COMP 3000 Midterm exam 2010

Question 1

Q: On an x86-class processor that supports hardware virtualization, it is possible in principle to virtualize any OS that can run on the native hardware. In practice, only certain operating system types and versions will run well on a given VM platform (e.g., VirtualBox, VMWare Player). What is one reason for this discrepancy?

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Question 2

Q: Typically, what does a guest OS's hard disk look like to the host OS?

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Question 3

Q: The execve system call gets three arguments: a full pathname to an executable, arguments, and environment variables. What are each of these for when "chmod a+r foo.txt" is run?

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Question 4

Q: Why doesn't ldd report all the libraries that are listed in /proc/$<$PID$>$/maps?

A: The ldd command only lists libraries that were statically linked at compile time. The proc mapping lists those and the ones linked dynamically at runtime.

Question 5

Q: When a process becomes undead (a zombie), who can send it to its grave permanently? What must it do?

A: The ancestor of all programs is init (PID = 1), and its job is to kill off any zombie children not properly killed by their parents before their own deaths. Gruesome stuff.

Question 6

Q: UNIX files each have permissions for three classes of users. What are they?

A: Owner, Group, Others. A file is typically created with owner set to the user running the program and the group set to that user's primary group. The value of the permissions (i.e. _rw_rw_r__) is set by the umask command. They can be changed after file creation with the chown command.

Comments

--Jjpwilso 14:19, 27 October 2010 (UTC) This question was truncated from what we had on the midterm. I tacked on "What are they?" Was there something else here?

Question 7

Q: A solution to the producer/consumer problem must take into account many conditions. What are three of them?

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Question 8

Q: What are three events that cause signals to be sent in UNIX?

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Question 9

Q: What system call do you use to receive data over an already existing pipe?

A: The read call receives data in a pipe. Note: this will block if the pipe is empty. When a sender issues the write command the receiver will unblock and receive a data from the pipe.

Comments

--Jjpwilso 14:28, 27 October 2010 (UTC) If I recall correctly, the amount of data read depends on: 1) a record separator such as a line-feed (possibly based on IFS?) or 2) the length of PIPEBUF as set during compile.

Question 10

Q: How is an exokernel library OS like a guest OS running on a VM? How are they different?

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Question 11

Q: What is one reason the UNIX system call interface has had to change in response to computer architectural changes?

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Question 12

Q: What is one reason why mainframes support near perfect hardware virtualization of CPU, memory, and all I/O devices?

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Question 13

Q: How can a cluster have higher overall reliability than the individual computers that it consists of?

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Question 14

Q: What was a key advantage of the Lisp machine OS, Genera, over current operating systems? What was a key disadvantage?

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Question 15

Q: Why did kernel extensions in SPIN need to be written in a "safe" language? Why don't kernel extensions in Windows and Linux (drivers, modules) need to be written in a safe language?

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Question 16

Q: What is an important reason why modern schedulers, like the ULE and recent Linux schedulers, require a scheduling algorithm which operates in sublinear time? Correspondingly, why was a linear-time scheduler okay for the original UNIX? Explain briefly.

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Question 17

Q: Describe a race condition error and a test that would catch that error in testing. (You may describe a ``well-known race condition or one that you make up.)

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Question 18

Q: Can a threading library like pthreads be implemented on a system that has no native support for threads, such as the original UNIX? What is required for such an implementation?

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Question 19

Q: Despite their performance disadvantage, what is one reason kernel space thread schedulers are generally favored over userspace implementations?

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Question 20

Q: Linux's logical volume manager (LVM) allows for the creation of synthetic block devices that span physical disks, e.g., the blocks in two hard disks can be appended or striped so they appear to be one large hard disk. LVM, however, implements no file system abstraction---the unified block device must be formatted with a regular filesystem. What is one way LVM plus ext4 (a standard Linux filesystem) is similar to ZFS? What is one way in which they are different?

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Question 21

Q: Hard disk-based filesystems must design around the slow seek times of hard disks (relative to their sequential transfer speed). What characteristic(s) of flash (solid state) storage must flash-based filesystems design around to achieve high performance and reliability?

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Question 22

Q: Storage area networks (SANs) allow many computers to share access to the same storage devices via a special trusted network. Standard SAN access protocols work in terms of blocks. By reading from multiple devices concurrently, SANs can provide impressive I/O performance. Given that SANs already are extremely high performance, why are many researchers arguing that they should transition to object-based stores?

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Question 23

Q: What key properties of a database are developers trying to capture with database-filesystem hybrids?

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Question 24

Q: Why aren't standard databases a drop-in replacement for filesystems?

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