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ThreadsThreads

• What are they?

• Why are they important?

• How are they implemented in OSes?

• How to use threads? (in Java)

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What are Threads?What are Threads?

• Recap: What are processes?– A process is a container (or execution environment)

for a program in execution.

• Threads (threads of execution/control)– A finer-grained container (or execution env) for a

program in execution, than a process.• A traditional process has a single thread of execution.• A process can contain multiple concurrent threads (of

execution).

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Process ImageProcess Image• Created for each process

• Each process image consists of 4 elements– User program

• The program to be executed.

– User data• The mutable part of memory. Includes program data

and a user stack.– Used to keep track of procedure calls and parameter

passing between procedures.

– Kernel stack• Used to keep track of system calls and parameter

passing.

– Process control block (PCB)• Data needed by an OS to control a process (a set of

attributes of a process)

Process Control Block

User Program

User Data/Stack

Kernel Stack

Process Image

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• PCB: the most important data structure in an OS – Process identification

• PID

– CPU state information• The contents of CPU registers• Program counter

– Indicates the address of the next instruction to be executed.

– Process control information• Event that a process is waiting on. • Scheduling info

– Current scheduling priority (pri) – User mode priority (usrpri)– CPU utilization (cpu)– Nice value (user-controllable adjustment factor; nice)

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A Little More Detailed Look at A Little More Detailed Look at Process ImagesProcess Images

thread

Single-threaded (traditional) process

Open files, open network connectionsdevices

User data ->User dataUser stackOS resources

PCB U K Stacks

U Prog U data OS Res

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thread

Single-threaded (traditional) process Multi-threaded process

• The CPU is scheduled against threads.• Concurrency (pseudo-parallelism) within a process

– Context switches occur among threads.

TCB

U K Stacks

U Prog U data OS Res

thread

TCB

U K Stacks

thread

PCB

TCB

U K Stacks

thread

PCB U K Stacks

U Prog U data OS Res

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• Thread Control Block (TCB)– Thread ID– The contents of CPU registers– Program counter

• Indicates the address of the next instruction to be executed.

– Event that a thread is waiting on.

– Scheduling info

• Different threads can share the memory and the resources used by a process.– User program– User data– OS resources

TCB

U K Stacks

U Prog U data OS Res

thread

TCB

U K Stacks

thread

PCB

TCB

U K Stacks

thread

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Why Threads?Why Threads?• Finer-grained concurrency

– Old, good days: coarse-grained concurrency• Writing a document with an editor, while sending/fetching emails

– Now: finer-grained concurrency• A program is expected to do different things at the same time.

– Word» Displaying text and images» Responding to keystrokes/mouse input from a user» Retreating data within the text » Accessing the MS web site to look for fancy document templates» Performing spelling and grammar checking in the background

– Web browser» Displaying text and images» Responding to keystrokes/mouse input» Checking and downloading software updates

– iTunes» Playing music» Downloading music and its info (album’s cover, song titles, lyrics…)

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• A program is expected to do the same or similar things at the same time

– Web server» Accepts and parses an HTTP request» Finds a target file» Makes an HTTP message (header, payload, etc.)» Returns a target file with the HTTP message

• Why not use multiple processes? – It was in common use 10 to 15 years ago

• before threading technology is well developed and it became popular

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PCB U K Stacks

U Prog U data OS Res

thread

Single-threaded (traditional) process Multi-threaded process

• Process-creation is heavyweight. – Time-consuming and resource intensive.

• Creating a process is 30 times slower than creating a thread.

• Process switching is 5 times slower than thread switching.

TCB

U K Stacks

U Prog U data OS Res

thread

TCB

U K Stacks

thread

PCB

TCB

U K Stacks

thread

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In Summary: Why Threads?In Summary: Why Threads?• Responsiveness

– Threads allow a program to continue running even if a part of it is blocked for I/O or is performing a long operation.

• Resource sharing– Threads share the memory and the resources of a

process that they belong to.

• Efficiency– Allocating memory and resources for process

creation is costly.– Switching processes is heavyweight.

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How are Threads Implemented in OSes?How are Threads Implemented in OSes?

• Two major strategies to implement threads in OSes– User-level threads (UTs)– Kernel level threads (KTs)

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User-level Threads (UTs)User-level Threads (UTs)

• Thread mgt done at user-level programs

• The kernel is not aware of the existence of threads.

Thread libraryUser level

Kernel level

UTs

KT

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UTs: Pros and ConsUTs: Pros and Cons• Pros

– Threads do not need to change their modes between the user mode to kernel mode.

• No overhead of mode switches

– Thread scheduling can be application specific. • Apps can tailor their sched algorithms without disturbing the

underlying OS scheduler.

– The thread library can be very portable.• No changes required to the underlying kernel.

• Cons– When a thread calls a system call, the thread AND all the

other threads in the same process are blocked.

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Example ImplementationsExample Implementations

• Pthreads– POSIX thread package

• Ported on every Unix OSes

• Solaris– With the “green thread” package

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Kernel-level Threads (KTs)Kernel-level Threads (KTs)• No thread mgt at user-level• System calls (APIs) provided to the kernel thread mgt facility. • One-to-one

– Even if a thread calls a system call, the kernel schedules all other threads in the same process to a CPU.

– With multiple processors, the kernel can schedule different threads in the same process to different CPUs.

User level

Kernel level

UTs

KT KTs

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– The number of UTs is same as the number of KTs.

– Example implementations• Linux• Windows (Win32)• Solaris

• Hybrid of UT-only and KT-one-to-one– Solaris

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• Many-to-many– Thread multiplexing– The number of KTs is a smaller or equal number of

UTs.

– Examples• HP-UX• IRIX• Tru64 Unix

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Java ThreadsJava Threads• All Java programs comprise at least one thread of

control. – main() runs as a single thread in a JVM.

• A thread is implicitly created when a JVM starts.

• Need to explicitly create additional threads.

• 4 things to do:– Define a class implementing the Runnable interface

(java.lang)• public abstract void run();

– Write a threaded task in run() in the class– Instantiate a thread and assign a Runnable object to the

thread– Start (call start() on) the instantiated thread.

• run() is executed on the thread.

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Code 1Code 1

• HelloWorld.java

• GreetingRunnable.java

• Output:– Mon Mar 26 15:14:43 EDT 2007 Hello World– Mon Mar 26 15:14:44 EDT 2007 Hello World– Mon Mar 26 15:14:45 EDT 2007 Hello World– Mon Mar 26 15:14:46 EDT 2007 Hello World– Mon Mar 26 15:14:47 EDT 2007 Hello World– Mon Mar 26 15:14:48 EDT 2007 Hello World– Mon Mar 26 15:14:49 EDT 2007 Hello World– Mon Mar 26 15:14:50 EDT 2007 Hello World– Mon Mar 26 15:14:51 EDT 2007 Hello World– Mon Mar 26 15:14:52 EDT 2007 Hello World

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Thread.start()Thread.start()

• Creating a Thread object does not create a new thread; rather, it is start() that actually create it.

• Start()– Allocates memory and initialies a new thread in a

JVM.– Calls run() of a specified Runnable object.

• Do not call run() directly. start() calls run() on our behalf.

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Thread Sleep and InterruptionThread Sleep and Interruption

• A sleeping thread can wake up via interruptions from other threads.– Call interrupt() on a target thread.

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Code 2Code 2• HelloWorld2.java and GreetingRunnable.java

• Output:– Mon Mar 26 15:28:45 EDT 2007 Goodbye World – Mon Mar 26 15:28:45 EDT 2007 Hello World– Mon Mar 26 15:28:46 EDT 2007 Hello World– Mon Mar 26 15:28:46 EDT 2007 Goodbye World– Mon Mar 26 15:28:47 EDT 2007 Hello World– Mon Mar 26 15:28:47 EDT 2007 Goodbye World– Mon Mar 26 15:28:48 EDT 2007 Goodbye World– Mon Mar 26 15:28:48 EDT 2007 Hello World– Mon Mar 26 15:28:49 EDT 2007 Goodbye World– Mon Mar 26 15:28:49 EDT 2007 Hello World– Mon Mar 26 15:28:50 EDT 2007 Goodbye World– Mon Mar 26 15:28:50 EDT 2007 Hello World– Mon Mar 26 15:28:51 EDT 2007 Goodbye World– Mon Mar 26 15:28:51 EDT 2007 Hello World– Mon Mar 26 15:28:52 EDT 2007 Hello World– Mon Mar 26 15:28:52 EDT 2007 Goodbye World– Mon Mar 26 15:28:53 EDT 2007 Hello World

• Two message sets (Hello and Goodbye) are not exactly interleaved.

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The Order of Thread ExecutionThe Order of Thread Execution

• The JVM thread scheduler gives NO guarantee about the order in which threads are executed.

• There are always slight variations in thread execution times,– especially when calling OS system calls (typically

I/O related system calls)

• Expect that the order of thread execution is somewhat random.