UCLA CS239--Advanced Operating Systems
Fall, 1998
Midterm Exam ("take home")
Exam Rules
You must complete the
academic integrity statement
prior to taking this exam.
If you have not already completed and submitted the statement, please complete
it and submit it along with your exam answers.
The exam answers are due at the beginning of class on Tuesday, November 24, at
the usual time and place.
This exam is open books, notes, libraries, archives, internet, etc. You are
free to consult any static resource. Fellow class members, other students,
faculty, friends (or enemies), etc., are NOT to be involved in any way.
Please limit each answer to 500 words. It is often a good idea to prepare a
"complete" answer without regard to the word limit, and then carefully edit
your answer down to meet the limit.
You are required to answer each question.
Questions
One
The advent of 64-bit architectures has the potential to dramatically
influence operating systems design. In particular, the concept of a
"single address space" system, in which all entities share a single
64-bit address space has been proposed. For example, suppose that the
operating system and all processes share a single virtual address space,
and that the OS supports a (new) service by which processes can be
"saved to disk" and resumed later, after rebooting the OS.
Please fully discuss the pros and cons of such a system in a
non-distributed environment, identifying any new issues that must be faced
as well as old problems that are no longer interesting.
Two
In the late 1970's in our department, a typical timesharing system was
built out of a PDP-11/45 (a 300ns/cycle CPU) with 512KB of RAM,
a 20MB hard drive, and a 300KB DECtape magnetic tape unit. DECtapes
were unusual in that they were bi-directional, direct-block access
(for both read and write) storage devices of about 600 0.5KB blocks,
with a maximum seek delay (end-to-end) of about 10 seconds.
A typical configuration had from 2-4 such tape units.
One day the hard drive failed (with a 3-5 day repair estimate),
and ongoing use of the system was absolutely critical. A few clever
programming tricks later, the DECtape units were each happily masquerading
as a separate hard drive.
How would you expect a disk-tuned file system to behave under these conditions?
How would you design/tune a file system for a DECtape-based secondary storage system?
Three
In the discussion on agreement protocols, the text asserts on page 184 that "if
authenticated messages are used, [the m<=(n-1)/3] bound is relaxed and a
consensus can be reached for any number of faulty processors." Provide an
informal justification for this assertion.
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