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Flertrådad programmering > Module 4: Monitors & Condition Synchronization > Flashcards

Module 4: Monitors & Condition Synchronization Flashcards

1
Q

Semaphore drawbacks

A
  1. Programmers gotta keep track of wait and signal. Thread has to release a lock before it dies.
  2. If the calls are done incorrectly => deadlocks.
  3. Unstructured => no encapsulation & data-hiding (not OOP).
  4. No connection: semaphore & data controlled by the semaphore.
  5. The multi-purpose of semahpores makes it difficult to identify what’s happening in the code
  6. Prone to bugs + hard to debug
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2
Q

How can Monitors be an alternative to semaphores?

A
  • More structure
  • Ties data, methods & sync as a class that encapsulates data & operations
  • Solves problem of a global shared data. All data is private.
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3
Q

How does the monitor sync data?

A
  • Threads trying to access shared data only interact through the monitor’s methods.
  • Only 1 thread in the monitor at a time
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4
Q

The process of a thread entering the monitor

A
  1. Entering the monitor (“enters building”. queues in “entry hall”)
  2. Aquiring the monitor (“enters the room from the entry hall”)
  3. Owning the monitor (“locks the room” and owns the monitor)
  4. Releasing the monitor (releases the lock when “leaving the room”. Wait queue)
  5. Exiting the monitor (“leaves the building”, dequeues)
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5
Q

Thread waiting on a condition in a monitor

A
  • Suspended thread = waits for an event to occur
  • Releases monitor & waits
  • Event occurs = thread is notified. Tries to aquire lock ASAP.
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6
Q

Why is a monitor good?

A
  • Allows threads to have mutual exclusion (only 1 thread is active in the monitor at a time)
  • Ability to suspend & have threads wait for a condition
  • Encapsulates data & operations
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7
Q

What does a monitor encapsulate?

A
  • Data: needs private variables (not outside monitor!)
  • Methods: accessing shared data only in the monitor
  • Synchronization: done thru condition variables (wait & signal capabilities)
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8
Q

How does a monitor ensure ME?

A

Each monitor has a lock and condition variable connected to it.

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9
Q

Condition variable in monitors

A
  • Wait()
  • Pulse()
  • PulseAll()
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10
Q

Wait() condition variable

A
  • If a resource not available, enter condition variable queue & sleep
  • Releases monitor lock
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11
Q

Pulse() condition variable

A
  • Wakes up a sleeping (waiting) thread
  • Thread resumes its work after the lock is released
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12
Q

PulseAll() condition variable

A
  • Wakes up ALL waiting threads
  • Thread allowed to executed is up to CPU
  • No thread waiting=> signal is lost
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13
Q

What is a condition variable?

A
  • Container of threads waiting for a condition
  • Type of queue, NOT boolean
  • Like a place for threads to wait
  • Waits or signals depending on the value
  • Each monitor has 1 or several queues: waiting to enter and waiting after releasing.
  • Queues are not directly visible to programmer
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14
Q

Thread or Monitor? Active and passive objects

A
  • Active: Objects that initiate actions. Implemented as threads.
  • Passive: Objects that respond to actions. Implemented as monitors.
  • Airport EX:
  • plane INITIATES action of arrival/departure (thread)
  • control tower RESPONDS to arrival/departure (monitor)
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15
Q

Mutual Exclusion through monitor?

A
  • No other thread will be in the executing region concurrently
  • Threads in entry-set must compete to acquire monitor
  • Threads in wait-set must compete to aquite monitor
  • Required to: first arrive at entry, get monitor, then enter region.
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16
Q

In what way do monitors support both mutual exclusion & condition sync?

A
  • ME: global lock. Mutex for all operations in monitor. Only one thread can execute in the monitor at a time.
  • Condition sync: Condition variables provide sync. While a thread waits for an event to happen, other threads are allowed to execute.
17
Q

How do waiting threads work in a monitor queue?

A
  • Threads wait on condition variable queue until other thread signals
  • Every monitor has a wait queue
  • If a thread tries to enter the monitor when its in use: blocks.
  • A suspended thread can reenter monitor when woken up (eg when a condition is met), resuming its work
18
Q

What is condition synchronization?

A
  • Coordination of multiple threads or processes in a way that allows them to cooperate & communicate with each other effectively
  • A thread in CS => what if it can’t continue cuz a resource is lacking? Can release the lock & wait until resource is available.
  • Think signaling with semaphores.
19
Q

How does condition synchronization work in monitors?

A
  • A thread may wanna sleep within a monitor, or allow a waiting thread to continue
  • Monitors have a signaling mechanism too = tells waiting threads their condition has become true
  • Condition variables provide sync within monitor => thread can wait & signal waiting threads to cont
  • Thread can suspend (wait) itself to cooperate with other threads, letting them execute instead
20
Q

What is the relationship between condition sync and condition variables?

A
  • Condition sync is done by means of condition variables
  • Condition variables are NOT booleans. They are DATA TYPES. Usually queue of waiting threads.
  • Monitors use conditional variables to suspend & signal threads
21
Q

What are the monitor methods in C#?

A
  • Monitor.Enter(lockObj) = Aquire lock & enter
  • Monitor.Exit(lockObj) = Release lock & exit
  • Monitor.Pulse(lockObj) = Notify 1 waiting thread
  • Monitor.PulseAll(lockObj) = Notify all waiting threads
  • Monitor.Wait(lockObj) = Suspend itself for a condition to become true
  • Monitor.TryEnter(lockObj) = Enters with a timeout time
22
Q

How is a monitor structured in C#?

A
  • Monitor has 2 items: critical section, condition variable
  • It’s an abstract class
  • Any object can be used as a monitor

object monitorObj = new object();
public double Method()
{
Monitor.Enter(monitorObj);
try
{
//critical section code
}
finally
{
Monitor.Exit(monitorObj);
}
}
}

23
Q

Condition variables vs mutex

A
  • A mutex controlls thread access to data
  • Condition variables allow threads to sync based on ACTUAL value of data
  • W/o condition variable => programmer has to continually check if condition is met (polling). Very resource consuming.
  • Condition variable = same goal WITHOUT polling. Much better!
24
Q

C# lock/Monitor example

A
  • Can use lock(lockObj) instead of Monitor.Enter & Exit while still using the other methods (Wait, Pulse, PulseAll..)

lock(lockObj) //same as .Enter
{
while(IsFull)
{
//thread blocks
Monitor.Wait(lockObj);
}
//More code
Monitor.Pulse(lockObj);
} //same as .Exit = the lock releases here

Flertrådad programmering (5 decks)

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