Questions and Answers
What impact does the rate at which threads are reaped have on the speed of thread creation?
Speeds up thread creation
How does the reaper running at high priority affect when it should be run?
Should be run rarely
What happens to a blocking unbound thread when a synchronization primitive is called?
Thread gets put to sleep
How are synchronization primitives handled in relation to asynchronous signals?
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What action does the scheduler take when an unbound thread is put to sleep on a synchronization variable's sleep queue?
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What is the purpose of putting the freed stacks on a cache of available thread stacks by the reaper?
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What is the main focus of the described threads library implementation?
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How are lightweight threads described in the text?
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What allows the process to make progress without using excessive kernel resources?
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What does the library manage to ensure progress of the process?
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How are user threads described in relation to kernel-supported threads?
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What is a key feature of the described threads library in terms of thread synchronization?
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What happens if one thread in a process calls exit()?
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How are synchronously generated signals handled in threads?
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What happens if all threads within a process mask a signal?
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In a process with multiple threads, what happens if a signal handler is set to SIG_IGN?
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How does forking differ between cloning the entire process and reproducing only the calling thread?
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What is a key characteristic of the signal handling in multithreaded processes?
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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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