Implementing Lightweight Threads - D.Stein, D.Shah

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Questions and Answers

What impact does the rate at which threads are reaped have on the speed of thread creation?

Halts thread creation

No impact

Speeds up thread creation (correct)

Slows down thread creation

How does the reaper running at high priority affect when it should be run?

Should be run rarely (correct)

Should be run frequently

Should be run at medium priority

Should be run at a fixed interval

What happens to a blocking unbound thread when a synchronization primitive is called?

Thread continues executing

Thread gets a priority boost

Thread is forcefully terminated

Thread gets put to sleep (correct)

How are synchronization primitives handled in relation to asynchronous signals?

They are safe with respect to signals Signup and view all the answers

What action does the scheduler take when an unbound thread is put to sleep on a synchronization variable's sleep queue?

Dispatches another unbound thread Signup and view all the answers

What is the purpose of putting the freed stacks on a cache of available thread stacks by the reaper?

Speeds up stack allocation for new threads Signup and view all the answers

What is the main focus of the described threads library implementation?

Enabling fully concurrent access to system resources Signup and view all the answers

How are lightweight threads described in the text?

Threads that can be created quickly Signup and view all the answers

What allows the process to make progress without using excessive kernel resources?

Pool of kernel-supported threads Signup and view all the answers

What does the library manage to ensure progress of the process?

Multiplexing user threads on kernel-supported threads Signup and view all the answers

How are user threads described in relation to kernel-supported threads?

User threads multiplex on the pool of kernel-supported threads Signup and view all the answers

What is a key feature of the described threads library in terms of thread synchronization?

Rapid synchronization capabilities Signup and view all the answers

What happens if one thread in a process calls exit()?

All threads are destroyed Signup and view all the answers

How are synchronously generated signals handled in threads?

Sent to the thread that caused them Signup and view all the answers

What happens if all threads within a process mask a signal?

The signal is set pending on the process until unmasked by a thread Signup and view all the answers

In a process with multiple threads, what happens if a signal handler is set to SIG_IGN?

Any received signals are discarded Signup and view all the answers

How does forking differ between cloning the entire process and reproducing only the calling thread?

One method includes all threads while the other only copies the main thread Signup and view all the answers

What is a key characteristic of the signal handling in multithreaded processes?

All threads share the set of signal handlers Signup and view all the answers

Study Notes

Thread Management

When one thread calls exit(), all threads are destroyed.
Each thread has its own signal mask, allowing it to block asynchronously generated signals while accessing shared state.

Signal Handling

Synchronously generated signals (e.g., SIGSEGV) are sent to the thread that caused them.
Externally generated signals are sent to one of the threads within a process that has it unmasked.
If all threads mask a signal, it is set pending on the process until a thread unmasks it.
Signals can be sent to particular threads within the same process.

Signal Handlers

All threads within a process share the same set of signal handlers.
If the handler is set to SIG_IGN, any received signals are discarded.
If the handler is set to SIG_DFL, the default action (e.g., stop, continue, exit) applies to the process as a whole.

Process Forking

A process can fork in one of two ways: cloning the entire process and all its threads, or reproducing only the calling thread in the new process.
The second way is useful when the process is simply going to call exec().

Threads Library Architecture

The threads library provides extremely lightweight threads within a single UNIX process.
Threads are multiplexed on a pool of kernel-supported threads of control, managed by the library.
The library ensures that the process will make progress while not using an excessive amount of kernel resources.

Thread Synchronization

The threads library implements two basic types of synchronization variables: process-local (the default) and process-shared.
The library ensures that synchronization primitives are safe with respect to asynchronous signals and can be called from signal handlers.

Process-Local Synchronization Variables

The default blocking behavior is to put the thread to sleep.
Each synchronization variable has a sleep queue associated with it.
Synchronization primitives put blocking threads on the sleep queue and surrender the executing thread to the scheduler.
If a thread is unbound, the scheduler dispatches another thread to the thread's underlying LWP.

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Description

Learn about the implementation of a lightweight threads library within a single UNIX process, allowing fully concurrent access to system resources. Explore how lightweight threads can be quickly created, with thousands present, and synchronization achieved rapidly.

Implementing Lightweight Threads - D.Stein, D.Shah - SunSoft, Inc.

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