50 points, due Feb. 16, 1998
- (10 pts.)
Consider a single processor timesharing system which supports a large
number of interactive users. Each time a process gets the processor, the
interrupt timer is set to interrupt after the quantum expires. Assume that
each process has the same quantum.
- What would be the effect of setting the quantum at a very large
value, say 10 minutes? What sort of process behavior(s) would preclude
- What would be the effect if the quantum were set to a very small
value, say a few processor cycles (machine instructions)?
- Obviously, an appropriate quantum must be between the values
previously mentioned. If you could vary the quantum, how would you
determine when you had chosen the ``right'' value? What factors make
this value right from the user's standpoint? What factors make it right
from the system's standpoint?
- (10 pts.)
In Unix, a signal is an abstraction of an interrupt. Non-privileged users
are permitted to install their own signal handlers. Recall that interrupt
handlers are run in supervisor mode.
- Explain why it would be a very bad idea to permit a user's signal
handler to run in supervisor mode.
- Assuming that the mechanism used to invoke a signal handler places
the CPU in supervisor mode, suggest a way (by writing pseudo-code) to
safely run user's signal handlers. (Hint: assume that the CPU has a
UserMode machine instruction which places the machine in user
- (15 pts.)
An alternative to the TAS instruction is the FAI (Fetch And Increment)
instruction. Here are its semantics:
int FAI(int& val)
return val++; // Performed atomically.
Devise a solution to the critical section problem for n processes using
this instruction. Instead, model your solution on the manner in which a
bakery, for instance, serializes customer service by means of a number
dispenser. Prove that your solution meets the three criteria of a solution
to the critical section problem. Solutions which treat the FAI instruction
as a ``glorified'' TAS instruction will be down-graded. Likewise, simple
ports of the bakery algorithm will be down-graded.
- (15 pts.)
A barbershop consists of a waiting room with N chairs, and the barber
room containing the barber chair. If there are no customers to be served
the barber goes to sleep. If a customer enters the barbershop and all
chairs are busy, then the customer leaves the shop. If the barber is busy,
then the customer sits in one of the available free chairs. If the barber
is asleep, the customer wakes the barber up. Write pseudo-code for the
cooperating barber and customer threads. In devising your solution, you
may make use of queueing semaphores.
Thomas P. Kelliher
Mon Feb 2 10:20:48 EST 1998