Operating Systems (CS 377):
Homework 3
Due: October 24, 2002 23:59
What to Hand In
Everyone must hand in their own work. Place in your Hand-In directory
(~/cs377/hw3/) one of the following: README (raw text file),
README.ps (a postscript file), README.pdf (pdf file), README.html.
The file must contain your name, user id, and answers to the questions
listed below.
Q1. Define "Hold and wait," one of the necessary conditions required for
deadlock to occur.
Q2.
In what situation does multilevel feedback queue
scheduling lead to starvation?
Readers/Writers problem with favored writers.
Below is the implementation of readers/writers
that we have been examining in class. However,
we will assume that the implementation of the semaphore is the decrement-last
implementation (the one that we are using in
Semaphore.java). The task for the next
set of questions is to trace the reader/writer code for multiple threads for
multiple scenarios and track the state of the 5 semaphores (both the semaphore
value and its corresponding wait queue) and the "readers" and "writers" state
variables. Assume for simplicity (except where noted otherwise) that
the threads maintain the CPU unless they explicitly block or are in the
middle of the reading or writing operation (R-10 and W-7, respectively).
LINE CODE
Write(){
W?-1 write_mutex.Wait(); // ensure mutual exclusion for writer bookkeeping
W?-2 writers += 1; // another pending writer
W?-3 if (writers == 1) // block readers
W?-4 read_block.Wait();
W?-5 write_mutex.Signal();
W?-6 write_block.Wait(); // ensure mutual exclusion for data write
W?-7 ##perform write##
W?-8 write_block.Signal();
W?-9 write_mutex.Wait(); // ensure mutual exclusion for writer bookkeeping
W?-10 writers -= 1; // writer done
W?-11 if (writers == 0) // enable readers
W?-12 read_block.Signal();
W?-13 write_mutex.Signal();
}
Read(){
R?-1 write_pending.Wait(); // ensures at most one reader will go
// before a pending write
R?-2 read_block.Wait();
R?-3 read_mutex.Wait(); // ensure mutual exclusion
R?-4 readers += 1; // another reader
R?-5 if (readers == 1) // synchronize with writers
R?-6 write_block.Wait();
R?-7 read_mutex.Signal();
R?-8 read_block.Signal();
R?-9 write_pending.Signal();
R?-10 ##perform read##
R?-11 read_mutex.Wait(); // ensure mutual exclusion
R?-12 readers -= 1; // reader done
R?-13 if (readers == 0) // enable writers
R?-14 write_block.Signal();
R?-15 read_mutex.Signal();
}
Example: Starting from the initial configuration, a writer begins to write
(we will refer to this as W-a). While the writing is taking place, three
different readers (R-a, R-b, R-c) attempt to read the database. Once all
three readers have blocked, the writer completes, allowing the three readers
to begin reading. One possible trace is as follows:
| thread and line
|
| write_mutex
|
| read_mutex
|
| write_pending
|
| write_block
|
| read_block
|
| writers
| readers
|
|
|
| value
| Q
|
| value
| Q
|
| value
| Q
|
| value
| Q
|
| value
| Q
|
|
|
|
| Init
|
| 1
| -
|
| 1
| -
|
| 1
| -
|
| 1
| -
|
| 1
| -
|
| 0
| 0
|
| W-a1
|
| 0
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| W-a2
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| 1
|
|
| W-a4
|
|
|
|
|
|
|
|
|
|
|
|
|
| 0
|
|
|
|
|
| W-a5
|
| 1
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| W-a6
|
|
|
|
|
|
|
|
|
|
| 0
|
|
|
|
|
|
|
|
| SWITCH: R-a
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| R-a1
|
|
|
|
|
|
|
| 0
|
|
|
|
|
|
|
|
|
|
|
| R-a2
|
|
|
|
|
|
|
|
|
|
|
|
|
| 0
| R-a
|
|
|
|
| SWITCH: R-b
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| R-b1
|
|
|
|
|
|
|
| 0
| R-b
|
|
|
|
|
|
|
|
|
|
| SWITCH: R-c
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| R-c1
|
|
|
|
|
|
|
| 0
| R-b, R-c
|
|
|
|
|
|
|
|
|
|
| SWITCH: W-a
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| W-a8
|
|
|
|
|
|
|
|
|
|
| 1
|
|
|
|
|
|
|
|
| W-a9
|
| 0
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| W-a10
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| 0
|
|
| W-a12
|
|
|
|
|
|
|
|
|
|
|
|
|
| 0
| -
|
|
|
|
| W-a13
|
| 1
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| SWITCH: R-a
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| R-a3
|
|
|
|
| 0
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| R-a4
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| 1
|
| R-a6
|
|
|
|
|
|
|
|
|
|
| 0
|
|
|
|
|
|
|
|
| R-a7
|
|
|
|
| 1
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| R-a8
|
|
|
|
|
|
|
|
|
|
|
|
|
| 1
|
|
|
|
|
| R-a9
|
|
|
|
|
|
|
| 0
| R-c
|
|
|
|
|
|
|
|
|
|
| SWITCH: R-b
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| R-b2
|
|
|
|
|
|
|
|
|
|
|
|
|
| 0
|
|
|
|
|
| R-b3
|
|
|
|
| 0
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| R-b4
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| 2
|
| R-b7
|
|
|
|
| 1
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| R-b8
|
|
|
|
|
|
|
|
|
|
|
|
|
| 1
|
|
|
|
|
| R-b9
|
|
|
|
|
|
|
| 0
| -
|
|
|
|
|
|
|
|
|
|
| SWITCH: R-c
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| R-c2
|
|
|
|
|
|
|
|
|
|
|
|
|
| 0
|
|
|
|
|
| R-c3
|
|
|
|
| 0
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| R-c4
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| 3
|
| R-c7
|
|
|
|
| 1
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| R-c8
|
|
|
|
|
|
|
|
|
|
|
|
|
| 1
|
|
|
|
|
| R-c9
|
|
|
|
|
|
|
| 1
|
|
|
|
|
|
|
|
|
|
|
Q3. Show the corresponding table for the following scenario:
From the initial configuration, a reader (R-a) starts reading. A writer
(W-a) then attempts to initiate a write; a second reader (R-b) next
attempts to initiate a read. The first reader then completes, which
allows the first writer to begin writing. On completion of the write, the 2nd
reader starts and then completes the read.
Q4. Show the corresponding table for the following scenario:
A reader (R-a) begins reading, but is swapped off of the CPU after
executing line 2. Two other readers (R-b, R-c) are then initiated.
Once these two block, R-a continues into the reading state (line 10).
R-b and R-c then move into the reading state. Then, one-by-one, the
three readers complete the reading process.
Q5.
How much time did you spend on this homework assignment?
fagg@cs.umass.edu
Last modified: Mon Oct 14 09:32:53 2002