Extra credit, due May 4, 2012 at noon
Each problem is worth eight points. Show all work.
- Assume a process has a fixed allocation of page frames. When a process
references a page that is not in one of its frames, a replacement
algorithm is invoked to select a page to replace. Consider the following
variant of the LRU replacement strategy:
if at least one page has not been modified
replace the least recently used unmodified page;
replace the least recently used page /* all have been modified */
This algorithm replaces unmodified pages in preference to modified ones.
Give a reference string which illustrates that this replacement algorithm
is not a stack algorithm. Mark modifying references with an asterisk to
distinguish them from non-modifying references. Show the memory contents
at the end of the reference for two values of (the number of page
frames) to illustrate the absence of memory inclusion. There are quite
short reference strings (six references) which will do the trick.
- Consider a demand-paged computer system which uses a paging drum, global
LRU replacement, and an allocation policy which shares frames equally among
processes (i.e., if there are frames and processes, then each
frames). The degree of
multiprogramming is currently
fixed at four. The system was recently measured to determine utilization
of the CPU and the paging drum. Consider each of the following to be the
results of the measurement. For each case, what is happening? Can the
degree of multiprogramming be increased to increase the CPU utilization?
Is the paging helping?
- CPU utilization 13%; disk utilization 97%.
- CPU utilization 87%; disk utilization 3%.
- CPU utilization 13%; disk utilization 3%.
- In a tree-structured file system such as Unix, permission bits are usually
associated with each directory and file. Consider the following
modification to the scheme. Associated with each directory is a password and a set of global permissions. For a user to access
a directory which she does not own, the correct password must first be
typed. Once this is performed, the user can manipulate the directory and
files within that directory according to the global permissions READ,
WRITE, and CREATE. If the global permission associated with the
directory is READ, then the user can list the associated file names and
read any file within the directory. If the permission is WRITE,
then the user can list file names as well as read and write any file. If
the permission is CREATE, then the user can list file names and create new
files within that directory, but cannot read or write any other
files. The owner of the directory can perform any action on any file in
Consider a system where class accounts are organized so that the
instructor and each student has a directory. A user logged onto the
instructor account can access any files in the student accounts, despite
the permissions associated with them. Among other things, this allows
the instructor to run a grading program, executing each student's
program against a common data file.
Given the protection scheme described above, describe how a class
account would be organized. In particular, describe how the instructor
can run the grading program without having to know the password
associated with each student's directory.
- An operating system only supports a single directory, but allows that
directory to have arbitrarily many files with arbitrarily long file names.
Explain how something approximating a hierarchical file system (including
relative and absolute pathnames and the concept of a current working
directory) can be created.
Thomas P. Kelliher