Rajkumar Buyya

1
Concurrent Programming with Threads

• Rajkumar Buyya
• School of Computer Science and Software
  Engineering
• Monash Technology
• Melbourne, Australia
• Email rajkumar_at_ieee.org
• URL http//www.dgs.monash.edu.au/rajkumar

2
Objectives

• Explain the parallel computing right from
  architecture, OS, programming paradigm, and
  applications
• Explain the multithreading paradigm, and all
  aspects of how to use it in an application
• Cover all basic MT concepts
• Explore issues related to MT
• Contrast Solaris, POSIX, Java threads
• Look at the APIs in detail
• Examine some Solaris, POSIX, and Java code
  examples
• Debate on MPP and Cluster Computing

3
Agenda

• Overview of Computing
• Operating Systems Issues
• Threads Basics
• Multithreading with Solaris and POSIX threads
• Multithreading in Java
• Distributed Computing
• Grand Challenges
• Solaris, POSIX, and Java example code

4
Computing Elements
Applications
Programming paradigms
Operating System
Hardware
5
Two Eras of Computing

• Architectures
• Compilers
• Applications
• P.S.Es
• Architectures
• Compilers
• Applications
• P.S.Es

Sequential Era
Parallel Era
1940 50 60 70 80
90 2000
2030
6
History of Parallel Processing

• PP can be traced to a tablet dated around 100 BC.
• Tablet has 3 calculating positions.
• Infer that multiple positions
• Reliability/ Speed

7
Motivating Factors

• Just as we learned to fly, not by constructing a
  machine that flaps its wings like birds, but
  by applying aerodynamics principles
  demonstrated by nature...
• We modeled PP after those of biological species.

8
Motivating Factors

• Aggregated speed with
• which complex calculations
• carried out by individual neurons
• response is slow (ms) - demonstrate
• feasibility of PP

9
Why Parallel Processing?

• Computation requirements are ever increasing --
  visualization, distributed databases,
  simulations, scientific prediction (earthquake),
  etc..
• Sequential architectures reaching physical
  limitation (speed of light, thermodynamics)

10
Technical Computing

• Solving technology problems using
• computer modeling, simulation and analysis

Life Sciences
Aerospace
Mechanical Design Analysis (CAD/CAM)
11
Computational Power Improvement
Multiprocessor
Uniprocessor
C.P.I.
1 2 . . . .
No. of Processors
12
Computational Power Improvement
Vertical
Horizontal
Growth
5 10 15 20 25 30 35 40
45 . . . .
Age
13
Why Parallel Processing?

• The Tech. of PP is mature and can be exploited
  commercially significant R D work on
  development of tools environment.
• Significant development in Networking technology
  is paving a way for heterogeneous computing.

14
Why Parallel Processing?

• Hardware improvements like Pipelining,
  Superscalar, etc., are non-scalable and requires
  sophisticated Compiler Technology.
• Vector Processing works well for certain kind of
  problems.

15
Parallel Program has needs ...

• Multiple processes active simultaneously
  solving a given problem, general multiple
  processors.
• Communication and synchronization of its
  processes (forms the core of parallel programming
  efforts).

16
Processing Elements Architecture
17
Processing Elements

• Simple classification by Flynn
• (No. of instruction and data streams)
• SISD - conventional
• SIMD - data parallel, vector computing
• MISD - systolic arrays
• MIMD - very general, multiple approaches.
• Current focus is on MIMD model, using general
  purpose processors.
• (No shared memory)

18
SISD A Conventional Computer

• Speed is limited by the rate at which computer
  can transfer information internally.

ExPC, Macintosh, Workstations
19
The MISDArchitecture

• More of an intellectual exercise than a practical
  configuration. Few built, but commercially not
  available

20
SIMD Architecture
Cilt Ai Bi

• Ex CRAY machine vector processing, Thinking
  machine cm

21
MIMD Architecture
Instruction Stream A
Instruction Stream B
Instruction Stream C
Data Output stream A
Data Input stream A
Processor A
Data Output stream B
Processor B
Data Input stream B
Data Output stream C
Processor C
Data Input stream C

• Unlike SISD, MISD, MIMD computer works
  asynchronously.
• Shared memory (tightly coupled) MIMD
• Distributed memory (loosely coupled) MIMD

22
Shared Memory MIMD machine
Processor A
Processor B
Processor C
Global Memory System

• Comm Source PE writes data to GM destination
  retrieves it
• Easy to build, conventional OSes of SISD can be
  easily be ported
• Limitation reliability expandability. A
  memory component or any processor failure affects
  the whole system.
• Increase of processors leads to memory
  contention.
• Ex. Silicon graphics supercomputers....

23
Distributed Memory MIMD
IPC channel
IPC channel
Processor A
Processor B
Processor C

• Communication IPC on High Speed Network.
• Network can be configured to ... Tree, Mesh,
  Cube, etc.
• Unlike Shared MIMD
• easily/ readily expandable
• Highly reliable (any CPU failure does not affect
  the whole system)

24
Laws of caution.....

• Speed of computers is proportional to the square
  of their cost.
• i.e.. cost Speed
• Speedup by a parallel computer increases as the
  logarithm of the number of processors.

Speedup log2(no. of processors)
25
Caution....

• Very fast development in PP and related area have
  blurred concept boundaries, causing lot of
  terminological confusion concurrent computing/
  programming, parallel computing/ processing,
  multiprocessing, distributed computing, etc..

26

• Its hard to imagine a field that changes as
  rapidly as computing.

27
Caution....
Computer Science is an Immature Science. (lack
of standard taxonomy, terminologies)
28
Caution....

• There is no strict delimiters for contributors to
  the area of parallel processing CA, OS, HLLs,
  databases, computer networks, all have a role to
  play.
• This makes it a Hot Topic of Research

29
Parallel Programming Paradigms

• Multithreading
• Task level parallelism

30
Serial Vs. Parallel
COUNTER 2
COUNTER
COUNTER 1
31
High Performance Computing
function1( ) //......function stuff
t1
function2( ) //......function stuff
t2
Parallel Machine MPP

• function1( ) function2 ( )
• massively parallel system
• containing thousands of CPUs
• Time max (t1, t2)

32
Single and Multithreaded Processes
Single-threaded Process
Multiplethreaded Process
Threads of Execution
Multiple instruction stream
Single instruction stream
Common Address Space
33
OSMulti-Processing, Multi-Threaded
Threaded Libraries, Multi-threaded I/O
Better Response Times in Multiple Application
Environments
Higher Throughput for Parallelizeable Applications
34
Multi-threading, continued...Multi-threaded OS
enables parallel, scalable I/O
Application
Application
Application
Multiple, independent I/O requests can be
satisfied simultaneously because all the major
disk, tape, and network drivers have been
multi-threaded, allowing any given driver to run
on multiple CPUs simultaneously.
OS Kernel
CPU
CPU
CPU
35
Basic Process Model
STACK
STACK
Shared memory segments, pipes, open files or
mmapd files
DATA
DATA
TEXT
TEXT
Shared Memory maintained by kernel
processes
processes
36
What are Threads?

• Thread is a piece of code that can execute in
  concurrence with other threads.
• It is a schedule entity on a processor

• Local state
• Global/ shared state
• PC
• Hard/Software Context

Thread Object
37
Threaded Process Model
THREAD STACK
SHARED MEMORY
THREAD DATA
Threads within a process
THREAD TEXT

• Independent executables
• All threads are parts of a process hence
  communication
• easier and simpler.

38
Levels of Parallelism
Code-Granularity Code Item Large grain (task
level) Program Medium grain (control
level) Function (thread) Fine grain (data
level) Loop Very fine grain (multiple
issue) With hardware
Task i-l
Task i
Task i1
func1 ( ) .... ....
func2 ( ) .... ....
func3 ( ) .... ....

• Task
• Control
• Data
• Multiple Issue

a ( 0 ) .. b ( 0 ) ..
a ( 1 ).. b ( 1 )..
a ( 2 ).. b ( 2 )..

x
Load
39
Simple Thread Example

• void func ( )
• / define local data /
• - - - - - - - - - - -
• - - - - - - - - - - - / function code /
• - - - - - - - - - - -
• thr_exit(exit_value)
• main ( )
• thread_t tid
• int exit_value
• - - - - - - - - - - -
• thread_create (0, 0, func (), NULL, tid)
• - - - - - - - - - - -
• thread_join (tid, 0, exit_value)
• - - - - - - - - - - -

40
Few Popular Thread Models

• POSIX, ISO/IEEE standard
• Mach C threads, CMU
• Sun OS LWP threads, Sun Microsystems
• PARAS CORE threads, C-DAC
• Java-Threads, Sun Microsystems
• Chorus threads, Paris
• OS/2 threads, IBM
• Windows NT/95 threads, Microsoft

41
Multithreading - Uniprocessors

• Concurrency Vs Parallelism

• Concurrency

P1
CPU
P2
P3
time
Number of Simulatneous execution units gt no of
CPUs
42
Multithreading - Multiprocessors
Concurrency Vs Parallelism
CPU
P1
CPU
P2
CPU
P3
time
No of execution process no of CPUs
43
Computational Model
User-Level Schedule (User)
Kernel-Level Schedule (Kernel)

• Parallel Execution due to
• Concurrency of threads on Virtual Processors
• Concurrency of threads on Physical Processor
• True Parallelism
• threads processor map 11

44
General Architecture ofThread Model

• Hides the details of machine architecture
• Maps User Threads to kernel threads
• Process VM is shared, state change in VM by one
  thread visible to other.

45
Process Parallelism

• int add (int a, int b, int result)
• // function stuff
• int sub(int a, int b, int result)
• // function stuff

Data
Processor
a b r1 c d r2
IS1
add
pthread t1, t2 pthread-create(t1, add, a,b,
r1) pthread-create(t2, sub, c,d,
r2) pthread-par (2, t1, t2)
Processor
IS2
sub
MISD and MIMD Processing
46
Data Parallelism
Data

• sort( int array, int count)
• //......
• //......

do dn/2 dn2/1 dn
Processor
Sort
pthread-t, thread1, thread2 pthread-create(
thread1, sort, array, N/2) pthread-create(
thread2, sort, array, N/2) pthread-par(2,
thread1, thread2)
IS
Processor
Sort
SIMD Processing
47
Process and Threaded models
Purpose
Threads Model
Process Model
Creation of a new thread
fork ( )
thr_create( )
exec( )
thr_create() builds the new thread and starts
the execution
Start execution of a new thread
Wait for completion of thread
thr_join()
wait( )
Exit and destroy the thread
exit( )
thr_exit()
48
Code Comparison

• Segment (Process)
• main ( )
• fork ( )
• fork ( )
• fork ( )

Segment(Thread) main() thread_create(0,0,func()
,0,0) thread_create(0,0,func(),0,0) thread_creat
e(0,0,func(),0,0)
49
Printing Thread
Editing Thread
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Independent Threads

• printing()
• - - - - - - - - - - - -
• editing()
• - - - - - - - - - - - -
• main()
• - - - - - - - - - - - -
• id1 thread_create(printing)
• id2 thread_create(editing)
• thread_run(id1, id2)
• - - - - - - - - - - - -

51
Cooperative threads - File Copy
reader() - - - - - - - - - - lock(buffi) read
(src,buffi) unlock(buffi) - - - - - - - - -
-
writer() - - - - - - - - - - lock(buffi) writ
e(src,buffi) unlock(buffi) - - - - - - - -
- -
buff0
buff1
Cooperative Parallel Synchronized Threads
52
RPC Call
Client
Network
Server
RPC(func)
func() / Body /
........
53
Multithreaded Server
Server Process
Client Process
Server Threads
Client Process
User Mode
Kernel Mode
Message Passing Facility
54
Multithreaded Compiler
Preprocessor Thread
Compiler Thread
55
Thread Programming models
1. The boss/worker model 2. The peer
model 3. A thread pipeline
56
The boss/worker model
Program
Resources
Workers
taskX
Files
Databases
Boss
taskY
main ( )
Input (Stream)
Disks
taskZ
Special Devices
57
Example

• main() / the boss /
• forever
• get a request
• switch( request )
• case X pthread_create(....,taskX)
• case X pthread_create(....,taskX)
• ....
• taskX() / worker /
• perform the task, sync if accessing shared
  resources
• taskY() / worker /
• perform the task, sync if accessing shared
  resources
• ....

58
The peer model
Program
Workers
Input (static)
taskX
taskY
59
Example

• main()
• pthread_create(....,thread1...task1)
• pthread_create(....,thread2...task2)
• ....
• signal all workers to start
• wait for all workers to finish
• do any cleanup
• task1() / worker /
• wait for start
• perform the task, sync if accessing shared
  resources
• task2() / worker /
• wait for start
• perform the task, sync if accessing shared
  resources

60
A thread pipeline
Program
Filter Threads
Stage 1
Stage 2
Stage 3
Input (Stream)
Resources
61
Example

• main()
• pthread_create(....,stage1)
• pthread_create(....,stage2)
• ....
• wait for all pipeline threads to finish
• do any cleanup
• stage1()
• get next input for the program
• do stage 1 processing of the input
• pass result to next thread in pipeline
• stage2()
• get input from previous thread in pipeline
• do stage 2 processing of the input
• pass result to next thread in pipeline
• stageN()

62
Multithreaded Matrix Multiply...
X

A
B
C
C1,1 A1,1B1,1A1,2B2,1.. . Cm,ns
um of product of corresponding elements in row of
A and column of B.
Each resultant element can be computed
independently.
63
Multithreaded Matrix Multiply

• typedef struct
• int id int size
• int row, column
• matrix MA, MB, MC
• matrix_work_order_t
• main()
• int size ARRAY_SIZE, row, column
• matrix_t MA, MB,MC
• matrix_work_order work_orderp
• pthread_t peersizezize
• ...
• / process matrix, by row, column /
• for( row 0 row lt size row )
• for( column 0 column lt size column)
• id column row ARRAY_SIZE
• work_orderp malloc( sizeof(matrix_work_order_t
  ))
• / initialize all members if wirk_orderp /

64
Multithreaded Server...

• void main( int argc, char argv )
• int server_socket, client_socket, clilen
• struct sockaddr_in serv_addr, cli_addr
• int one, port_id
• ifdef _POSIX_THREADS
• pthread_t service_thr
• endif
• port_id 4000 / default port_id /
• if( (server_socket socket( AF_INET,
  SOCK_STREAM, 0 )) lt 0 )
• printf("Error Unable to open socket in parmon
  server.\n")
• exit( 1 )
• memset( (char) serv_addr, 0,
  sizeof(serv_addr))
• serv_addr.sin_family AF_INET
• serv_addr.sin_addr.s_addr htonl(INADDR_ANY)
• serv_addr.sin_port htons( port_id )

65
Multithreaded Server...

• if( bind( server_socket, (struct sockaddr
  )serv_addr, sizeof(serv_addr)) lt 0 )
• printf( "Error Unable to bind socket in parmon
  server-gtd\n",errno )
• exit( 1 )
• listen( server_socket, 5)
• while( 1 )
• clilen sizeof(cli_addr)
• client_socket accept( server_socket, (struct
  sockaddr )serv_addr, clilen )
• if( client_socket lt 0 )
• printf( "connection to client failed in
  server.\n" ) continue
• ifdef POSIX_THREADS
• pthread_create( service_thr, NULL,
  service_dispatch, client_socket)
• else
• thr_create( NULL, 0, service_dispatch,
  client_socket, THR_DETACHED, service_thr)
• endif

66
Multithreaded Server

• // Service function -- Thread Funtion
• void service_dispatch(int client_socket)
• Get USER Request
• if( readline( client_socket, command, 100 ) gt
  0 )
• IDENTIFY USER REQUEST
• .Do NECESSARY Processing
• ..Send Results to Server
• CLOSE Connect and Terminate THREAD
• close( client_socket )
• ifdef POSIX_THREADS
• pthread_exit( (void )0)
• endif

67
The Value of MT

• Program structure
• Parallelism
• Throughput
• Responsiveness
• System resource usage
• Distributed objects
• Single source across platforms (POSIX)
• Single binary for any number of CPUs

68
To thread or not to thread

• Improve efficiency on uniprocessor systems
• Use multiprocessor Hardware
• Improve Throughput
• Simple to implement Asynchronous I/O
• Leverage special features of the OS

69
To thread or not to thread

• If all operations are CPU intensive do not go far
  on multithreading
• Thread creation is very cheap, it is not free
• thread that has only five lines of code would not
  be useful

70
DOS - The Minimal OS
Stack Stack Pointer
Program Counter
User Code Global Data DOS Code
User Space Kernel Space DOS Data
DOS
Hardware
71
Multitasking OSs
Process User Space Kernel Space
Process Structure
UNIX
Hardware
(UNIX, VMS, MVS, NT, OS/2 etc.)
72
Multitasking Systems
Processes
P3
P4
P1
P2
The Kernel
Hardware
(Each process is completely independent)
73
Multithreaded Process
T1s SP T3sPC T1sPC T2sPC
T1s SP
User Code Global Data
T2s SP
Process Structure
The Kernel
(Kernel state and address space are shared)
74
Kernel Structures
Traditional UNIX Process Structure
Solaris 2 Process Structure
Process ID UID GID EUID EGID CWD.
Signal Dispatch Table
Memory Map
File Descriptors
LWP 2
LWP 1
75
Scheduling Design Options
M1 HP-UNIX
11 DEC, NT, OS/1, AIX. IRIX
MM
2-level
76
SunOS Two-Level Thread Model
Traditional process
Proc 1
Proc 2
Proc 3
Proc 4
Proc 5
User
LWPs
Kernel threads
Kernel
Hardware
Processors
77
Thread Life Cycle
T1
pthread_create(...func...)
pthread_exit()
T2

• main() main()
• ...
• pthread_create( func, arg) thr_create(
  ..func..,arg..)
• ... ...
• void func()
• ....

POSIX
Solaris
78
Waiting for a Thread to Exit
T1
pthread_join()
pthread_exit()
T2

• main() main()
• ...
• pthread_join(T2) thr_join(
  T2,val_ptr)
• ... ...
• void func()
• ....

POSIX
Solaris
79
Scheduling States Simplified View of Thread
State Transitions
RUNNABLE
Wakeup
Stop
Continue
Preempt
SLEEPING
STOPPED
Stop
ACTIVE
Stop
Sleep
80
Preemption

• The process of rudely interrupting a thread and
  forcing it to relinquish its LWP (or CPU) to
  another.
• CPU2 cannot change CPU3s registers directly.
  It can only issue a hardware interrupt to CPU3.
  It is up to CPU3s interrupt handler to look at
  CPU2s request and decide what to do.
• Higher priority threads always preempt lower
  priority threads.
• Preemption ! Time slicing
• All of the libraries are preemptive

81
EXIT Vs. THREAD_EXIT

• The normal C function exit() always causes the
  process to exit. That means all of the process
  -- All the threads.
• The thread exit functions
• UI thr_exit()
• POSIX pthread_exit()
• OS/2 DosExitThread() and _endthread()
• NT ExitThread() and endthread()
• all cause only the calling thread to exit,
  leaving the process intact and all of the other
  threads running. (If no other threads are
  running, then exit() will be called.)

82
Cancellation

• Cancellation is the means by which a thread can
  tell another thread that it should exit.
• main() main() main()
• ... ... ...
• pthread_cancel (T1) DosKillThread(T1) TerminateT
  hread(T1)
• There is no special relation between the killer
  of a thread and the victim. (UI threads must
  roll their own using signals)

(pthread exit)
T1
(pthread cancel()
T2
Windows NT
POSIX
OS/2
83
Cancellation State and Type

• State
• PTHREAD_CANCEL_DISABLE (Cannot be cancelled)
• PTHREAD_CANCEL_ENABLE (Can be cancelled, must
  consider type)
• Type
• PTHREAD_CANCEL_ASYNCHRONOUS (any time
  what-so-ever) (not generally used)
• PTHREAD_CANCEL_DEFERRED
• (Only at cancellation points)
• (Only POSIX has state and type)
• (OS/2 is effectively always enabled
  asynchronous)
• (NT is effectively always enabled asynchronous)

84
Cancellation is Always Complex!

• It is very easy to forget a lock thats being
  held or a resource that should be freed.
• Use this only when you absolutely require it.
• Be extremely meticulous in analyzing the possible
  thread states.
• Document, document, document!

85
Returning Status

• POSIX and UI
• A detached thread cannot be joined. It cannot
  return status.
• An undetached thread must be joined, and can
  return a status.
• OS/2
• Any thread can be waited for
• No thread can return status
• No thread needs to be waited for.
• NT
• No threads can be waited for
• Any thread can return status

86
Suspending a Thread
T1
suspend()
continue()
T2
Solaris

• main()
• ...
• thr_suspend(T1)
• ...
• thr_continue(T1)
• ...

POSIX does not support thread suspension
87
Proposed Uses of Suspend/Continue

• Garbage Collectors
• Debuggers
• Performance Analysers
• Other Tools?
• These all must go below the API, so they dont
  count.
• Isolation of VM system spooling (?!)
• NT Services specify that a service should b
  suspendable (Questionable requirement?)
• Be Careful

88
Do NOT Think about Scheduling!

• Think about Resource Availability
• Think about Synchronization
• Think about Priorities
• Ideally, if youre using suspend/ continue,
  youre making a mistake!

89
Synchronization

• Websters To represent or arrange events to
  indicate coincidence or coexistence.
• Lewis To arrange events so that they occur in
  a specified order.
• Serialized access to controlled resources.
• Synchronization is not just an MP issue. It is
  not even strictly an MT issue!

90

• Threads Synchronization
• On shared memory shared variables - semaphores
• On distributed memory
• within a task semaphores
• Across the tasks By passing messages

91
Unsynchronized Shared Data is a Formula for
Disaster

• Thread1 Thread2
• temp Your - gt BankBalance
• dividend temp InterestRate
• newbalance dividend temp
• Your-gtDividend dividend
  Your-gtBankBalance deposit
• Your-gtBankBalance newbalance

92
Atomic Actions

• An action which must be started and completed
  with no possibility of interruption.
• A machine instruction could need to be atomic.
  (not all are!)
• A line of C code could need to be atomic. (not
  all are)
• An entire database transaction could need to be
  atomic.
• All MP machines provide at least one complex
  atomic instruction, from which you can build
  anything.
• A section of code which you have forced to be
  atomic is a Critical Section.

93
Critical Section(Good Programmer!)
T1
T2
reader() - - - - - - - - - - lock(DISK) .......
.... ........... ........... unlock(DISK) - - -
- - - - - - -
writer() - - - - - - - - - - lock(DISK) .......
....... .............. unlock(DISK) - - - - - -
- - - -
Shared Data
94
Critical Section(Bad Programmer!)
T1
T2
reader() - - - - - - - - - - lock(DISK) .......
.... ........... ........... unlock(DISK) - - -
- - - - - - -
writer() - - - - - - - - - - .............. ....
.......... - - - - - - - - - -
Shared Data
95
Lock Shared Data!

• Globals
• Shared data structures
• Static variables
• (really just lexically scoped global variables)

96
Mutexes
Thread2
Thread 1

• item create_and_fill_item()
• mutex_lock( m )
• item-gtnext list
• list item
• mutex_unlock(m)

mutex_lock( m ) this_item list list
list_next mutex_unlock(m) .....func(this-item)

• POSIX and UI Owner not recorded, block in
  priority order.
• OS/2 and NT. Owner recorded, block in FIFO order.

97
Synchronization Variables in Shared Memory (Cross
Process)
Process 1
Process 2
Synchronization Variable
S
S
Shared Memory
S
S
Thread
98
SynchronizationProblems
99
Deadlocks
Thread 2
Thread 1
lock( M2 ) lock( M1 )

• lock( M1 )
• lock( M2 )

Thread1 is waiting for the resource(M2) locked by
Thread2 and Thread2 is waiting for the resource
(M1) locked by Thread1
100
Avoiding Deadlocks

• Establish a hierarchy Always lock Mutex_1
  before Mutex_2, etc..,.
• Use the trylock primitives if you must violate
  the hierarchy.
• while (1)
• pthread_mutex_lock (m2)
• if( EBUSY pthread mutex_trylock
  (m1))
• break
• else
• pthread _mutex_unlock (m1)
• wait_around_or_do_something_else()
• do_real work() / Got em both! /
• Use lockllint or some similar static analysis
  program to scan your code for hierarchy
  violations.

101
Race Conditions

• A race condition is where the results of a
  program are different depending upon the timing
  of the events within the program.
• Some race conditions result in different answers
  and are clearly bugs.
• Thread 1 Thread 2
• mutex_lock (m) mutex_lock (m)
• v v - 1 v v 2
• mutex_unlock (m) mutex_unlock (m)
• --gt if v 1, the result can be 0 or 1based on
  which thread gets chance to enter CR first

102
Operating System Issues
103
Library Goals

• Make it fast!
• Make it MT safe!
• Retain UNIX semantics!

104
Are Libraries Safe ?

• getc() OLD implementation
• extern int get( FILE p )
• / code to read data /
• getc() NEW implementation
• extern int get( FILE p )
• pthread_mutex_lock(m)
• / code to read data /
• pthread_mutex_unlock(m)

105
ERRNO

• In UNIX, the distinguished variable errno is
  used to hold the error code for any system calls
  that fail.
• Clearly, should two threads both be issuing
  system calls around the same time, it would not
  be possible to figure out which one set the value
  for errno.
• Therefore errno is defined in the header file to
  be a call to thread-specific data.
• This is done only when the flag_REENTRANT (UI)
• _POSIX_C_SOURCE199506L (POSIX) is passed to the
  compiler, allowing older, non-MT programs to
  continue to run.
• There is the potential for problems if you use
  some libraries which are not reentrant. (This is
  often a problem when using third party libraries.)

106
Are Libraries Safe?

• MT-Safe This function is safe
• MT-Hot This function is safe and fast
• MT-Unsafe This function is not MT-safe, but was
  compiled with _REENTRANT
• Alternative Call This function is not safe, but
  there is a similar function (e.g.
  getctime_r())
• MT-Illegal This function wasnt even compiled
  with _REENTRANT and therefore can only be
  called from the main thread.

107
Threads Debugging Interface

• Debuggers
• Data inspectors
• Performance monitors
• Garbage collectors
• Coverage analyzers
• Not a standard interface!

108
The APIs
109
Different Thread Specifications

• Functionality UI Threads POSIX Thteads NT
  Threads OS/2 Threads
• Design Philosophy Base Near-Base Complex Comple
  x
• Primitives Primitives Primitives Primitives
• Scheduling Classes Local/ Global Local/Global Glob
  al Global
• Mutexes Simple Simple Complex Complex
• Counting Semaphores Simple Simple Buildable Bui
  ldable
• R/W Locks Simple Buildable Buildable Buildable
• Condition Variables Simple Simple Buildable Bui
  ldable
• Multiple-Object Buildable Buildable Complex Com
  plex
• Synchronization
• Thread Suspension Yes Impossible Yes Yes
• Cancellation Buildable Yes Yes Yes
• Thread-Specific Data Yes Yes Yes Yes
• Signal-Handling
• Primitives Yes Yes n/a n/a
• Compiler Changes
• Required No No Yes No
• Vendor Libraries MT-safe? Moat Most All? All?

110
POSIX and Solaris API Differences
POSIX API
Solaris API
continue suspend semaphore vars
concurrency setting
reader/ writer vars daemon threads
thread cancellation scheduling
policies sync attributes thread attributes
join exit key creation priorities sigmask
create thread specific data mutex
vars kill condition vars
111
Error Return Values

• Many threads functions return an error value
  which can be looked up in errno.h.
• Very few threads functions set errno(check man
  pages).
• The lack of resources errors usually mean that
  youve used up all your virtual memory, and your
  program is likely to crash very soon.

112
Attribute Objects

• UI, OS/2, and NT all use flags and direct
  arguments to indicate what the special details of
  the objects being created should be. POSIX
  requires the use of Attribute objects
• thr_create(NULL, NULL, foo, NULL, THR_DETACHED)
• Vs
• pthread_attr_t attr
• pthread_attr_init(attr)
• pthread_attr_setdetachstate(attr,PTHREAD_CREATE_D
  ETACHED)
• pthread_create(NULL, attr, foo, NULL)

113
Attribute Objects

• Although a bit of pain in the compared to
  passing all the arguments directly, attribute
  objects allow the designers of the threads
  library more latitude to add functionality
  without changing the old interfaces. (If they
  decide they really want to, say, pass the signal
  mask at creation time, they just add a function
  pthread_attr_set_signal_mask() instead of adding
  a new argument to pthread_create().)
• There are attribute objects for
• Threads
• stack size, stack base, scheduling policy,
  scheduling class, scheduling scope, scheduling
  inheritance, detach state.
• Mutexes
• Cross process, priority inheritance
• Condition Variables
• Cross process

114
Attribute Objects

• Attribute objects must be
• Allocated
• Initialized
• Values set (presumably)
• Used
• Destroyed (if they are to be freed)
• pthread_attr_t attr
• pthread_attr_init (attr)
• pthread_attr_setdetachstate(attr,
  PTHREAD_CREATE_DETACHED)
• pthread_create(NULL, attr, foo, NULL)
• pthread_attr_destroy (attr)

115
Thread Attribute Objects

• pthread_attr_t
• Thread attribute object type
• pthread_attr_init (pthread_mutexattr_t attr)
• pthread_attr_destroy (pthread_attr_t attr)
• pthread_attr_getdetachstate (pthread_attr_t
  attr, in state)
• pthread_attr_setdetachstate (pthread_attr_t
  attr, int state)
• Can the thread be joined?
• pthread_attr_getscope(pthread_attr_t attr, in
  scope)
• pthread_attr_setscope(pthread_attr_t attr, int
  scope)

116
Thread Attribute Objects

• pthread_attr_getinheritpolicy(pthread_attr_t
  attr, int policy)
• pthread_attr_setinheritpolicy(pthread_attr_t
  attr, int policy)
• Will the policy in the attribute object be used?
• pthread_attr_getschedpolicy(pthread_attr_t attr,
  int policy)
• pthread_attr_setschedpolicy(pthread_attr_t attr,
  int policy)
• Will the scheduling be RR, FIFO, or OTHER?
• pthread_attr_getschedparam(pthread_attr_t attr,
  struct sched param param)
• pthread_attr_setschedparam(pthread attr_t attr,
  struct sched param param)
• What will the priority be?

117
Thread Attribute Objects

• pthread_attr_getinheritsched(pthread_attr_t
  attr, int inheritsched)
• pthread_attr_setinheritsched(pthread_attr_t
  attr, int inheritsched)
• Will the policy in the attribute object be used?
• pthread_attr_getstacksize(pthread_attr_t attr,
  int size)
• pthread_attr_setstacksize(pthread_attr_t attr,
  int size)
• How big will the stack be?
• pthread_attr_getstackaddr (pthread_attr_t attr,
  size_t base)
• pthread_attr_setstackaddr(pthread_attr_t attr,
  size_t base)
• What will the stacks base address be?

118
Mutex Attribute Objects

• pthread_mutexattr_t
• mutex attribute object type
• pthread_mutexattr_init(pthread_mutexattr_t attr)
• pthread_mutexattr_destroy(pthread_mutexattr_t
  attr)
• pthread_mutexattr_getshared(pthread_mutexattr_tat
  tr, int shared)
• pthread_mutexattr_setpshared (pthread_mutex
  attr_t attr,
• int shared)
• Will the mutex be shared across processes?

119
Mutex Attribute Objects

• pthread_mutexattr_getprioceiling(pthread_mutexattr
  _t
• attr, int ceiling)
• pthread_mutexattr_setprioceiling(pthread_mutexattr
  _t
• attr, int ceiling)
• What is the highest priority the thread owning
  this mutex can acquire?
• pthread_mutexattr_getprotocol (pthread_mutexattr_t
  
• attr, int protocol)
• pthread_mutexattr_setprotocol (pthread_mutexattr_t
  
• attr, int protocol)
• Shall the thread owning this mutex inherit
  priorities from waiting threads?

120
Condition Variable Attribute Objects

• pthread_condattr_t
• CV attribute object type
• pthread_condattr_init(pthread_condattr_t attr)
• pthread_condattr_destroy(pthread_condattr_t
  attr)
• pthread_condattr_getpshared (pthread_condattr_t
• attr, int shared)
• pthread_condattr_setpshared (pthread_condattr_t
• attr, int shared)
• Will the mutex be shared across processes?

121
Creation and Destruction (UI POSIX)

• int thr_create(void stack_base, size_t
  stacksize,
• void (start_routine) (void ), void
• arg, long flags, thread_t thread)
• void thr_exit (void value_ptr)
• int thr_join (thread_t thread, void value_ptr)
• int pthread_create (pthread_t thread, const
• pthread_attr_t attr, void
• (start_routine) (void ), void arg)
• void pthread_exit (void value_ptr)
• int pthread_join (pthread_t thread, void
• value_ptr)
• int pthread_cancel (pthread_t thread)

122
Suspension (UI POSIX)

• int thr_suspend (thread_t target)
• int thr_continue (thread_t target)

123
Changing Priority (UI POSIX)

• int thr_setpriority(thread_t thread, int
  priority)
• int thr_getpriority(thread_t thread, int
  priority)
• int pthread_getschedparam(pthread_t thread, int
• policy, struct sched param
• param)
• int pthread_setschedparam(pthread_t thread, int
• policy, struct sched param param)

124
Readers / Writer Locks (UI)

• int rwlock_init (rwlock_t rwlock, int type,
  void arg)
• int rw_rdlock (rwlock_t rwlock)
• int rw_wrlock (rwlock_t rwlock)
• int rw_tryrdlock (rwlock_t rwlock)
• int rw_trywrlock (rwlock_t rwlock)
• int rw_unlock (rwlock_t rwlock)
• int rw_destroy (rwlock_t rwlock)

125
(Counting) Semaphores (UI POSIX)

• int sema_init (sema_t sema,
• unsigned int sema_count,
• int type, void arg)
• int sema_wait (sema_t sema)
• int sema_post (sema_t sema)
• int sema_trywait (sema_t sema)
• int sema_destroy (sema_t sema)
• int sem_init (sem_t sema, int pshared, unsigned
  int count)
• int sem_post (sem_t sema)
• int sem_trywait (sem_t sema)
• int sem_destroy (sem_t sema)
• (POSIX semaphores are not part of pthread. Use
  the libposix4.so and posix4.h)

126
Condition Variables (UI POSIX)

• int cond_init(contd_t cond, int type, void arg)
• int cond_wait(cond_t cond, mutex_t mutex)
• int cond_signal(cond_t cond)
• int cond_broadcast(cond_t cond)
• int cond_timedwait(cond_t cond, mutex_t mutex,
  timestruc_t abstime)
• int cond_destroy (cond_t cond)
• int pthread_cond_init(pthread_cond_t
  cond,pthread_condattr_t attr)
• int pthread_cond_wait(pthread_cond_t cond,
  pthread_mutex_t mutex)
• int pthread_cond_signal (pthread_cond_t cond)
• int pthread_cond_broadcast(pthread_cond_t cond,
  pthread_mutex_t mutex, struct timespec abstime)
• int pthread_cond_destroy(pthread_cond_t cond)

127
Signals (UI POSIX)

• int thr_sigsetmask(int how, const sigset_t set,
  sigset_t oset)
• int thr_kill(thread_t target thread, int sig)
• int sigwait(sigset_t set)
• int pthread_sigmask(int how, const sigset_t set,
  sigset_t oset)
• int pthread_kill(thread_t target_thread, int sig)
• int sigwait(sigset_t set, int sig)

128
Cancellation (POSIX)
int pthread_cancel (pthread_thread_t
thread) int pthread cleanup_pop (int
execute) int pthread_cleanup_push (void
(funtion) (void ), void arg) int
pthread_setcancelstate (int state, int
old_state) int pthread_testcancel (void)
129
Other APIs

• thr_self(void)
• thr_yield()
• int pthread_atfork (void (prepare) (void),
• void (parent) (void),
• void (child) (void)
• pthread_equal (pthread_thread_t tl,
  pthread_thread_t t2)
• pthread_once (pthread_once_t once_control, void
• (init_routine) (void))
• pthread_self (void)
• pthread_yield()
• (Thread IDs in Solaris recycle every 232
  threads, or about once a month if you do
  create/exit as fast as possible.)

130
Compiling
131
Solaris Libraries

• Solaris has three libraries libthread.so,
  libpthread.so, libposix4.so
• Corresponding new include files synch.h,
  thread.h, pthread.h, posix4.h
• Bundled with all O/S releases
• Running an MT program requires no extra effort
• Compiling an MT program requires only a compiler
  (any compiler!)
• Writing an MT program requires only a compiler
  (but a few MT tools will come in very handy)

132
Compiling UI under Solaris

• Compiling is no different than for non-MT
  programs
• libthread is just another system library in
  /usr/lib
• Example cc -o sema sema.c -lthread
  -D_REENTRANT cc -o sema sema.c -mt
• All multithreaded programs should be compiled
  using the _REENTRANT flag
• Applies for every module in a new application
• If omitted, the old definitions for errno, stdio
  would be used, which you dont want
• All MT-safe libraries should be compiled using
  the _REENTRANT flag, even though they may be used
  single in a threaded program.

133
Compiling POSIX under Solaris

• Compiling is no different than for non-MT
  programs
• libpthread is just another system library in
  /usr/lib
• Example cc-o sema sema.c -lpthread
  -lposix4 -D_POSIX_C_SOURCE19956L
• All multithreaded programs should be compiled
  using the _POSIX_C_SOURCE199506L flag
• Applies for every module in a new application
• If omitted, the old definitions for errno, stdio
  would be used, which you dont want
• All MT-safe libraries should be compiled using
  the _POSIX_C_SOURCE199506L flag, even though
  they may be used single in a threaded program

134
Compiling mixed UI/POSIX under Solaris

• If you just want to use the UI thread functions
  (e.g., thr_setconcurrency())
• cc-o sema sema.c -1thread -1pthread -1posix4
  D_REENTRANT -
• _POSIX_PTHREAD_SEMANTICS
• If you also want to use the UI semantics for
  fork(), alarms, timers, sigwait(), etc.,.

135
Summary

• Threads provide a more natural programming
  paradigm
• Improve efficiency on uniprocessor systems
• Allows to take full advantage of multiprocessor
  Hardware
• Improve Throughput simple to implement
  asynchronous I/O
• Leverage special features of the OS
• Many applications are already multithreaded
• MT is not a silver bullet for all programming
  problems.
• Threre is already standard for multithreading--POS
  IX
• Multithreading support already available in the
  form of language syntax--Java
• Threads allows to model the real world object
  (ex in Java)

136
Java

• Multithreading in Java

137
Java - An Introduction

• Java - The new programming language from Sun
  Microsystems
• Java -Allows anyone to publish a web page with
  Java code in it
• Java - CPU Independent language
• Created for consumer electronics
• Java - James , Arthur Van , and others
• Java -The name that survived a patent search
• Oak -The predecessor of Java
• Java is C --

138
Object Oriented Languages -A comparison
139
Sun defines Java as

• Simple and Powerful
• Safe
• Object Oriented
• Robust
• Architecture Neutral and Portable
• Interpreted and High Performance
• Threaded
• Dynamic

140

• Java Integrates
• Power of Compiled Languages
• and
• Flexibility of Interpreted Languages

141
Classes and Objects

• Classes and Objects
• Method Overloading
• Method Overriding
• Abstract Classes
• Visibility modifiers
• default
• public
• protected
• private protected , private

142
Threads

• Java has built in thread support for
  Multithreading
• Synchronization
• Thread Scheduling
• Inter-Thread Communication
• currentThread start setPriority
• yield run getPriority
• sleep stop suspend
• resume
• Java Garbage Collector is a low-priority thread

143
Ways of Multithreading in Java

• Create a class that extends the Thread class
• Create a class that implements the Runnable
  interface
• 1st Method Extending the Thread class
• class MyThread extends Thread
• public void run()
• // thread body of execution
• Creating thread
• MyThread thr1 new MyThread()
• Start Execution
• thr1.start()

144
2nd method Threads by implementing Runnable
interface

• class ClassName implements Runnable
• .....
• public void run()
• // thread body of execution
• Creating Object
• ClassName myObject new ClassName()
• Creating Thread Object
• Thread thr1 new Thread( myObject )
• Start Execution
• thr1.start()

145
Thread Class Members...

• public class java.lang.Thread extends
  java.lang.Object
• implements java.lang.Runnable
• // Fields
• public final static int MAX_PRIORITY
• public final static int MIN_PRIORITY
• public final static int NORM_PRIORITY
• // Constructors
• public Thread()
• public Thread(Runnable target)
• public Thread(Runnable target, String name)
• public Thread(String name)
• public Thread(ThreadGroup group, Runnable
  target)
• public Thread(ThreadGroup group, Runnable
  target, String name)
• public Thread(ThreadGroup group, String name)
• // Methods
• public static int activeCount()
• public void checkAccess()
• public int countStackFrames()

146
...Thread Class Members.

• public final int getPriority() // 1 to 10
  priority-pre-emption at mid.
• public final ThreadGroup getThreadGroup()
• public void interrupt()
• public static boolean interrupted()
• public final boolean isAlive()
• public final boolean isDaemon()
• public boolean isInterrupted()
• public final void join()
• public final void join(long millis)
• public final void join(long millis, int nanos)
• public final void resume()
• public void run()
• public final void setDaemon(boolean on)
• public final void setName(String name)
• public final void setPriority(int newPriority)
• public static void sleep(long millis)
• public static void sleep(long millis, int
  nanos)
• public void start()
• public final void stop()

147
Manipulation of Current Thread

• // CurrentThreadDemo.java
• class CurrentThreadDemo
• public static void main(String arg)
• Thread ct Thread.currentThread()
• ct.setName( "My Thread" )
• System.out.println("Current Thread "ct)
• try
• for(int i5 igt0 i--)
• System.out.println(" " i)
• Thread.sleep(1000)
• catch(InterruptedException e)
• System.out.println("Interrupted.")
• Run
• Current Thread ThreadMy Thread,5,main
• 5

148
Creating new Thread...

• // ThreadDemo.java
• class ThreadDemo implements Runnable
• ThreadDemo()
• Thread ct Thread.currentThread()
• System.out.println("Current Thread "ct)
• Thread t new Thread(this,"Demo Thread")
• t.start()
• try
• Thread.sleep(3000)
• catch(InterruptedException e)
• System.out.println("Interrupted.")
• System.out.println("Exiting main thread.")

149
...Creating new Thread.

• public void run()
• try
• for(int i5 igt0 i--)
• System.out.println(" " i)
• Thread.sleep(1000)
• catch(InterruptedException e)
• System.out.println("Child
  interrupted.")
• System.out.println("Exiting child
  thread.")
• public static void main(String args)
• new ThreadDemo()
• Run
• Current Thread Threadmain,5,main
• 5
• 4

150
Thread Priority...

• // HiLoPri.java
• class Clicker implements Runnable
• int click 0
• private Thread t
• private boolean running true
• public Clicker(int p)
• t new Thread(this)
• t.setPriority(p)
• public void run()
• while(running)
• click
• public void start()
• t.start()

151
...Thread Priority

• class HiLoPri
• public static void main(String args)
• Thread.currentThread().setPriority(Thread.MA
  X_PRIORITY)
• Clicker Hi new Clicker(Thread.NORM_PRIORIT
  Y2)
• Clicker Lo new Clicker(Thread.NORM_PRIORIT
  Y-2)
• Lo.start()
• Hi.start()
• try
• Thread.sleep(10000)
• catch (Exception e)
• Lo.stop()
• Hi.stop()
• System.out.println(Lo.click " vs. "
  Hi.click)

152
The Java monitor model
Method 1
Method 2
Key
Block 1
Threads
Monitor (synchronised) solves race-condition
problem
153
Threads Synchronisation...

• // Synch.java race-condition without
  synchronisation
• class Callme
• // Check synchronized and unsynchronized
  methods
• / synchronized / void call(String msg)
• System.out.print(""msg)
• try
• Thread.sleep(1000)
• catch(Exception e)
• System.out.println("")
• class Caller implements Runnable
• String msg
• Callme Target
• public Caller(Callme t, String s)

154
...Threads Synchronisation.

• public void run()
• Target.call(msg)
• class Synch
• public static void main(String args)
• Callme Target new Callme()
• new Caller(Target, "Hello")
• new Caller(Target, "Synchronized")
• new Caller(Target, "World")
• Run 1 With unsynchronized call method (race
  condition)
• HelloSynchronizedWorld
• Run 2 With synchronized call method
• Hello
• Synchronized

155
Queue (no inter-threaded communication)...

• // pc.java produce and consumer
• class Queue
• int n
• synchronized int get()
• System.out.println("Got "n)
• return n
• synchronized void put(int n)
• this.n n
• System.out.println("Put "n)
• class Producer implements Runnable
• Queue Q
• Producer(Queue q)

156
Queue (no inter-threaded communication)...

• public void run()
• int i 0
• while(true)
• Q.put(i)
• class Consumer implements Runnable
• Queue Q
• Consumer(Queue q)
• Q q
• new Thread( this, "Consumer").start()
• public void run()
• while(true)

157
...Queue (no inter-threaded communication).

• class PC
• public static void main(String args)
• Queue Q new Queue()
• new Producer(Q)
• new Consumer(Q)
• Run
• Put 1
• Got 1
• Got 1
• Got 1
• Put 2
• Put 3
• Got 3
• C

158
Queue (interthread communication)...

• // PCnew.java produce-consumenr with interthread
  communication
• class Queue
• int n
• boolean ValueSet false
• synchronized int get()
• try
• if(!ValueSet)
• wait()
• catch(InterruptedException e)
• System.out.println("Got "n)
• ValueSet false
• notify()
• return n

159
Queue (interthread communication)...

• synchronized void put(int n)
• try
• if(ValueSet)
• wait()
• catch(InterruptedException e)
• this.n n
• System.out.println("Put "n)
• ValueSet true
• notify()
• class Producer implements Runnable
• Queue Q
• Producer(Queue q)

160
Queue (interthread communication)...

• public void run()
• int i 0
• while(true)
• Q.put(i)
• class Consumer implements Runnable
• Queue Q
• Consumer(Queue q)
• Q q
• new Thread( this, "Consumer").start()
• public void run()
• while(true)
• Q.get()

161
...Queue (no interthread communication).

• class PCnew
• public static void main(String args)
• Queue Q new Queue()
• new Producer(Q)
• new Consumer(Q)
• Run
• Put 0
• Got 0
• Put 1
• Got 1
• Put 2
• Got 2
• Put 3
• Got 3
• Put 4

162
Deadlock...

• // DeadLock.java
• class A
• synchronized void foo(B b)
• String name Thread.currentThread().getName
  ()
• System.out.println(name " entered
  A.foo")
• try
• Thread.sleep(1000)
• catch(Exception e)
• System.out.println(name " trying to call
  B.last()")
• b.last()
• synchronized void last()

163
Deadlock...

• class B
• synchronized void bar(A a)
• String name Thread.currentThread().getName
  ()
• System.out.println(name " entered
  B.bar")
• try
• Thread.sleep(1000)
• catch(Exception e)
• System.out.println(name " trying to call
  A.last