操作系统导论习题解答（8. Multi-level Feedback）

0. 文件地址

Homework

1. MLFQ: Basic Rules

2. Attempt #1: How To Change Priority

2.1 Example 1: A Single Long-Running Job

2.2 Example 2: Along Came A Short Job

In this example, there are two jobs: A, which is a long-running CPU-intensive job, and B, which is a short-running interactive job. Assume A has been running for some time, and then B arrives. What will happen? Will MLFQ approximate SJF for B?

A (shown in black) is running along in the lowest-priority queue (as would any long-running CPU intensive jobs); B (shown in gray) arrives at time T = 100, and thus is inserted into the highest queue; as its run-time is short (only 20 ms), B completes before reaching the bottom queue, in two time slices; then A resumes running (at low priority).

2.3 Example 3: What About I/O?

2.4 Problems With Our Current MLFQ

1. starvation: if there are “too many” interactive jobs in the system, they will combine to consume all CPU time, and thus long-running jobs will never receive any CPU time (they starve).
2. game the scheduler: Gaming the scheduler generally refers to the idea of doing something sneaky to trick the scheduler into giving you more than your fair share of the resource.
3. a program may change its behavior over time: what was CPU-bound may transition to a phase of interactivity.

3. Attempt #2: The Priority Boost

4. Attempt #3: Better Accounting

5. Tuning MLFQ And Other Issues

6. Summary

7. Homework (Simulation)

Question & Answer

1. Run a few randomly-generated problems with just two jobs and two queues; compute the MLFQ execution trace for each. Make your life easier by limiting the length of each job and turning off I/Os.

2. How would you run the scheduler to reproduce each of the examples in the chapter?

3. How would you configure the scheduler parameters to behave just like a round-robin scheduler?

in order to reflect RR scheduling, this parameter should have the following points:
1. Multiple jobs
2. One queue
3. There are the time slice

// x >= 2，y > 0
python mlfq.py -n 1 -j x -q y

4. Craft a workload with two jobs and scheduler parameters so that one job takes advantage of the older Rules 4a and 4b (turned on with the -S flag) to game the scheduler and obtain 99% of the CPU over a particular time interval.

to be continue....

5. Given a system with a quantum length of 10 ms in its highest queue, how often would you have to boost jobs back to the highest priority level (with the -B flag) in order to guarantee that a single long running (and potentially-starving) job gets at least 5% of the CPU?

T(boost) <= T(time slice) / 0.05 = 10ms / 0.05 = 200ms

6. One question that arises in scheduling is which end of a queue to add a job that just finished I/O; the -I flag changes this behavior for this scheduling simulator. Play around with some workloads and see if you can see the effect of this flag.

to be continue....