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8/13/2019 Priority PRESENT1

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PRIORITYSCHEDULING

NUR SYAHIRAH F2004

NUR AQILAH F2018

MUHAMMAD AFIQ F2019


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Priority Scheduling

The SJF algorithm is a special case of thegeneral priority scheduling algorithm

A priority number (integer) is associatedwith each process

The CPU is allocated to the process with

the highest priority (smallest integer =highest priority)

Priority scheduling can be eitherpreemptive or non-preemptive


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Priority Scheduling Each process is assigned a priority and the runnable process with the

highest priority is allowed to run

To prevent high-priority processes from running indefinitely, the

scheduler may decrease the priority of currently running process at

each clock tick.

I/O bound processes are given highest priority


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Pre-emptive priority

processes in highest priority queue alwaysget run first

those in lower priority queues alwayswait

starvationlikely

Problem: Starvation

low priority processes may never execute.

Solution: Aging

as time progresses increase the priority of the process.


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Non-Pre-Emptive Priority

higher priority processes still favored but not exclusively

every so often, take a process from a lower priority queue apriority ratio table(used in EMAS medium-term scheduling)

1 2 1 3 1 2 1 4 1 2 1 3 1 2 1

make priorities dynamic lower a processes priority after each time it has been run for a quantum used in Windows NT

process given an initial boost in priority, then gradual decay

favors short interactions

boost a processes priority if it has not had a go on the CPU lately

prioritypurchase ?

a user may wish topaymore to get better service whether funny money i.e. computing time allocations, or real money

Higher and lower priority bands kernel processes v. background job stream

real-time processes (NT)


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Set Priority

Two approaches

- Static (for system with well know and

regular application behaviors)

- Dynamic (otherwise)

Priority may be based on:

- Cost to user

- Importance of user

- Aging

- Percentage of CPU time used in last hours.


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When you set a 100 CPU program to real-timepriority, you get what you asked for

Real-time priority is really dangerous. It's higher priority than nearly everything else. It's higherpriority than mouse input, keyboard input, and the disk cache. If you foolishly set the priority classof a CPU-intensive program to real-time, it will suck up your entire processor, leaving no cycles foranything else.

In particular, since not even input runs at real-time priority, you can't stop it via any interactive

means, because the thread that manages input can't even run to process your input.

Mind you, even if the input thread did run at real-time priority, that wouldn't really help you any.Sure, it could receive your input and distribute it to the appropriate application queues, but thoseapplications are themselves not running with real-time priority, so all that happens is that yourinput gets quickly transferred to the input queues, where it then sits waiting for the applications toprocess them (which will never happen since the applications are not running with high enoughpriority).

One might argue that Task Manager should run with real-time priority, so it can extricate you fromthis situation, but that won't help, and it would be wrong. It won't help because you first need to beable to launch Task Manager (or switch to it, if you were prescient enough to have it alreadyrunning), and none of the ways of launching Task Manager run with real-time priority. (Becausenothing in the user interface runs with real-time priority.)


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Second, even if there were a special code path that enabled you to launchTask Manager at real-time priority, it would be wrong, because the factthat Task Manager is running with real-time priority means that it is nowstealing CPU cycles from that other process which set itself to real-timepriority, which defeats the purpose of that process setting itself to real-time priority in the first place: It set itself to real-time priority because itdidn't want anybody stealing CPU time from it!

What could be done is to have Task Manager display an extra warningdialog when somebody uses it to change a process's priority. The warningdialog would say something like "Fiddling with priority can result in youbeing totally screwed. Are you sure you want to take this risk?" (Oh wait,that dialog box already exists.) Our friend Igor probably clicked right onpast that warning dialog, because he's thinking, "Of course I want to set itto real-time priority, you stupid program, that's why I clicked Real-TimePriority! This is another in a long string of examples of how Windows is one

of those coddling operating systems that gets in the way of advanced userslike me, throwing up frustrating obstacles which prevent me from gettingthings done."

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