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22 Cards in this Set
- Front
- Back
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What is the dispatcher? |
A CPU module that gives control of the CPU to a process selected by the short-term scheduler. |
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What parameters should the scheduler consider? |
CTTWR 1) CPU Utilization 2) Throughput 3) Turnaround Time 4) Waiting Time 5) Response Time |
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What is the Convoy Effect? |
When a few slow processes are holding up lots of smaller faster processes. |
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What is a CPU burst? |
The block of time that a process needs to use the CPU without stopping for I/O, etc. |
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How does Shortest Job First work? |
CPU will do the waiting job that has lowest amount of CPU burst time |
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SJF is theoretically optimal. Why isn't it in practice? |
Because it's hard to know what the burst time will be |
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How does the CPU estimate burst time? |
Using exponential averaging. (Last burst time * alpha) + (Last guess * (1 - alpha)) |
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What is the problem with Priority Scheduling? |
Starvation
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What is the solution to the problem of Starvation with Priority Scheduling? |
Incrementing priority as the process ages |
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True or False: The Round Robin scheduler is a "preemptive algorithm". |
True, since it relies on kicking processes off the CPU when their time expires. |
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How does Round Robin scheduler work? |
A "quanta" is decided and each process is assigned that amount of time to use the CPU and gets kicked off and moved to the back of the line when its quanta runs out |
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What's the problem with small quanta in RR scheduling? |
Lots of time will be used on context switching relative to actual CPU work |
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What is a Multilevel Queue and what does it aim to do? |
A Multilevel queue maintains separate process queues with different scheduling strategies for different types of processes (for example, one for background processes one for foreground processes)
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What is a Multilevel Feedback Queue? |
It's a form of Multilevel Queue that allows processes to move between queues. For example, it starts in Q1, and if it doesn't finish in the allotted time, it moves to Q2, etc. |
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What is System Contention Scope? |
It's what happens kernel threads are competing for time on the CPU. Its counterpart is the Process Contention Scope which is when multiple user threads in the same user thread are competing for the user thread. |
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What are the two basic forms of Multi-CPU architecture? |
1) Asymmetric Multiprocessing 2) Symmetric Multiprocessing (each processor is self scheduling, all processes wait in common queue) |
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What is Processor Affinity? |
The idea that in multi-processor systems, a process should be run by the same CPU since it's cache memory could still be saved on that CPU which would save the time it would take to rewrite it |
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What is NUMA? |
Non-Uniform Memory Addressing. The idea is that CPU's can access different memory locations in different amounts of time. It accesses its own memory faster than it accesses memory around other CPUs |
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What is hyperthreading? |
When the OS "thinks" there are two cores, but really there aren't. |
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What is the difference between hard real-time systems and soft-real-time systems? |
Hard real-time systems are not allowed to miss a single processing deadline, soft can. |
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What is the Completely Fair Scheduler? |
A scheduler included in newer Linux versions. It uses a red-black to organize the tasks by estimated required run time. |
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What type of scheduling does Windows use? |
Preemptive priority-based |
