Environment
o Basic: Provides simple tools for
• Program tracing facility for debugging and performance monitoring
• Graphic mechanism for specifying dependence graphs
o Limited: Provides tools for
• Parallel Debugging
• Performance monitoring
• Program visualization beyond capability of basic environments
o Well-developed: Provides intensive tools for
• Debugging programs
• Interaction of textual/graphical representations of parallel program
• Visualization support for performance monitoring, program visualization, parallel I/O, etc.

26

• Environment Features
o Design Issues
• Compatibility
• Expressiveness
• Ease of use
• Efficiency
• Portability
o Parallel languages may developed as extension to existing sequential languages
o A new parallel programming language has the advantage of:
• Using high-level parallel concepts or constructs for parallelism instead of imperative
(algorithmic) languages which are inherently sequential
o Special optimizing compilers detect parallelism and transform sequential constructs into parallel.

27

• Environment Features
o Special optimizing compilers detect parallelism and transform sequential constructs into parallel.
o High Level parallel constructs can be:
• implicitly embedded in syntax, or
• explicitly specified by users
o Compiler Approaches
• Pre-processors:
o use compiler directives or macroprocessors
• Pre-compilers:
o include automated and semi-automated parallelizers
• Parallelizing Compilers


28

• Fig 11.2


29

• Summary of Important Environment Features

•

o Control flow graph generation
o Integrated textual/graphical map
o Parallel debugger at source code level
o Performance Monitoring by either software or hardware means
o Performance Prediction Model
o Program visualizer for displaying program structures and data flow patterns
o Parallel I/O for fast data movement
o Visualization support for program development and guidance for parallel computations
o OS support for parallelism in front-end or back-end environments
o Communication support in a network environment
Refer to Table 11.1 for attributes of Representative Parallel Programming Tools

30

Case Studies
• Cray Y-MP Software
• Intel Paragon XP/S Software
o Refer Table 11.2 for its characteristic attributes

• CM-5 Software
o Refer Figure 11.3 for several software layers of Connection Machine System


31

Case Studies


32

That’s All Folks!
Attempt your best in exams


33

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