Operating system questions
What are the basic functions of an operating system? - Operating system controls and coordinates the use of the hardware among the various applications programs for various uses. Operating system acts as resource allocator and manager. Since there are many possibly conflicting requests for resources the operating system must decide which requests are allocated resources to operating the computer system efficiently and fairly. Also operating system is control program which controls the user programs to prevent errors and improper use of the computer. It is especially concerned with the operation and control of I/O devices.
Why paging is used? - Paging is solution to external fragmentation problem which is to permit the logical address space of a process to be noncontiguous, thus allowing a process to be allocating physical memory wherever the latter is available.
While running DOS on a PC, which command would be used to duplicate the entire diskette? diskcopy
What resources are used when a thread created? How do they differ from those when a process is created? - When a thread is created the threads does not require any new resources to execute the thread shares the resources like memory of the process to which they belong to. The benefit of code sharing is that it allows an application to have several different threads of activity all within the same address space. Whereas if a new process creation is very heavyweight because it always requires new address space to be created and even if they share the memory then the inter process communication is expensive when compared to the communication between the threads.
What is virtual memory? - Virtual memory is hardware technique where the system appears to have more memory that it actually does. This is done by time-sharing, the physical memory and storage parts of the memory one disk when they are not actively being used.
What is Throughput, Turnaround time, waiting time and Response time? - Throughput – number of processes that complete their execution per time unit. Turnaround time – amount of time to execute a particular process. Waiting time – amount of time a process has been waiting in the ready queue. Response time – amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment).
What is the state of the processor, when a process is waiting for some event to occur? - Waiting state
What is the important aspect of a real-time system or Mission Critical Systems? - A real time operating system has well defined fixed time constraints. Process must be done within the defined constraints or the system will fail. An example is the operating system for a flight control computer or an advanced jet airplane. Often used as a control device in a dedicated application such as controlling scientific experiments, medical imaging systems, industrial control systems, and some display systems. Real-Time systems may be either hard or soft real-time. Hard real-time: Secondary storage limited or absent, data stored in short term memory, or read-only memory (ROM), Conflicts with time-sharing systems, not supported by general-purpose operating systems. Soft real-time: Limited utility in industrial control of robotics, Useful in applications (multimedia, virtual reality) requiring advanced operating-system features.
What is the difference between Hard and Soft real-time systems? - A hard real-time system guarantees that critical tasks complete on time. This goal requires that all delays in the system be bounded from the retrieval of the stored data to the time that it takes the operating system to finish any request made of it. A soft real time system where a critical real-time task gets priority over other tasks and retains that priority until it completes. As in hard real time systems kernel delays need to be bounded
What is the cause of thrashing? How does the system detect thrashing? Once it detects thrashing, what can the system do to eliminate this problem? - Thrashing is caused by under allocation of the minimum number of pages required by a process, forcing it to continuously page fault. The system can detect thrashing by evaluating the level of CPU utilization as compared to the level of multiprogramming. It can be eliminated by reducing the level of multiprogramming.
What is multi tasking, multi programming, multi threading? - Multi programming: Multiprogramming is the technique of running several programs at a time using timesharing. It allows a computer to do several things at the same time. Multiprogramming creates logical parallelism. The concept of multiprogramming is that the operating system keeps several jobs in memory simultaneously. The operating system selects a job from the job pool and starts executing a job, when that job needs to wait for any i/o operations the CPU is switched to another job. So the main idea here is that the CPU is never idle. Multi tasking: Multitasking is the logical extension of multiprogramming .The concept of multitasking is quite similar to multiprogramming but difference is that the switching between jobs occurs so frequently that the users can interact with each program while it is running. This concept is also known as time-sharing systems. A time-shared operating system uses CPU scheduling and multiprogramming to provide each user with a small portion of time-shared system. Multi threading: An application typically is implemented as a separate process with several threads of control. In some situations a single application may be required to perform several similar tasks for example a web server accepts client requests for web pages, images, sound, and so forth. A busy web server may have several of clients concurrently accessing it. If the web server ran as a traditional single-threaded process, it would be able to service only one client at a time. The amount of time that a client might have to wait for its request to be serviced could be enormous. So it is efficient to have one process that contains multiple threads to serve the same purpose. This approach would multithread the web-server process, the server would create a separate thread that would listen for client requests when a request was made rather than creating another process it would create another thread to service the request. To get the advantages like responsiveness, Resource sharing economy and utilization of multiprocessor architectures multithreading concept can be used.
What is hard disk and what is its purpose? - Hard disk is the secondary storage device, which holds the data in bulk, and it holds the data on the magnetic medium of the disk.Hard disks have a hard platter that holds the magnetic medium, the magnetic medium can be easily erased and rewritten, and a typical desktop machine will have a hard disk with a capacity of between 10 and 40 gigabytes. Data is stored onto the disk in the form of files.
What is fragmentation? Different types of fragmentation? - Fragmentation occurs in a dynamic memory allocation system when many of the free blocks are too small to satisfy any request. External Fragmentation: External Fragmentation happens when a dynamic memory allocation algorithm allocates some memory and a small piece is left over that cannot be effectively used. If too much external fragmentation occurs, the amount of usable memory is drastically reduced. Total memory space exists to satisfy a request, but it is not contiguous. Internal Fragmentation: Internal fragmentation is the space wasted inside of allocated memory blocks because of restriction on the allowed sizes of allocated blocks. Allocated memory may be slightly larger than requested memory; this size difference is memory internal to a partition, but not being used
What is DRAM? In which form does it store data? - DRAM is not the best, but it’s cheap, does the job, and is available almost everywhere you look. DRAM data resides in a cell made of a capacitor and a transistor. The capacitor tends to lose data unless it’s recharged every couple of milliseconds, and this recharging tends to slow down the performance of DRAM compared to speedier RAM types.
What is Dispatcher? - Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves: Switching context, Switching to user mode, Jumping to the proper location in the user program to restart that program, dispatch latency – time it takes for the dispatcher to stop one process and start another running.
What is CPU Scheduler? - Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them. CPU scheduling decisions may take place when a process: 1.Switches from running to waiting state. 2.Switches from running to ready state. 3.Switches from waiting to ready. 4.Terminates. Scheduling under 1 and 4 is non-preemptive. All other scheduling is preemptive.
What is Context Switch? - Switching the CPU to another process requires saving the state of the old process and loading the saved state for the new process. This task is known as a context switch. Context-switch time is pure overhead, because the system does no useful work while switching. Its speed varies from machine to machine, depending on the memory speed, the number of registers which must be copied, the existed of special instructions(such as a single instruction to load or store all registers).
What is cache memory? - Cache memory is random access memory (RAM) that a computer microprocessor can access more quickly than it can access regular RAM. As the microprocessor processes data, it looks first in the cache memory and if it finds the data there (from a previous reading of data), it does not have to do the more time-consuming reading of data from larger memory.
What is a Safe State and what is its use in deadlock avoidance? - When a process requests an available resource, system must decide if immediate allocation leaves the system in a safe state. System is in safe state if there exists a safe sequence of all processes. Deadlock Avoidance: ensure that a system will never enter an unsafe state.
What is a Real-Time System? - A real time process is a process that must respond to the events within a certain time period. A real time operating system is an operating system that can run real time processes successfully
Friday, December 1, 2006
Semaphore
Defination of Semaphore:
A mechanism for restricting access to critical sections of code
to a single user or process at a time.
Defination of Critical section :
A segment of code in which a thread uses resources (such as certain instance variables) that can be used by other threads, but that must not be used by them at the same time.
The Major activities of Operating system :
1.What are the five major activities of an OS in regard to process management?
Creation and deletion of both user and system processes
Suspension and resumption of processes
Mechanisms for processes
Mechanisms for process synchronization
Mechanisms for process communication
Mechanisms for deadlock handling
2.What are three major activities of an operating system in regard to memory
management?
Keep track of which parts of memory are currently being used & by whom
Decided which processes are to be loaded into memory
Allocated and de-allocate memory space as needed.
3. What are the three major activities of an operating system in regard to Secondary storage management
Free-space management
Storage allocation
Disk scheduling
4. What are the five major activities of an operating system in regard to file management?
Creation and deletion of files
Creation and deletion of directories
Supporting manipulating files and directories
Mapping of files onto secondary storage
Backup of files on stable storage media
5. List five services provided by an operating system.
Program execution
I/O operations
File-system manipulation
Communications
Error detection
6. List three methods for passing parameters needed by system calls.
Pass parameters in registers
Registers pass starting addresses of blocks of parameters
Parameters can be placed, or pushed, onto the stack by the program, and
popped off the stack by the operating system
7. Describe the differences among short-term, medium-term and long-term scheduling.
Short-term (CPU schedular) –
Selects from jobs in memory, those jobs which are ready to execute and allocates the CPU to them
Medium-term
Used especially with time-sharing systems as an intermediate scheduling level. A swapping scheme is implemented to remove partially run programs from memory and reinstate them later to continue where they left off
Long-term (job schedular)
Determines which jobs are brought into memory for processing.
8. What two advantages do threads have over multiple processes? What major disadvantages do they have?
Threads are very inexpensive to create and destroy, and they use very little resources while they exist. They do use CPU time but they don't have totally separate memory spaces.
Threads must trust each other not to damage shared data
Any program that may do more than one task at once could benefit from multitasking. For instance, a program that reads input, processes it and outputs it could have three threads, one for each task.
"Single-minded" process would not benefit from multiple threads; for instance, a program that displays the time of day.
9. What resources are used when a thread is created? How do they differ from those used when a process is created?
A context must be created, including a register set storage location for storage during context switching, and a local stack to record the procedure call arguments, return values, and return address, and thread-local storage. A process creation results in memory being allocated for program instructions and data, as well as thread-like storage. Code may also be loaded into the allocated memory
10. What are the differences between user-level threads and kernel-supported threads? Under what circumstances is one type"better" than the other?
User-level threads have no-kernel support, so they are very inexpensive to create, destroy and switch among. However, if one blocks, the whole process blocks. Kernel-supported threads are more expensive because system calls are needed to create and destroy them and the kernel must schedule them. They are more powerful because they are independently scheduled and block individually.
11. Describe five implementations of the create-new process mechanism.
a. parent continues executing
b. parent stops executing until children are done
c. parent and children share all variables
d. children share only a subset of parent's variables
e. parent and children share no common resources
12. Define the difference between preemptive and nonpreemptive scheduling?
State why strict nonpreemptive scheduling is unlikely to be used in a
computer centre.
Preemptive scheduling allows a process to be interrupted in the midst of its execution, taking the CPU away and allocating it to another process.
Nonpreemptive scheduling ensures that a process relinquishes control of the CPU only when it finishes with its current CPU burst.
13. What is indefinite blocking? How can it occur?
Also call starvation. A process with low priority that never gets a chance to execute. Can occur if CPU is continually busy with higher priority jobs.
14. What are the advantages and disadvantages of deterministic modelling?
Advantage: simple to compute
Disadvantage: results apply only to the specified job set.
15. What are the advantaes and disadvanteages of using implementation to compare various scheduling algorithm?
Advantage: completely accurate
Disadvantage: cost in coding, modifying operating system, modifying data structures
16. What is the meaning of the term busy waiting? What other kinds of waiting are there? Can busy waiting be avoid together?
A process is waiting an event to occur and it does so by executing instructions
A process is waiting for an event to occur in some waiting queue
(e.g. I/O, semaphore) and it does so without having the CPU assigned to it.
17. What is a critical section?
A section of code that only one process at a time can be executing.
18. What is the critical-section problem?
To design an algorithm that allows at most one process into the critical section at a time, without deadlock.
19. What three requirements must a solution to the critical section problem satisfy?
Mutual exclusion, progress, bounded waiting
20. What does execute " atomically" mean?
Execute as a group, with no context switches possible until all of the statements in the group are completed.
21. Define the wait-(postpone) operation, wait(S).
While semaphore is nonpositive, wait; when semaphore becomes positive, subtract one and exit.
22. Define the wakeup operation, signal(S).
Add 1 to the semaphore
23. Is it possible to have a deadlock involving only one single process?
No. this follows directly from the hold-and-wait condition.
24. List types of resources we might consider in deadlock problems on computers.
CPU cycles, memory spaces, files, I/O devices, tape drive, printers
25. What are the four necessary conditions needed before deadlock can occur?
a. at least one resource must be held in a nonsharable mode
b. a process holding at least one resources is waiting for more resources held by other processes
c. resources cannot be preempted
d. there must be a circular waiting
26. What is a system resource-allocation graph (SRAG) in general?
A graph which shows the resources and processes, and the relationships among them.
27. List three overall strategies in handling deadlocks.
a. ensure system will never enter a deadlock state
b. allow deadlocks, but devise schemes to recover from them
c. pretend deadlocks don't happen
28. To avoid deadlock, what information do we need on the current process ?
Simplest scheme: each process declares the maximum number of resources it may need.
29. What is a safe state?
A set of resource allocations such that the system can allocate resources to each process (up to its max.) in some order, and still avoid a deadlock.
30. List the data structure needed for the banker's algorithm.
available vector: Available(m)
demand matrix: Max(n, m)
allocation matrix: Allocation(n, m)
needed matrix: Need(n, m)
31. Summarize the banker's algorithm.
If request for process i exceeds its need, error has occurred
If request of process i exceeds available resources, process i must wait.
The system temporarily allocates the resources process i wants; if the state is unsafe, the allocation is postponed.
32. How can we determine whether current state is "safe" in systems with only one instance of each resource type?
State is unsafe if any cycle exists
33. List three options for breaking an existing deadlock.
Violate mutual exclusion, risking data
Abort a process
Preempt resources of some process
34. What three issues must be considered in the case of preemption?
Select a victim to be preempted
Determine how far back to rollback the victim
Determine means for preventing that process from being "starved."
A mechanism for restricting access to critical sections of code
to a single user or process at a time.
Defination of Critical section :
A segment of code in which a thread uses resources (such as certain instance variables) that can be used by other threads, but that must not be used by them at the same time.
The Major activities of Operating system :
1.What are the five major activities of an OS in regard to process management?
Creation and deletion of both user and system processes
Suspension and resumption of processes
Mechanisms for processes
Mechanisms for process synchronization
Mechanisms for process communication
Mechanisms for deadlock handling
2.What are three major activities of an operating system in regard to memory
management?
Keep track of which parts of memory are currently being used & by whom
Decided which processes are to be loaded into memory
Allocated and de-allocate memory space as needed.
3. What are the three major activities of an operating system in regard to Secondary storage management
Free-space management
Storage allocation
Disk scheduling
4. What are the five major activities of an operating system in regard to file management?
Creation and deletion of files
Creation and deletion of directories
Supporting manipulating files and directories
Mapping of files onto secondary storage
Backup of files on stable storage media
5. List five services provided by an operating system.
Program execution
I/O operations
File-system manipulation
Communications
Error detection
6. List three methods for passing parameters needed by system calls.
Pass parameters in registers
Registers pass starting addresses of blocks of parameters
Parameters can be placed, or pushed, onto the stack by the program, and
popped off the stack by the operating system
7. Describe the differences among short-term, medium-term and long-term scheduling.
Short-term (CPU schedular) –
Selects from jobs in memory, those jobs which are ready to execute and allocates the CPU to them
Medium-term
Used especially with time-sharing systems as an intermediate scheduling level. A swapping scheme is implemented to remove partially run programs from memory and reinstate them later to continue where they left off
Long-term (job schedular)
Determines which jobs are brought into memory for processing.
8. What two advantages do threads have over multiple processes? What major disadvantages do they have?
Threads are very inexpensive to create and destroy, and they use very little resources while they exist. They do use CPU time but they don't have totally separate memory spaces.
Threads must trust each other not to damage shared data
Any program that may do more than one task at once could benefit from multitasking. For instance, a program that reads input, processes it and outputs it could have three threads, one for each task.
"Single-minded" process would not benefit from multiple threads; for instance, a program that displays the time of day.
9. What resources are used when a thread is created? How do they differ from those used when a process is created?
A context must be created, including a register set storage location for storage during context switching, and a local stack to record the procedure call arguments, return values, and return address, and thread-local storage. A process creation results in memory being allocated for program instructions and data, as well as thread-like storage. Code may also be loaded into the allocated memory
10. What are the differences between user-level threads and kernel-supported threads? Under what circumstances is one type"better" than the other?
User-level threads have no-kernel support, so they are very inexpensive to create, destroy and switch among. However, if one blocks, the whole process blocks. Kernel-supported threads are more expensive because system calls are needed to create and destroy them and the kernel must schedule them. They are more powerful because they are independently scheduled and block individually.
11. Describe five implementations of the create-new process mechanism.
a. parent continues executing
b. parent stops executing until children are done
c. parent and children share all variables
d. children share only a subset of parent's variables
e. parent and children share no common resources
12. Define the difference between preemptive and nonpreemptive scheduling?
State why strict nonpreemptive scheduling is unlikely to be used in a
computer centre.
Preemptive scheduling allows a process to be interrupted in the midst of its execution, taking the CPU away and allocating it to another process.
Nonpreemptive scheduling ensures that a process relinquishes control of the CPU only when it finishes with its current CPU burst.
13. What is indefinite blocking? How can it occur?
Also call starvation. A process with low priority that never gets a chance to execute. Can occur if CPU is continually busy with higher priority jobs.
14. What are the advantages and disadvantages of deterministic modelling?
Advantage: simple to compute
Disadvantage: results apply only to the specified job set.
15. What are the advantaes and disadvanteages of using implementation to compare various scheduling algorithm?
Advantage: completely accurate
Disadvantage: cost in coding, modifying operating system, modifying data structures
16. What is the meaning of the term busy waiting? What other kinds of waiting are there? Can busy waiting be avoid together?
A process is waiting an event to occur and it does so by executing instructions
A process is waiting for an event to occur in some waiting queue
(e.g. I/O, semaphore) and it does so without having the CPU assigned to it.
17. What is a critical section?
A section of code that only one process at a time can be executing.
18. What is the critical-section problem?
To design an algorithm that allows at most one process into the critical section at a time, without deadlock.
19. What three requirements must a solution to the critical section problem satisfy?
Mutual exclusion, progress, bounded waiting
20. What does execute " atomically" mean?
Execute as a group, with no context switches possible until all of the statements in the group are completed.
21. Define the wait-(postpone) operation, wait(S).
While semaphore is nonpositive, wait; when semaphore becomes positive, subtract one and exit.
22. Define the wakeup operation, signal(S).
Add 1 to the semaphore
23. Is it possible to have a deadlock involving only one single process?
No. this follows directly from the hold-and-wait condition.
24. List types of resources we might consider in deadlock problems on computers.
CPU cycles, memory spaces, files, I/O devices, tape drive, printers
25. What are the four necessary conditions needed before deadlock can occur?
a. at least one resource must be held in a nonsharable mode
b. a process holding at least one resources is waiting for more resources held by other processes
c. resources cannot be preempted
d. there must be a circular waiting
26. What is a system resource-allocation graph (SRAG) in general?
A graph which shows the resources and processes, and the relationships among them.
27. List three overall strategies in handling deadlocks.
a. ensure system will never enter a deadlock state
b. allow deadlocks, but devise schemes to recover from them
c. pretend deadlocks don't happen
28. To avoid deadlock, what information do we need on the current process ?
Simplest scheme: each process declares the maximum number of resources it may need.
29. What is a safe state?
A set of resource allocations such that the system can allocate resources to each process (up to its max.) in some order, and still avoid a deadlock.
30. List the data structure needed for the banker's algorithm.
available vector: Available(m)
demand matrix: Max(n, m)
allocation matrix: Allocation(n, m)
needed matrix: Need(n, m)
31. Summarize the banker's algorithm.
If request for process i exceeds its need, error has occurred
If request of process i exceeds available resources, process i must wait.
The system temporarily allocates the resources process i wants; if the state is unsafe, the allocation is postponed.
32. How can we determine whether current state is "safe" in systems with only one instance of each resource type?
State is unsafe if any cycle exists
33. List three options for breaking an existing deadlock.
Violate mutual exclusion, risking data
Abort a process
Preempt resources of some process
34. What three issues must be considered in the case of preemption?
Select a victim to be preempted
Determine how far back to rollback the victim
Determine means for preventing that process from being "starved."
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