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> The THREADS parameter in Config.sys
> 
> The task management of OS/2 is divided in to three seperate entities:
> 
> *  Threads
> 
> *  Processes
> 
> *  Sessions
> 
> A thread is the only unit to get its own time slice of the CPU. All threads belonging to
> a process are contained within the process, and each thread has its own stack and 
> registers. Each thread consumes with its stack space 4096 (4k) bytes of the resident
> memory. 
> 
> There is a systemwide limit of 4096 threads(OS/2 4.x); however, Config.sys contrains a
> THREADS parameter that is usually set to a significantly smaller number - 64 is the
> default(Source: OS/2 Warp Version 4 Certification Handbook, Oct. 1996, SG24-4869-00,
> p. 1061). The base operating system uses approximatlely 40 threads.
> 
> Typically, a thread should have one distinct function; for example, file I/O, asynchronous
> communication, or heavy number crunching. Each thread has a thread identifier - a TID.
> Each thread has also a priority(later lession shows more about this priority). The higher
> the priority, the more time slices are given to the thread. A thread is much quicker to
> create than a process or sessions and has less system overhead.
> 
> All threads within a process run in the same virtual address space; therefore, global
> resources, such as file handles, and global variables are accessible from all threads
> in the process. Threads can be created through an OS/2 API like DosCreateThread, with
> the first thread created automatically by the operating sytem. When a thread is created
> it is assigned the same priority class as the thread that created it(i.e. a thread can create
> another thread).
> 
> A process is a collection of threads and resources that are owned by those threads. 
> Each process occupies a unique address space in memory that cannot be addressed by
> other processes in the system. Two processes can access the same area in memory only
> by using 'shared memory'. A process also contains file handles, semaphores, and other
> resources. All processes contain at least one thread, the 'main thread'. 
> 
> A process also contains a unique identifier - a PID(Process IDentifier). A process contains
> its own set of memory pages that can be swapped in and out (called 'paging') as the
> kernel switches from one process to the other. A process can create other processes;
> however, this must be the same session type. For instance, a full-screen process can
> only create other full-screen processes. 
> 
> The 5 types of processes are:
> 
> 1. OS/2 Full screen
> 2. OS/2 windowed
> 3. DOS Full screen
> 4. DOS windowed
> 5. Presentation Manager(PM)
> 
> A session is similar to a process except a session also contains the ownership of the
> mouse, keyboard, and video(screen). A session can contain either one process or multiple processes.
> 
> The task list(accesed by CRT-ESC) contains a list of all running sessions. When a 
> process or a session  creates a new session using the OS/2 API DosStartSession, the
> keyboard, screen, and mouse are responsive only to the session in the foreground.
> The session chosen as the background can gain control of the three resources only by
> switching to the foreground.
> 
> 
> Boosts from the Scheduler
> **************************
> 
> The OS/2 Scheduler runs on a round-robin type of disbursement of CPU time. The 
> Scheduler works  only with threads, not with processes or sessions. Threads have
> 4 different priority levels:
> 
> 1. time-critical
> 2. server class or fixed high
> 3. regular
> 4. idle-time
> 
> The first threads to run are the time-critical threads. All time-critical threads will run
> until there are no more time-critical threads waitung to be run. 
> After all time-critical threads are finished, the server-class threads are to run. After
> server-class, the regular class of threads are run. 
> After the regular class of threads are run, idle-time therads are run. 
> 
> Within each class of priorities are 32 sublevels. 
> 
> A thread that is not running is called a 'blocked' thread(more in lession "Process/Thread
> Model").
> 
> The OS/2 Scheduler works a lot around with thread priorities. Threads are given "boosts"
> by the scheduler to make OS/2 multitasking smarter. Three types of artifical priority
> boosts are given to threads:
> 
> 1. Foreground boosts
> 2. I/O boosts
> 3. Starvation boosts
> 
> The foreground boost is given to the user interface thread of the process that is in
> foreground. This is usually the main thread. The foreground process is the process with
> which the user is currently interacting. This makes the system respond quickly when the
> user clicks a mouse button or types in characters at a keyboard. This boost is a full 
> boost in priority. Also, a Presentation Manager thread has a boost applied to it while
> it is processing a message.
> 
> An I/O boost is given after an I/O operation is completed. An I/O boost does not change
> a thread's priority but will bump it up to level 31(the highest level of 32 stages) within
> its own priority class.
> 
> A starvation boost is given to a thread in the regular class that has not been able to
> run. The MAXWAIT parameter in Config.sys is used to define how long, in seconds, a
> thread must not(!) run before it is given a starvation boost. This duration for waiting is
> also called the "ready-to-run state". The default value is 3 seconds.
> 
> The time slices for threads that are given a starvation boost or an I/O boost are different
> from a normal time slice. Because of the tinkering the scheduler does with their priorities,
> they do not get run as long as a nonadjusted thread would run. 
> 
> The length of time for the 'short' and normal time slices is controlled by the TIMESLICE
> parameter in Config.sys. The first value repressents the 'short' time slice lenght; the
> default amount of time is set to 32 ms(milliseconds). The second value represents the
> normal slice lenght; the default amount of time is set to 65 536 ms.  
>  
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