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CHAPTER
4
Multithreading
This chapter describes the locking primitives and thread synchronization
mechanisms of the SunOS multithreaded kernel.
Threads
A thread of control, or thread, is a sequence of instructions executed within a program.
A thread can share data and code with other threads and can run concurrently with
other threads. There are two kinds of threads: user threads and kernel threads. See
Multithreaded Programming Guide for more information on threads.
User Threads
Each process in the SunOS operating system has an address space that contains one
or more lightweight processes (LWPs), each of which in turn runs one or more user
threads. Figure 4–1 shows the relationship between threads, LWPs, and processes. An
LWP schedules its user threads and runs one user thread at a time, though multiple
LWPs may run concurrently. User threads are handled in user space.
The LWP is the interface between user threads and the kernel. The LWP can be
thought of as a virtual CPU that schedules user thread execution. When a user
thread issues a system call, the LWP running the thread calls into the kernel and
remains bound to the thread at least until the system call is complete. When an LWP
is running in the kernel, executing a system call on behalf of a user thread, it runs
one kernel thread. Each LWP is therefore associated with exactly one kernel thread.
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