Thursday, July 16, 2009
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1:47 AM
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Scheduling Queues
*Job queue – set of all processes in the system.
*Ready queue – set of all processes residing in main memory, ready and waiting to execute.
*Device queues – set of processes waiting for an I/O device.
*Processes migrate between the various queues.
*Short-term scheduler (or CPU scheduler) – selects whichprocess should be executed next and allocates CPU.
* Short-term scheduler is invoked very frequently (milliseconds) fi (must be fast).
* Long-term scheduler is invoked very infrequently (seconds, minutes) fi (may be slow).
* The long-term scheduler controls the degree of multiprogramming.
* Processes can be described as either:
* I/O-bound process – spends more time doing I/O than computations, many short CPU bursts.
* CPU-bound process – spends more time doing computations; few very long CPU bursts
Context Switch
* When CPU switches to another process, the ssave the state of the old process and load the for the new process.
* Context-switch time is overhead; the system douseful work while switching.
* Time dependent on hardware support.
*Job queue – set of all processes in the system.
*Ready queue – set of all processes residing in main memory, ready and waiting to execute.
*Device queues – set of processes waiting for an I/O device.
*Processes migrate between the various queues.
Schedulers
*Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue.*Short-term scheduler (or CPU scheduler) – selects whichprocess should be executed next and allocates CPU.
* Short-term scheduler is invoked very frequently (milliseconds) fi (must be fast).
* Long-term scheduler is invoked very infrequently (seconds, minutes) fi (may be slow).
* The long-term scheduler controls the degree of multiprogramming.
* Processes can be described as either:
* I/O-bound process – spends more time doing I/O than computations, many short CPU bursts.
* CPU-bound process – spends more time doing computations; few very long CPU bursts
Context Switch
* When CPU switches to another process, the ssave the state of the old process and load the for the new process.
* Context-switch time is overhead; the system douseful work while switching.
* Time dependent on hardware support.
Posted by
Roger
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os 4
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