ThreadsThreadsThreadsThreads

operatingsystems

ThreadsThreadsThreadsThreadsA Thread, or thread of execution, is the sequenceof instructions being executed.

A process may have one or more threads.

Each thread has its own * program counter * set of registers * execution stack

But, all threads within a process share the same address space the same files, the same signals, and the same child processes!

operatingsystems

code data files

registers stack

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single thread process

code data files

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Why Threads?Why Threads?Why Threads?Why Threads?

It allows an application to do multiple things at once * Real parallelism with multiple cpu’s * overlap I/O, user interaction with processing * share the same address space

Threads are cheaper than processes because they haveno resources associated with them. Context switchesare cheaper.

operatingsystems

Benefits of using threads

Responsiveness: One thread can run while another is blocked *

Resource sharing

Economy: On a Solaris (SUN) O/S Thread creation is 30 times faster than process creation

Context switching for a thread is 5 times faster than process context switching

Exploitation of multi-processor architectures

* Assuming that threads are implemented in the kernel

operatingsystems

Implementing ThreadsImplementing ThreadsImplementing ThreadsImplementing Threads

There are two approaches to implementing threads,and the differences are quite controversial.

Support for threads can be provided either at theuser level or by the kernel.

Virtually all contemporary operating systems supportkernel threads.

operatingsystems

Implementing ThreadsImplementing ThreadsImplementing ThreadsImplementing ThreadsImplementing Threads in User Space

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A run-time systemprovides the interface to manage threads

Each process has it’sown thread table

No trap to kernel spaceis needed, no contextswitch is needed

operatingsystems

-- many to One Model --

Implementing ThreadsImplementing ThreadsImplementing ThreadsImplementing ThreadsImplementing Threads in User Space

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operatingsystems

Problems:

1. A blocking system call issued in one thread will block all threads in the process.

2. If a page fault occurs in one thread, all threads are blocked while the system pages.

3. Once a thread starts running there is no way to interrupt it. It must give up the cpu voluntarily.

Implementing ThreadsImplementing ThreadsImplementing ThreadsImplementing ThreadsImplementing Threads in the Kernel

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No run time system. Usekernel calls to manage threads

Thread table is kept in thekernel

Kernel schedules threads,no blocking issues

But … system calls are much more expensive thanusing a run-time system.

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operatingsystems

-- one to one model --

Implementing ThreadsImplementing ThreadsImplementing ThreadsImplementing ThreadsThere is a hybrid approach to thread support used by Linux.

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The kernel has a small setof re-usable threads that itmanages.

Threads in user space aremapped onto one of the available kernel threads.

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operatingsystems

Thread Libraries

Thread libraries provide the programming interfacefor creating and managing threads.

Thread libraries may be implemented completely inuser space, or in kernel space.

Thread Libraries

There are three main thread libraries in use today

* POSIX pthreads * win32 threads * Java threads

Thread Issues

When a thread issues a fork( ) system call, does the newprocess contain just one thread, or all of the threads ofthe original process?

Thread Issues

When a signal occurs, should the signal

* be delivered to all threads in the process? * deliver the signal only to the thread to which it applies? * deliver the signal to certain threads in the process? * Assign a specific thread to handle all signals for the process?