L7 - Concurrency

Question 1

Semaphores are ________________ that can only accessed by _________ and _________.

Answer 1

Semaphores are kernel data structures, that can only be accessed by wait and signal.

Question 2

What is Push Through Blocking?

Answer 2

In push through blocking, the medium priority task is blocked by a lower priority task which inherited a higher priority from a task it directly blocks.

Question 3

Name the three problems that can occur while multi-threading.

Answer 3

• Race Conditions
• Priority Inversions
• Starvation

Question 4

What does NPP stand for?

Answer 4

In NPP (Non-Preemptive Protocol), a task that enters a critical section gets the highest priority level and therefore prevents a deadlock. NPP is easy to implement and denies preemption during execution of any critical section

Question 5

Describe Starvation.

Answer 5

Process is denied necessary resources and waits forever.

Question 6

Multi-Tasking

Answer 6

Parallel execution of independent processes is called multi-tasking.

Question 7

Priority Ceiling Protocol (PCP) avoids ____________, ____________ and ____________. What is the concept of PCP?

Answer 7

Priority Ceiling Protocol (PCP) avoids priority inversion, deadlocks and chained blocking. Task can only enter critical section if priority is higher than priority ceiling of all currently locked semaphores

The protocol has:

• medium permission
• deadlock prevention and medium implementation

Question 8

What is a Race Condition?

Answer 8

Threads/processes are reading or writing shared data. This leads to results depending on the exact execution order of instructions.

Solution: critical sections and mutual exclusion

Question 9

What is understood under the term of performance degradation?

Answer 9

• Slow Memory Access
• changing threads
• time consuming thread synchronization

Question 10

What is not allowed in a critical section?

Answer 10

In critical sections shared resources are not allowed to be simultaneously accessed by competing tasks. There is only one process in the critical section.

Question 11

Thread

Answer 11

Different execution paths that operate on the same data.

Question 12

What is generally known as Memory Wall?

Answer 12

Memory speed does not keep up with processor speed.

Question 13

How is Concurrency defined?

Answer 13

In concurrent processes code or instructions are executable in parallel.

Question 14

Power Wall

Answer 14

High clock frequencies lead to high thermal output that can’t be handled.

Question 15

Define HLP.

Answer 15

HLP stands for Highest Locker Priority Protocol: task in critical section is given highest dynamic priority. With an easy implementation and medium permission, HLP prevents a possible deadlock.

Question 16

Multi-Threading

Answer 16

Parallel execution of independent subsequences is called multi-threading.

Question 17

Direct Blocking

Answer 17

Higher priority wants to access resource that is held by a lower priority task.

Question 18

In which protocol are tasks scheduled based on their active priority?

Answer 18

Priority Inheritance Protocol (PIP):

• Low permission
• no deadlock prevention and difficult implementation.

Question 19

Which resources does an execution of a process demand and on which factors does the execution time depend on?

Answer 19

• CPU time
• memory
• hardware resources

The execution time depends on:

1. CPU performance
2. Availability of resources
3. Input parameters
4. Delays

Question 20

Priority Inversion

Answer 20

Low-priority task blocks high priority process.

Question 21

How many memory regions do threads use?

Answer 21

Threads always use the same memory region which leads to…

• very low management effort
• high efficiency
• but also to conflicts (common memory space)

Question 22

Name limiting factors of memory development.

Answer 22

• Memory Wall
• Power Wall
• Performance Degradation.