Advanced Operating Systems - Lecture 11: Producer consumer. This lecture will cover the following: producer consumer problem with a bounded buffer; solving producer consumer problem using locks and semaphores; semaphores used for two purposes; condition variables and monitors—introduction; condition variables—definition; hoare and mesa style monitors;...
Trang 1CS703 Advanced Operating Systems
By Mr Farhan Zaidi
Trang 211
Trang 3 Producer consumer problem with a bounded buffer
Solving producer consumer problem using locks and semaphores
Semaphores used for two purposes
Condition variables and monitors—introduction
Condition variables—definition
Hoare and Mesa style monitors
Recap of lecture
Trang 4 Problem definition
Producer puts things into a shared buffer, consumer takes them out Need
synchronization for coordinating producer and consumer.
Multimedia processing:Producer creates MPEG video frames, consumer
renders the frames
Event-driven graphical user interface: Producer detects mouse clicks, mouse movements, and keyboard hits and inserts corresponding events in buffer
Consumer retrieves events from buffer and paints the display.
Don't want producer and consumer to have to operate in lockstep, so put a
fixed-size buffer between them; need to synchronize access to this buffer
Producer needs to wait if buffer is full; consumer needs to wait if buffer is empty.
Solution uses semaphores for both mutex and scheduling.
Trang 51) Consumer must wait for producer to fill buffers, if none full (scheduling constraint)
2) Producer must wait for consumer to empty buffers, if all full (scheduling constraint)
3) Only one thread can manipulate buffer queue at a time (mutual exclusion)
Use a separate semaphore for each constraint;
Note semaphores being used in multiple ways
Semaphore fullBuffers;
// if 0, no item in buffer
Semaphore emptyBuffers;
// if 0, nowhere to put more item
Semaphore mutex; // mutual exclusion
Trang 6 Semaphore fullBuffers = 0 // initially, no item!
Semaphore emptyBuffers = numBuffers;
// initially, number of empty slots
// semaphore used to count how many
// resources there are!
Semaphore mutex = 1; // no one using the buffer
Producer() {
emptyBuffers.P();
mutex.P(); // make sure no one else is accessing the buffer
put 1 item in buffer
mutex.V(); // ok for others to use buffer
fullBuffers.V();
}
Consumer() {
fullBuffers.P();
mutex.P(); // make sure no one else is accessing the buffer
take 1 item out;
mutex.V(); // next person's turn
emptyBuffers.V();
Trang 7 Why does producer does P & V operations on different semaphores than the consumer?
Is order of P's important?
Is order of V's important?
What if we have 2 producers or 2 consumers? Do we need to
change anything?
Trang 8 Mutual exclusion
When semaphores are used for mutual exclusion, the semaphore has an initial value of 1, and P() is called before the critical section, and V() is called after the critical section
semaphore->P();
// critical section goes here
semaphore->V();
Scheduling constraints
Semaphores can also be used to express generalized scheduling constraints in other words, semaphores provide a way for a
thread to wait for something Usually, in this case, the initial value of the semaphore is 0, but not always!
For example, you can implement Thread’s join (reaping) using
semaphores:
Initial value of semaphore = 0
ThreadJoin calls P
ThreadFinish calls V
Trang 9Monitor: a lock and zero or more condition variables for
managing concurrent access to shared data
Note: Tanenbaum and Silberschatz both describe monitors as a
programming language construct, where the monitor lock is
acquired automatically on calling any procedure in a C++ class,
for example
No widely-used language actually does this, however! So in many real-life operating systems, such as Windows, Linux, or Solaris, monitors are used with explicit calls to locks and condition variables
Trang 10A simple example:
AddToQueue() {
lock.Acquire(); // lock before using shared data
put item on queue; // ok to access shared data
lock.Release(); // unlock after done with shared
// data
}
RemoveFromQueue() {
lock.Acquire(); // lock before using shared data
if something on queue // ok to access shared data
remove it;
lock.Release(); // unlock after done with shared data
return item;
}
Trang 11 How do we change RemoveFromQueue to wait until something
is on the queue? Logically, want to go to sleep inside of critical section, but if we hold lock when going to sleep, other threads won't be able to get in to add things to the queue, to wake up the sleeping thread Key idea with condition variables: make it
possible to go to sleep inside critical section, by atomically
releasing lock at same time we go to sleep
inside a critical section.
Trang 12 Condition variables support three operations:
Wait() release lock, go to sleep, re-acquire lock
Note: Releasing lock and going to sleep is atomic
Signal() wake up a waiter, if any
Broadcast() wake up all waiters
Rule: must hold lock when doing condition variable operations
Trang 13A synchronized queue, using condition variables:
AddToQueue() {
lock.Acquire();
put item on queue;
condition.signal();
lock.Release();
}
RemoveFromQueue() {
lock.Acquire();
while nothing on queue
condition.wait(&lock);// release lock; go to sleep; re-acquire lock remove item from queue;
lock.Release();
Trang 14 Need to be careful about the precise definition of signal and wait.
Mesa-style: (most real operating systems) Signaller keeps lock and processor
Waiter simply put on ready queue, with no special priority (in other words,
waiter may have to wait for lock)
Hoare-style: (most textbooks)
Signaller gives up lock, CPU to waiter; waiter runs immediately Waiter gives lock and processor back to signaller when it exits critical section or if it waits again
Trang 15Shared database (for example, bank balances, or airline seats) Two classes of users:
Readers never modify database
Writers read and modify database
Using a single lock on the database would be overly restrictive Want:
many readers at same time
only one writer at same time