Operating Systems
Clicker Questions
Kevin Webb & Cynthia Taylor
The operating system kernel…
A. Executes as a process
B. Is always executing, in support of other processes
C. Should execute as little as possible
D. More than one of the above
Why shouldn’t processes control
context switching?
A. It would cause too much overhead
B. They could refuse to give up the CPU
C. They don’t have enough information about
other processes
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Which CPU scheduling policy is the best?
A. Processes keep CPU until done (maximize
throughput)
B. Processes use a fraction of CPU and pass it on
(ensure fairness)
C. Processes receive CPU in proportion to their
priority or what they pay (prioritize importance)
D. Other (explain)
Which is true of round-robin schedules
relative to first-come first-served.
A. Round-robin requires knowledge of process run times.
B. Round-robin schedules have lower average turnaround times.
C. Round-robin schedules have higher average turnaround times.
D. Round-robin has higher overhead.
E. More than one of these
Multi-level feedback queues will
naturally prioritize…
A. Compute-bound processes
B. I/O-bound processes
C. Long-running processes
D. Short processes
Which component of a process might
we replicate to take advantage of
multiple CPUs?
A. The address space (memory)
B. OS resources (open files, etc.)
C. Execution state (PC, SP, registers, etc.)
D. More than one of these
E. Some other component(s)
If you call thread_create() on a
modern OS (Linux/Mac/Windows), which
type of thread would you expect to
receive? (Why? Which would you pick?)
A. Kernel thread
B. User thread
C. Some other sort of thread
For processes (or threads) to
communicate, we must have:
A. a data transfer mechanism
B. a synchronization mechanism
C. either synchronization or data transfer
D. both synchronization and data transfer
Suppose we’re using message passing,
will this code operate correctly?
/* NO SHARED MEMORY */
Producer
int item;
Consumer
int item;
while (TRUE) {
while (TRUE) {
item = Produce ();
receive (Producer, &item);
send (Consumer, &item);
Consume (item);
}
}
A. No, there is a race condition.
B. No, we need to protect item.
C. Yes, this code is correct.
This code is correct and relatively
simple. Why don’t we always just use
message passing (vs semaphores)?
/* NO SHARED MEMORY */
Producer
int item;
Consumer
int item;
while (TRUE) {
while (TRUE) {
item = Produce ();
receive (Producer, &item);
send (Consumer, &item);
Consume (item);
}
}
A.
B.
C.
D.
Message passing copies more data.
Message passing only works across a network.
Message passing is a security risk.
We usually do use message passing!
Which of these conditions is easiest to
give up to prevent deadlocks?
A. Mutual exclusion (make everything sharable)
B. Hold and wait (must get all resources at once)
C. No preemption (resources can be taken away)
D. Circular wait (total order on resource requests)
E. I’m not willing to give up any of these!
Given the following table, for which
values of X, Y, and Z, is this a safe state
in the Banker's Algorithm?
Claim
Allocation
P1
P2
P3
P1
P2
P3
Availability
R1
3
6
1
Y
4
1
Z
7
R2
2
2
X
2
1
1
1
5
A.
B.
C.
D.
X = 3, Y = 1, Z = 1
X = 2, Y = 0, Z = 2
Both of these are safe states
Neither of these are safe states
Total
Which type of deadlock-handling
scheme would you expect to see in a
modern OS (Linux/Windows/OS X) ?
A. Deadlock prevention
B. Deadlock avoidance
C. Deadlock detection/recovery
D. Something else
Which form of fragmentation is easiest
for the OS to reduce/eliminate?
A. Internal fragmentation
B. External fragmentation
C. Neither
Which memory allocation algorithm
leaves the smallest fragments (external)?
A. first-fit
B. worst-fit
C. best-fit
Where would worst-fit place this
memory chunk?
A.
5 MB
B.
7 MB
C.
9 MB
5 MB
How would the buddy system allocate
the following set of requests? Assume
8 MB free initially.
Alloc 3 MB, Alloc 1.5 MB, Alloc 1 MB
8 MB
4 MB
B.
4 MB
2 MB
8 MB
2 MB
8 MB
4 MB
1 MB
4 MB
A.
1 MB
3 MB
5 MB
C.
2.5M
2 MB
2.5M
2 MB
1.5M
1 MB
Given what we currently know about
memory, what must we do during a
context switch?
A. Allocate memory to the switching process
B. Load the base and bound registers
C. Convert logical to physical memory addresses
Segment Address Translation
Segment Table
Index
Valid
Base
Bound
Perm
1
0
120
600
r
2
1
8000
1000
rw
3
1
9000
512
wx
• Physical address of
2
1200
?
A. 8000
B. 9200
C. Error
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5 bit segment address, 32 bit logical
address, 1 GB Physical memory.
How many entries will we have in our
segment table?
A. 32: The logical address size is 32 bits
B. 32: The segment address is five bits
C. 30: We need to address 1 GB of physical memory
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Which of these would be true about a
multi-level page table, compared to a
single-level page table?
A. Multi-level usually uses less memory.
B. Multi-level usually has faster lookup times.
C. Both cause the same amount of fragmentation.
D. More than one of the above is true.
22
A page fault occurs. What must we do
in response?
A. Find the faulting page on disk.
B. Evict a page from memory and write it to disk.
C. Bring in the faulting page and retry the operation.
D. Two of the above
E. All of the above
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Handing faults from disk seems very
expensive. How can we get away with
this in practice?
A. We have lots of memory, and it isn’t usually full.
B. We use special hardware to speed things up.
C. We tend to use the same pages over and over.
D. This is too expensive to do in practice!
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Your closet is full, but you just got a
new shirt. How do you make room for
it in your closet?
A. Throw out another shirt at random.
B. Throw out your newest shirt.
C. Throw out your oldest shirt.
D. Throw out a shirt you haven’t worn for a while.
E. I govern my closet with some other shirt replacement policy.
25
Which is true of the Clock algorithm?
A. Clock will always have fewer faults than FIFO.
B. Clock will always have the same fault count as LRU.
C. Clock will always have more faults than Optimal.
D. More than one of the above.
E. None of the above.
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Why are disks slow?
A. We have to move a disk head.
B. We have to spin the disk platters.
C. Disk data is farther away from the CPU/Mem.
D. More than one of these.
E. All of these.
27
Regarding file types…
A. The OS distinguishes between file types.
B. The user distinguishes between file types.
C. Both the OS and users distinguish between file
types.
D. Nobody distinguishes between file types, files
are all just files.
28
Which of the following is commonly
represented by a file?
A.
B.
C.
D.
E.
A mouse
A link to another file
A directory
More than one of the above
All of the above
29
A file’s “path” determines where it is
stored on a disk device.
A. True
B. False
C. Sometimes
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Why do we have an open() call, as
opposed to just read()ing or
write()ing a file path?
A.
B.
C.
D.
E.
To check file permissions
To improve performance
To lookup the file’s location on disk
Two of the above
All of the above
31
For a memory mapped file…
A. The entire file must be in memory.
B. Pages are brought in as accessed.
C. All pages will eventually be written back to disk.
D. More than one of the above.
32
Which allocation strategy has the
smallest space overhead? Easiest to
resize a file?
Answer Choice
Smallest Overhead
Easiest to Resize a File
A
Extents
Extents
B
Non-contiguous
Contiguous
C
Non-contiguous
Extents
D
Contiguous
Non-contiguous
33
Suppose a block is 1 KB (210 bytes).
How many pointers do we need in our
block map to support a maximum file
size of 16 GB (234 bytes)?
Block size: 1 KB
…
?
A: 210
B: 216
C: 224
E: Some other number of pointers
D: 234
34
What about writes? What if we lose
power, and our modified data is only in
memory? When should we write it to
the disk to maintain reliability?
A. Write blocks to disk immediately, to ensure safety.
B. Write blocks when we evict them from the cache,
to maximize performance.
C. Write blocks when the user explicitly tells us to,
since the user/application knows best.
D. Write blocks to disk at some other time.
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