OS comps 10 – 11:30am, 4140 CSE bldg. October 1, 2007
Answer each of the four numbered questions in a separate blue book.
1. These five questions concern scheduling. For each, answer the question and provide an explanation for it.
a. Given 3 processes X = (0,4), Y = (0,2) and Z = (3,1), where (a, s) specifies a process’s arrival and
service times, what is the average turnaround time when using shortest remaining time?
b. Given 3 processes X = (0,4), Y = (0,2) and Z = (3,1), where (a, s) specifies a process’s arrival and
service times, assuming preemption, what is the largest the average turnaround time can ever be?
c. Multi-level Feedback-Queue Scheduling approximates which of the following scheduling policies: FIFO,
Priority, Round Robin, Shortest Process Next?
d. If a set of processes can be scheduled to meet all deadlines using Earliest Deadline First, can all deadlines
also be met using Rate Monotonic Scheduling?
e. When using the proportional-share scheduling algorithm, say process X is to receive 70% of the CPU
and process Y has received 30%, and thus far X has received 50% and Y has received 20%, which process
should be given the CPU for the next quantum?
2. As you have seen in your readings, file system performance is all about avoiding disk seeks – either by
locating data blocks close together on the disk while reading and writing large numbers of blocks at a time,
or avoiding writing to disk altogether. Answer the following questions about the Log-Based File System
(LFS) and the Soft Updates system:
a. Recall that LFS writes data a segment-at-a-time, requiring an entire segment’s worth of free space.
Hence, LFS required a cleaner that perio dically compacted the live data in old segments. Describe the
tradeoff in deciding how many segments to clean at once. That is, what are the pros and cons of cleaning
many segments at the same time? Few segments?
b. Upon experimentation, Rosenblum discovered that a ‘hot-and-cold’ file access pattern caused the greedy
cleaner to clean segments at a higher average utilization than a uniform access pattern. Why did the
authors claim this occurred? Describe the ‘cost-benefit’ cleaning policy the authors devised to address
the issue.
c. Unlike LFS, Soft Updates continues to use the standard FFS on-disk structures, instead altering the
rules for when data blocks are written to disk. What invariants does Soft Updates enforce with respect
to meta-data it writes to disk?
d. When evaluating Soft Updates, the authors discovered that its performance decreased when the size
of the benchmark increased. That is, when the benchmarks operated on either larger data sets or for
longer periods of time, the relative throughput of Soft Updates (as compared to FFS) often decreased
considerably when compared to smaller data sets over shorted periods of time. List two causes of this
effect.
3. Lazy evaluation is an optimization technique frequently used in the implementation of operating system
services.
a. Please explain what lazy evaluation is in general and when it will be an optimization.
b. For each of the following systems, describe how lazy evaluation is used as an optimization and under
what particular circumstances it will be effective: Mach VM, LRPC & FFS.
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