Comparing POSIX Multithreading to z/OS Multitasking

最新推荐文章于 2018-09-07 17:35:25 发布

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#multithreading #subroutine #variables #constants #thread #allocation

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本文深入探讨了z/OS操作系统中任务的概念与POSIX线程之间的关键区别，解释了堆、静态区域、进程和线程的概念，并详细比较了两者在资源管理和执行方式上的差异。

Although the z/OS operating system has the ability to dispatch multiple threads of execution, it is different from the POSIX definition. To compare the two, you must first understand the meaning of some key C and POSIX constructs.

Heap

The area of memory from which dynamic storage is allocated using the ANSI memory allocation routines like malloc(), calloc(), free(), and so on.

Static Area

The area of memory where global variables (variables declared outside the scope of the main() program or outside the scope of any subroutines) and string constants (string values that appear within double quotation marks within a program) are stored. Variables that are declared static within the scope of a subroutine are also stored in this area.

Stack

The area of memory used for parameter passing and for storing automatic variables (variables defined within the scope of main() program or any subroutine).

Process

A single program that consists of one main() program and any number of subroutines. Each process has its own heap and static area.

Thread

A subroutine dispatched as a separate executable entity from the main() program (which itself is also a thread).

task

A TASK (in z/OS) is a unit of execution. POSIX threads, as used by DCE, are mapped to z/OS tasks (TCBs). These mappings occur in two flavors:

heavy weight
pthread creation is the same as task creation, and pthread deletion is the same as task deletion (TCB DETACH). z/OS resource managers are run.
medium weight
pthread creation is an association of a thread with an existing z/OS TCB on a one-for-one basis. pthreaddeletion ends this association; the z/OS task is not DETACHed and z/OS resource managers are not run.

DCE uses medium weight threads.

The key distinction between z/OS tasking and POSIX threading is that each z/OS task is close to being a separate process, whereas each POSIX thread is actually a program subroutine. Refer to below for a comparison of threads to processes. In the following sections, the term thread refers to the POSIX execution entity, while the term task refers to the z/OS execution entity.