Overview
资源按进程分配，调度以线程为基本单位。
Benefits
- Responsiveness
- Resource Sharing
- Economy
- Utilization of MultiProcessor Architectures
Multithreading Models
There are two supporting levels for multithreading:
User Threads
Thread management done by user-level threads library.
Three primary thread libraries:
- POSIX Pthreads
- Win32 threads
- Java threads
Kernel Threads
Supported by the Kernel
Examples
- Windows 95/98/NT/2000/XP
- Solaris
- Tru64 UNIX
- BeOS
- Linux
Note: No One-to-Many Model exists
Many-to-One
Many user-level threads mapped to single kernel thread.
Used on systems that do not support kernel threads.
- Green Threads
- GNU Portable Threads
One-to-One
Each user-level thread maps to kernel thread.
- Windows 95/98/NT/2000/XP
- OS/2
Many-to-Many
Allows many user level threads to be mapped to many kernel threads.
Allows the operating system to create a sufficient number of kernel threads.
- Solaris 2
- Windows NT/2000 with the ThreadFiber package
One popular variation on the many-to-many model still multiplexes many user-level threads to a smaller or equal number of kernel threads but also allows a user-level thread to be bound to a kernel thread. This variation, sometimes referred to as the two-level model, is supported by operating systems such as IRIX, HP-UX, and Tru64 UNIX.
Thread Libraries
POSIX Pthreads
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)
Windows Threads
Implements the one-to-one mapping
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
Linux 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).
Java Threads
Java threads may be created by:
- Extending Thread class
- Implementing the Runnable interface
Java threads are managed by the JVM.