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Chapter 4: ThreadsChapter 4: Threads
4.2 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Chapter 4: ThreadsChapter 4: Threads
Overview
Multithreading Models
Threading Issues
Pthreads
Windows XP Threads
Linux Threads
Java Threads
4.3 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
3
Thread: IntroductionThread: Introduction
Each process has
1. Own Address Space
2. Single thread of control
A process model has two concepts:
1. Resource grouping
2. Execution
Sometimes it is useful to separate them
4.4 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
4
Unit of Resource OwnershipUnit of Resource Ownership
A process has an
Address space
Open files
Child processes
Accounting information
Signal handlers
Etc
If these are put together in a form of a process, can be managed more easily
4.5 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
5
Unit of DispatchingUnit of Dispatching Path of execution
Program counter: which instruction is running
Registers: holds current working variables
Stack: Contains the execution history, with one entry for
each procedure called but not yet returned
State
Processes are used to group resources together
Threads are the entities scheduled for execution on the CPU
Threads are also called lightweight process
4.6 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
6
Its better to distinguish between the two conceptsIts better to distinguish between the two concepts
Address space/Global VariablesOpen filesChild processesAccounting infoSignal handlersProgram counterRegistersStackState
Address space/Global VariablesOpen filesChild processesAccounting infoSignal handlers
Program counterRegistersStackState
Program counterRegistersStackState
Split
Program counterRegistersStackState
Unit of Resource
Unit of Dispatch
In case of multiple threads per process
Share
Light weight processes
Hea
vy w
eigh
t pr
oces
s
4.7 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Threads allow you to multiplex which Threads allow you to multiplex which resources?resources?
CPU
Mem
ory
PCBs
Open fi
les
User a
uthen
ticat
ion s
...
149%
81%
38%
62%60%
1. CPU
2. Memory
3. PCBs
4. Open files
5. User authentication structures
4.8 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
8
ThreadThread A thread is a basic unit of CPU utilization. It consist of
A thread ID
A program Counter
A register Set
A stack
Threads share something with its peer threads (all the other5 threads in this particular task) the things that it share are
Its code section
Its data section
Any OS resources, available for the task.
The first thread starts execution with
int main(int argc, char *argv[])
The threads appear to the Scheduling part of an OS just like any other process
Allow multiple execution paths in the same process environment
4.9 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Threads vs. ProcessesThreads vs. Processes
Threads A thread has no data segment or
heap A thread cannot live on its own, it
must live within a process There can be more than one thread
in a process, the first thread calls main & has the process’s stack
Inexpensive creation Inexpensive context switching If a thread dies, its stack is
reclaimed Inter-thread communication via
memory.
ProcessesA process has code/data/heap & other segments
There must be at least one thread in a process
Threads within a process share code/data/heap, share I/O, but each has its own stack & registers
Expensive creation
Expensive context switching
If a process dies, its resources are reclaimed & all threads die
Inter-process communication via OS and data copying.
4.10 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
10
Process Vs. ThreadsProcess Vs. Threads
(a) Three threads, each running in a separate address space
(b) Three threads, sharing the same address space
4.11 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Context switch time for which entity is Context switch time for which entity is greater?greater?
Proce
ss
Thread
21%
79%1. Process
2. Thread
4.12 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
12
The Thread ModelThe Thread Model
Each thread has its own stack
4.13 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Single and Multithreaded ProcessesSingle and Multithreaded Processes
A traditional heavy weight process is same as task
with one thread. It has a single thread of control.
If a process is multi thread, then that means more than
one part of the thread is executing at one time.
Multi threading can be useful in programs such as web
browsers where you can wish to download a file , view an
animation and print something at the same time.
4.14 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Single and Multithreaded ProcessesSingle and Multithreaded Processes
4.15 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Implementing ThreadsImplementing Threads
Processes define an address space; threads share the address space
Process Control Block (PCB) contains process-specific information
Owner, PID, heap pointer, priority, active thread, and pointers to thread information
Thread Control Block (TCB) contains thread-specific information
Stack pointer, PC, thread state (running, …), register values, a pointer to PCB, …
Code
Initialized data
Heap
DLL’s
mapped segments
Process’s address space
Stack – thread1
PCSP
StateRegisters
…
PCSP
StateRegisters
…
TCB for Thread1
Stack – thread2
PCSP
StateRegisters
…
PCSP
StateRegisters
…
TCB for Thread2
4.16 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Threads’ Life CycleThreads’ Life Cycle
Threads (just like processes) go through a sequence of start, ready, running, waiting, and done states
RunningRunningReadyReady
WaitingWaiting
StartStart DoneDone
4.17 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Benefits Of Multi-ThreadingBenefits Of Multi-Threading
Responsiveness
Multithreading increase the responsiveness .As the process
consist of more than one thread, if one thread block or busy in
lengthy calculation, some other thread still executing of the
process. So the user get response from executing process.
Resource Sharing
All threads, which belongs to one process, share the memory and
resources of that process. Secondly it allow the application to have
several different threads within the same address space.
4.18 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Benefits Of Multi-ThreadingBenefits Of Multi-Threading
Economy
Allocation of memory and resources or process creation is costly.
All threads of a process share the resources of that process so it is
more economical to create and context switch the thread.
Utilization of Multi processor Architectures
MP architecture allows the facility of parallel processing, which is
most efficient way of processing. A single process can run on one
CPU even if we have more processors. Multi-threading on MP
system increase the concurrency. If a process is dividing into
multiple threads , these threads can execute simultaneously on
different processors.
4.19 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Types of ThreadsTypes of Threads
There are two types of threads
Kernel Threads
User Threads
4.20 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
User ThreadsUser Threads
User level threads are not seen by operating system and also very fast
(switching from one thread to another thread in a single process does
not require context switch since same process is still executing).
However, if the thread that is currently executing blocks, the rest of the
process may also blocked( if OS is using only one single kernel thread
for this process i.e. the thread that kernel sees is same as blocked
thread , hence kernel assume that whole process is blocked).
Thread management done by user-level threads library
Three primary thread libraries:
POSIX Pthreads
Win32 threads
Java threads
4.21 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Kernel ThreadsKernel Threads
Kernel Supported threads are seen by operating system and must be scheduled by the operating system. One multi thread may have multiple kernel threads.
Examples
Windows XP/2000
Solaris
Linux
Tru64 UNIX
Mac OS X
4.22 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
User-Level vs. Kernel ThreadsUser-Level vs. Kernel Threads
User-Level Managed by application
Kernel not aware of thread
Context switching cheap
Create as many as needed
Must be used with care
Kernel-Level Managed by kernel
Consumes kernel resources
Context switching expensive
Number limited by kernel resources
Simpler to use
Key issue: kernel threads provide virtual processors to user-level threads, but if all of kthreads block, then all user-level threads will block even if the program logic allows them to proceed
4.23 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Thread LibrariesThread Libraries
Thread library provides programmer with API for creating and managing threads
Two primary ways of implementing Library entirely in user space with no kernel support. All the
code and data structure exists in user space. This means that invoking of a function in a the library results in a local function call in user space and not a system call.
Kernel-level library supported by the OS. In this case code and data structures for the library exits in kernel space. Invoking a function in the API of library typically results in a system call to a kernel.
4.24 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Thread Libraries Thread Libraries
Three main thread libraries are used in today POSIX Pthreads
Win32
Java
4.25 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Multithreading ModelsMultithreading Models
Many-to-One
One-to-One
Many-to-Many
4.26 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Many-to-OneMany-to-One
Many user-level threads mapped to single kernel thread.
It is efficient because it is implemented in user space. A process using
this model blocked entirely if a thread makes a blocking system call.
Only one thread can access the kernel at a time so it can not be run
in parallel on multiprocessor.
Examples:
Solaris Green Threads
GNU Portable Threads("Genuinely Not Unix" ; GNU is an
operating system composed of free software)
4.27 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Many-to-One ModelMany-to-One Model
4.28 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
One-to-OneOne-to-One
Each user-level thread maps to kernel thread.
It provides more concurrency because it allows another thread to
execute when threads invoke the blocking system call.
It facilitates the parallelism in multiprocessor systems.
Each user thread requires a kernel thread, which may affect the
performance of the system.
Creation of threads in this model is restricted to certain number.
Examples
Windows NT/XP/2000
Linux
Solaris 9 and later
4.29 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
One-to-one ModelOne-to-one Model
4.30 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Many-to-Many ModelMany-to-Many Model
Allows many user level threads to be mapped to many kernel threads
Allows the operating system to create a sufficient number of kernel
threads
Number of kernel threads may be specific to a either a particular
application or a particular machine.
The user can create any number of threads and corresponding kernel
level threads can run in parallel on multiprocessor.
When a thread makes a blocking system call, the kernel can execute
another thread.
Solaris prior to version 9
Windows NT/2000 with the ThreadFiber package
4.31 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Many-to-Many ModelMany-to-Many Model
4.32 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Multithreading Models: ComparisonMultithreading Models: Comparison
The many-to-one model allows the developer to create as many user threads as he/she wishes, but true concurrency can not be achieved because only one kernel thread can be scheduled for execution at a time
The one-to-one model allows more concurrence, but the developer has to be careful not to create too many threads within an application
The many-to-many model does not have these disadvantages and limitations: developers can create as many user threads as necessary, and the corresponding kernel threads can run in parallel on a multiprocessor
4.33 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Two-level ModelTwo-level Model
Similar to M:M, except that it allows a user thread to be bound to kernel thread
Examples
IRIX
HP-UX
Tru64 UNIX
Solaris 8 and earlier
4.34 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Two-level ModelTwo-level Model
LWP
user-levelthreads
LWP LWP LWP
• Combination of one-to-one + “strict” many-to-many models• Supports both bound and unbound threads
– Bound threads - permanently mapped to a single, dedicated LWP– Unbound threads - may move among LWPs in set
• Thread creation, scheduling, synchronization done in user space• Flexible approach, “best of both worlds”• Used in Solaris implementation of Pthreads and several other Unix
implementations (IRIX, HP-UX)
4.35 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Two-level ModelTwo-level Model
4.36 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
PthreadsPthreads
A POSIX standard (IEEE 1003.1c) API for thread creation and synchronization
API specifies behavior of the thread library, implementation is up to development of the library
Common in UNIX operating systems (Solaris, Linux, Mac OS X)
4.37 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Java ThreadsJava Threads
Java threads are managed by the JVM
Java threads may be created by:
Extending Thread class
Implementing the Runnable interface
4.38 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Java Thread States Java Thread States
4.39 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Threading IssuesThreading Issues
Semantics of fork() and exec() system calls
Thread cancellation
Signal handling
Thread pools
Thread specific data
Scheduler activations
4.40 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Semantics of fork() and exec()Semantics of fork() and exec()
As the fork() system call is used to create a separate, duplicate process.
The semantics of the fork() and exec() system calls change in a multithreaded program.
If one thread in a program calls the fork(), does the new process duplicate all the threads or is the new process single threaded?
4.41 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Semantics of fork() and exec()Semantics of fork() and exec()
Does fork() duplicate only the calling thread or all threads?
• Two versions of fork() in UNIX:, one that duplicates all threads and another that
duplicates only the thread that invoked the fork() system call.
• If a thread invokes the exec() system call, the program specified in the parameter
to exec() will replace the entire process – including all threads.
• If exec() is called immediately after forking, then duplicating all threads is
unnecessary, as the program specified in the parameters to exec() will replace
the process. In this instance, duplicating only the calling thread is appropriate.
• If the separate process does not call exec() after forking, the separate process
should duplicate all threads.
4.42 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Thread CancellationThread Cancellation
Thread cancellation is the task of terminating a thread before it has finished. For example, if multiple threads are concurrently searching through a database and one thread return the result, the remaining threads might be canceled.
A thread that is to be often canceled is referred as the target thread.
Two general approaches:
Asynchronous cancellation terminates the target thread immediately
Deferred cancellation allows the target thread to periodically check if it should be cancelled, allowing it an opportunity to terminate itself in an orderly fashion.
4.43 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Problem in Thread CancellationProblem in Thread Cancellation
The difficulty with cancellation occurs in a situations where resources have been allocated to a cancel thread
Where thread has been cancelled in the midst of updating data it share with other threads.
This becomes especially difficult with asynchronous cancellation. Often operating system will reclaim system resources from the canceled thread but will not reclaim all the resources. Therefore, canceling a thread asynchronously may not free a necessary system-wide resource.
4.44 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Signal HandlingSignal Handling
A signal is used in a UNIX system to notify a process that a particular event has occurred. Signal may be received either asynchronously or synchronously. All Signal follow the same pattern.
A signal handler is used to process signals
1. Signal is generated by particular event
2. Signal is delivered to a process
3. Signal is handled
4.45 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Signal HandlingSignal Handling
Examples of synchronous signals include illegal memory access
and division by 0. if a running program perform either of these
operations a signal is generated. Synchronous signals are
delivered to the same process that performed the operation that
caused the signals.
When a signal is generated by some event external to a running
process, that process receive the signal asynchronously.
Examples of such signals include terminating a process with
specific key strokes (such as <control><C>) and having a timer
expire. Typically , asynchronous signal is sent to another process.
4.46 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Signal Handling in UNIXSignal Handling in UNIX
Every signal may be handled by one of two possible handlers:
1. A default signal handler that is run by the kernel when handling that signal.
2. A user-defined signal handler that is called to handle a signal.
4.47 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Signal HandlingSignal Handling
Delivering the signal in multithreaded programs is more complicated, where a process may have several threads. Following Options exist: Deliver the signal to the thread to which the
signal applies Deliver the signal to every thread in the
process Deliver the signal to certain threads in the
process Assign a specific thread to receive all signals
for the process.
4.48 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Thread PoolsThread Pools
The general idea is to create a number of threads at process
startup and place them into a pool where they sit and wait for
work
Advantages:
Usually slightly faster to service a request with an existing
thread than create a new thread
Allows the number of threads in the application(s) to be
bound to the size of the pool
4.49 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Thread Specific DataThread Specific Data
Allows each thread to have its own copy of data
Useful when you do not have control over the thread creation process (i.e., when using a thread pool)
4.50 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Scheduler ActivationsScheduler Activations
A final issue to be considered with multithreaded programs concerns
with communication between kernel level and user level thread
library.
Both M:M and Two-level models require communication to maintain
the appropriate number of kernel threads allocated to the
application.
Many system implementing either M:M or Two level model place an
intermediate data structure between user and kernel threads. This
data structure is typically known as light weight process or (LWP).
4.51 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Scheduler ActivationsScheduler Activations
One scheme for communication between user thread library and kernel
thread library is known as Scheduler Activation. It works as follow:
The kernel provides an application with a set of virtual processor (LWPs), and the application can schedule user threads onto an available virtual processor.
The kernel must inform an application about certain events. This
procedure is known as upcalls - a communication mechanism from the
kernel to the thread library.
Upcalls are handled by thread library with an upcall handler.
Upcall handler must run on virtual processor.
This communication allows an application to maintain the correct number
of kernel threads.
4.52 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Operating System ExamplesOperating System Examples
Windows XP Threads
Linux Thread
4.53 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Windows XP ThreadsWindows XP Threads
Implements the one-to-one mapping, kernel-level
Each thread contains
A thread id
Register set
Separate user and kernel stacks
Private data storage area
The register set, stacks, and private storage area are known as the context of the threads
The primary data structures of a thread include:
ETHREAD (executive thread block)
KTHREAD (kernel thread block)
TEB (thread environment block)
4.54 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Windows XP ThreadsWindows XP Threads
4.55 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Linux ThreadsLinux Threads
Linux refers to them as tasks rather than threads
Thread creation is done through clone() system call
clone() allows a child task to share the address space of the parent task (process)
4.56 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Linux ThreadsLinux Threads
4.57 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
57
Thread UsageThread Usage Less time to create a new thread than a process
the newly created thread uses the current process address space
no resources attached to them
Less time to terminate a thread than a process. Less time to switch between two threads within the same
process, because the newly created thread uses the current process address space.
Less communication overheads threads share everything: address space, in particular. So, data
produced by one thread is immediately available to all the other threads
Performance gain Substantial Computing and Substantial Input/output
Useful on systems with multiple processors
End of Chapter 4End of Chapter 4