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ser334_unit8_exercise_sample.lyx
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#LyX 2.3 created this file. For more info see http://www.lyx.org/
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\begin_body
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\noindent
Arizona State University
\begin_inset space \hfill{}
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SER334: Operating Systems & System Programming
\end_layout
\begin_layout Standard
\noindent
Lecturer Acuña (5), UGTA Lisonbee (3)
\begin_inset space \hfill{}
\end_inset
Revised 12/10/2021
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\begin_layout Standard
\noindent
\begin_inset CommandInset line
LatexCommand rule
offset "0.5ex"
width "100col%"
height "1pt"
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\size huge
Unit 8 Sample Problems - Process Synchronization II
\end_layout
\begin_layout Standard
\begin_inset VSpace 0.5cm
\end_inset
\end_layout
\begin_layout Standard
In this exercise, we will review the concepts of Process Synchronization.
\end_layout
\begin_layout Itemize
Length: 1:15 minutes with discussion.
\end_layout
\begin_layout Itemize
Questions: Q1-Q3, Q5-Q8
\end_layout
\begin_layout Standard
\begin_inset VSpace 0.5cm
\end_inset
\end_layout
\begin_layout Section
\series bold
\size larger
Semaphores
\end_layout
\begin_layout Enumerate
[Acuña] Consider each of the following resource scenarios:
\end_layout
\begin_deeper
\begin_layout Enumerate
Sending an email to an email server.
\end_layout
\begin_layout Enumerate
Managing a file sharing client's capped connections to peers.
\end_layout
\begin_layout Enumerate
A printer connected to a network.
\end_layout
\begin_layout Enumerate
Managing a local database that can be read or changed by multiple external
clients.
\end_layout
\begin_layout Standard
For each of these,
\series bold
select
\series default
the appropriate synchronization tool: mutex, semaphore, or none.
\end_layout
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status collapsed
\begin_layout Standard
\begin_inset VSpace 4cm
\end_inset
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\end_inset
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inverted 0
status collapsed
\begin_layout Standard
\series bold
Ans: [Acuña]
\end_layout
\begin_layout Enumerate
None.
\end_layout
\begin_layout Enumerate
A semaphore.
\end_layout
\begin_layout Enumerate
A mutex.
\end_layout
\begin_layout Enumerate
A mutex.
\end_layout
\end_inset
\end_layout
\end_deeper
\begin_layout Enumerate
[Acuña]
\series bold
Implement
\series default
the wait() operation for a semaphore using the test_and_set() instruction.
Assume that you have two methods available: add_this_process(list) which
adds the current process to a list, and block() which pauses the current
process until it is signaled.
\end_layout
\begin_deeper
\begin_layout Standard
\begin_inset Branch blank
inverted 0
status collapsed
\begin_layout Standard
\begin_inset listings
inline false
status open
\begin_layout Plain Layout
struct semaphore* S {
\end_layout
\begin_layout Plain Layout
static boolean lock;
\end_layout
\begin_layout Plain Layout
struct process_node* list;
\end_layout
\begin_layout Plain Layout
int value;
\end_layout
\begin_layout Plain Layout
}
\end_layout
\begin_layout Plain Layout
void wait(semaphore* S) {
\end_layout
\end_inset
\end_layout
\end_inset
\begin_inset Branch soln
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status collapsed
\begin_layout Standard
\series bold
Ans: [Acuña, Silberschatz]
\end_layout
\begin_layout Standard
\begin_inset listings
inline false
status open
\begin_layout Plain Layout
struct semaphore* S {
\end_layout
\begin_layout Plain Layout
static int lock;
\end_layout
\begin_layout Plain Layout
struct process_node* list;
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\begin_layout Plain Layout
int value;
\end_layout
\begin_layout Plain Layout
}
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\begin_layout Plain Layout
void wait(semaphore* S) {
\end_layout
\begin_layout Plain Layout
while(test_and_set(&S->lock));
\end_layout
\begin_layout Plain Layout
S->value--;
\end_layout
\begin_layout Plain Layout
\end_layout
\begin_layout Plain Layout
if (S->value < 0) {
\end_layout
\begin_layout Plain Layout
add_this_process(S->list);
\end_layout
\begin_layout Plain Layout
S->lock = 0; //false
\end_layout
\begin_layout Plain Layout
block();
\end_layout
\begin_layout Plain Layout
}
\end_layout
\begin_layout Plain Layout
else
\end_layout
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S->lock = 0;
\end_layout
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}
\end_layout
\end_inset
\end_layout
\end_inset
\end_layout
\begin_layout Standard
\begin_inset Newpage pagebreak
\end_inset
\end_layout
\end_deeper
\begin_layout Enumerate
[Lisonbee] Consider the following partially implemented code:
\end_layout
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\begin_layout Standard
\begin_inset listings
lstparams "language=C"
inline false
status open
\begin_layout Plain Layout
#define ROW 3
\end_layout
\begin_layout Plain Layout
#define COL 10
\end_layout
\begin_layout Plain Layout
\end_layout
\begin_layout Plain Layout
semaphore S = ROW;
\end_layout
\begin_layout Plain Layout
int index = 0;
\end_layout
\begin_layout Plain Layout
int data[ROW][COL];
\end_layout
\begin_layout Plain Layout
\end_layout
\begin_layout Plain Layout
void funct(void* param) {
\end_layout
\begin_layout Plain Layout
wait(&S); //LINE 1
\end_layout
\begin_layout Plain Layout
int i;
\end_layout
\begin_layout Plain Layout
for (i = 0; i < 10; i++) {
\end_layout
\begin_layout Plain Layout
printf(
\begin_inset Quotes eld
\end_inset
%d
\backslash
n
\begin_inset Quotes erd
\end_inset
, data[index][i]);
\end_layout
\begin_layout Plain Layout
}
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index = ++index % 3;
\end_layout
\begin_layout Plain Layout
signal(&S); //LINE 2
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\begin_layout Plain Layout
pthread_exit(0);
\end_layout
\begin_layout Plain Layout
}
\end_layout
\begin_layout Plain Layout
\end_layout
\begin_layout Plain Layout
int main() {
\end_layout
\begin_layout Plain Layout
int num_threads = 12;
\end_layout
\begin_layout Plain Layout
pthread_t threads[num_threads];
\end_layout
\begin_layout Plain Layout
for (int i = 0; i < num_threads; i++) {
\end_layout
\begin_layout Plain Layout
pthread_create(threads[i], NULL, funct, NULL);
\end_layout
\begin_layout Plain Layout
}
\end_layout
\begin_layout Plain Layout
for (int i = 0; i < num_threads; i++) {
\end_layout
\begin_layout Plain Layout
pthread_join(threads[i], NULL);
\end_layout
\begin_layout Plain Layout
}
\end_layout
\begin_layout Plain Layout
return 0;
\end_layout
\begin_layout Plain Layout
}
\end_layout
\end_inset
The provided code is attempting to guard the data resource so that only
one thread can access a row of the 2D data array at a time.
Is the semaphore properly guarding this resource as is? If not, how could
you fix this problem? Justify your answer with regard to how a semaphore
guards a resource and/or critical section.
\end_layout
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inverted 0
status collapsed
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\begin_inset VSpace 3cm
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\end_inset
\begin_inset Branch soln
inverted 0
status open
\begin_layout Standard
\series bold
Ans: [Lisonbee]
\end_layout
\begin_layout Standard
No, the semaphore in its current state will only ensure that no more than
three threads can touch the data array at a time.
Because you are accessing data with a global variable and then incrementing
it, this becomes a critical section problem.
The semaphore allows for multiple threads to run on this crtitical section,
and thus mutual exclusion is broken.
To fix this issue you would need to either limit the semaphore's initial
value to 1 (so it essentially becomes a mutex lock), or add a mutex lock
around the critical section.
Only then would the program work as intended.
\end_layout
\end_inset
\end_layout
\begin_layout Section
\series bold
\size larger
Classic Problems of Synchronization
\end_layout
\end_deeper
\begin_layout Enumerate
[Acuña] Consider the following solution to the producer consumer problem
from Silberschatz:
\end_layout
\begin_deeper
\begin_layout Standard
\begin_inset listings
inline false
status open
\begin_layout Plain Layout
//shared data.
[Operating Systems Concepts by Silberschatz.]
\end_layout
\begin_layout Plain Layout
int n;
\end_layout
\begin_layout Plain Layout
//also a buffer data structure
\end_layout
\begin_layout Plain Layout
semaphore buf_mutex = 1;
\end_layout
\begin_layout Plain Layout
semaphore empty = n;
\end_layout
\begin_layout Plain Layout
semaphore full = 0
\end_layout
\begin_layout Plain Layout
\end_layout
\begin_layout Plain Layout
...
\end_layout
\end_inset
\end_layout
\begin_layout Standard
This solution to the producer consumer problem with a bounded buffer requires
three semaphores - can the problem be solved with less? Explain.
\end_layout
\begin_layout Standard
\begin_inset Branch blank
inverted 0
status collapsed
\begin_layout Standard
\begin_inset VSpace 3cm
\end_inset
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\end_inset
\begin_inset Branch soln
inverted 0
status collapsed
\begin_layout Standard
\series bold
Ans: [Acuña]
\end_layout
\begin_layout Standard
Sure.
We only need to use one semaphore to guard a critical section that is basically
the entire producer or consumer.
That is, instead of invidually managing resources with a semaphore, we
just need a single one to act as a lock which guards empty/full/buffer/fill/use.
The downside is that the producer and consumer won't be able to execute
at the same time, they will have to take turns obtaining the global lock.
\end_layout
\end_inset
\end_layout
\end_deeper
\begin_layout Enumerate
[Acuña] Consider the following code which uses a semaphore to solve the
reader-writer problem:
\end_layout
\begin_deeper
\begin_layout LyX-Code
\begin_inset listings
inline false
status open
\begin_layout Plain Layout
//writer process.
[Operating Systems Concepts by Silberschatz.]
\end_layout
\begin_layout Plain Layout
do {
\end_layout
\begin_layout Plain Layout
wait(rw_mutex);
\end_layout
\begin_layout Plain Layout
/* writing is performed */
\end_layout
\begin_layout Plain Layout
signal(rw_mutex);
\end_layout
\begin_layout Plain Layout
} while (true);
\end_layout
\begin_layout Plain Layout
\end_layout
\begin_layout Plain Layout
//reader process
\end_layout
\begin_layout Plain Layout
do {
\end_layout
\begin_layout Plain Layout
wait(rc_mutex);
\end_layout
\begin_layout Plain Layout
read_count++;
\end_layout
\begin_layout Plain Layout
if (read_count == 1)
\end_layout
\begin_layout Plain Layout
wait(rw_mutex);
\end_layout
\begin_layout Plain Layout
signal(rc_mutex);
\end_layout
\begin_layout Plain Layout
/* reading is performed */
\end_layout
\begin_layout Plain Layout
wait(rc_mutex);
\end_layout
\begin_layout Plain Layout
read_count--; //THIS LINE
\end_layout
\begin_layout Plain Layout
if (read_count == 0)
\end_layout
\begin_layout Plain Layout
signal(rw_mutex);
\end_layout
\begin_layout Plain Layout
signal(rc_mutex);
\end_layout
\begin_layout Plain Layout
} while (true);
\end_layout
\begin_layout Plain Layout
\end_layout
\end_inset
\end_layout
\begin_layout Standard
\series bold
Explain
\series default
how the program would act differently if the indicated line was commented
out.
\end_layout
\begin_layout Standard
\begin_inset Branch blank
inverted 0
status collapsed
\begin_layout Standard
\begin_inset VSpace 5cm
\end_inset
\end_layout
\end_inset
\begin_inset Branch soln
inverted 0
status collapsed
\begin_layout Standard
\series bold
Ans: [Lisonbee]
\end_layout
\begin_layout Standard
Removing the decrement for
\shape italic
read_count
\shape default
means that the program never becomes aware when a reader is finished reading.
\shape italic
rw_mutex
\shape default
guards writing, where a given process cannot write if there is already
another process writing, or if one or more processes are reading from the
file.
This means as soon as the first reader starts reading, then no process
will ever get to write again (since it will always get stuck waiting on
\shape italic
rw_mutex
\shape default
).
Reading the file would never be a problem as this code allows for any number
of readers to read the file at a time.
\end_layout
\end_inset
\end_layout
\begin_layout Standard
\begin_inset Newpage pagebreak
\end_inset
\end_layout
\end_deeper
\begin_layout Enumerate
[Lisonbee] The bounded buffer problem is a result of producing and consuming
work asynchronously at different and/or variable rates.
Provided below is are the producer and consumer functions used by a program:
\end_layout
\begin_deeper
\begin_layout Standard
\begin_inset listings
lstparams "language=C"
inline false
status open
\begin_layout Plain Layout
int data[15], i0 = 0, i1 = 0;
\end_layout
\begin_layout Plain Layout
\end_layout
\begin_layout Plain Layout
semaphore s1 = ...
;
\end_layout
\begin_layout Plain Layout
semaphore s2 = ...
;
\end_layout
\begin_layout Plain Layout
semaphore s3 = ...
;
\end_layout
\begin_layout Plain Layout
\end_layout
\begin_layout Plain Layout
void producer() {
\end_layout
\begin_layout Plain Layout
while (1) {
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\begin_layout Plain Layout
\end_layout
\begin_layout Plain Layout