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A mutual exclusion object, or MUTEX, is an object created for use in computer programs. The essential function of MUTEX is to make it possible for a multiple program thread to make use of a single resource. Generally, the functionality of mutual exclusion objects is to allow this use of a single resource by creating an environment where the access to the resource is continually passed back and forth between the various aspects of the program.
A single process can have multiple threads that share global data and address space with other threads running in the same process, and therefore can operate on the same data set easily. Processes do not share address space and a different mechanism must be used if they are to share data.
If we consider running a word processing program to be a process, then the auto-save and spell check features that occur in the background are different threads of that process which are all operating on the same data set (your document).
process:
In computing, a process is an instance of a computer program that is being sequentially executed[1] by a computer system that has the ability to run several computer programs concurrently.
Thread:
A single process may contain several executable programs (threads) that work together as a coherent whole. One thread might, for example, handle error signals, another might send a message about the error to the user, while a third thread is executing the actual task of the...
INODE is a pointer to a block on the disk and it is unique.
Inode is an unique number. Inode holds metadata of files.
Virtual memory like as a temporary storage area.It consists of page table.In this pages are divided into frames.It is a continuous memory allocation.It is also called logical memory.
Preemptive multitask
A hardware or software flag.
In multitasking systems, a semaphore is a variable with a value that indicates the status of a common resource.
Its used to lock the resource that is being used.
A process needing the resource checks the semaphore to determine the resource's status
and then decides how to proceed.
Process Termination:
->Abort all deadlocked processes.
->Abort one process at a time until the deadlock cycle is eliminated.
->In which order should we choose to abort?
Priority of the process.
How long process has computed, and how much longer to completion.
Resources the process has used.
Resources process needs to complete.
How many processes will need to be terminated?
Is process interactive or batch?
Resource Preemption:
->Selecting a victim ? minimize cost.
->Rollback ? return to some safe state, restart process for that state.
->Starvation ? same process may always be picked as victim, include number of rollback in cost factor.
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.
Semiconductor memories are of two types: RAM (random access memory) and ROM (read only memory).
RAM is a read/write memory. Information can be written into and read from a RAM. It is volatile memory.
It stores information so long as power supply is on.
ROM is permanent type memory. Its contents are not lost when power supply goes off. the user cannot write into a ROM.Its contents are decided by the manufacturer and written at the time of manufacture. Programmable ROMs are also available. They are called PROMs.
Address binding of instructions and data to memory addresses can happen at three different stages
Compile time: If memory location known a priori, absolute code can be generated; must recompile code if starting location changes.
Load time: Must generate relocatable code if memory location is not known at compile time.
Execution time: Binding delayed until run time if the process can be moved during its execution from one memory segment to another. Need hardware support for address maps (e.g., base and limit registers).
Multistep Processing of a User Program
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.
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.
->Sequence <P1, P2? Pn> is safe if for each Pi, the resources that Pi can still request can be satisfied by currently available resources + resources held by all the Pj, with j<I.
If Pi resource needs are not immediately available, then Pi can wait until all Pj have finished.
When Pj is finished, Pi can obtain needed resources, execute, return allocated resources, and terminate.
When Pi terminates, Pi+1 can obtain its needed resources, and so on.
->Deadlock Avoidance ?ensure that a system will never enter an unsafe state.
Micro-Kernel: A micro-kernel is a minimal operating system that performs only the essential functions of an operating system. All other operating system functions are performed by system processes.
Monolithic: A monolithic operating system is one where all operating system code is in a single executable image and all operating system code runs in system mode.
An interpreter reads one instruction at a time and carries out the actions implied by that instruction. It does not perform any translation. But a compiler translates the entire instructions.
Process system time (turn around time) = process cpu time + process waiting time
Deadlock can arise if four conditions hold simultaneously.
Mutual exclusion:
only one process at a time can use a resource.
Hold and wait:
a process holding at least one resource is waiting to acquire additional resources held by other processes.
No preemption:
a resource can be released only voluntarily by the process holding it, after that process has completed its task.
Circular wait:
there exists a set {P0, P1, ?, P0} of waiting processes such that P0 is waiting for a resource that is held by P1, P1 is waiting for a resource that is held by P
2, Pn1 is waiting for a resource that is held by
Pn, and P0 is waiting for a resource that is held by P0.
For the conventional time ?shared processes, Linux uses a prioritized, credit-based algorithm. Each process possesses a certain number of scheduling credits; when a new task must be chosen to run, the process with most credits is selected. Every time that a timer interrupt occurs, the currently running process loses one credit; when its credits reaches zero, it is suspended and another process is chosen. If no runnable processes have any credits, then Linux performs a recrediting operation, adding credits to every process in the system (rather than just to the runnable ones), according to the following rule:
Credits = credits/2 + priority
The above scheduling class is used for time-shared process and the in Linux for the real-time scheduling is simpler it uses scheduling classes: first come, first served
(FCFS), and round-robin (RR) .In both cases, each process has a priority in addition to its scheduling class. In time-sharing scheduling, however, processes of different priorities can still compete with one another to some extent; in real-time scheduling, the scheduler always runs the process with the highest priority. Among processes of
equal priority, it runs the process that has been waiting longest. The only difference between FCFS and RR scheduling is that FCFS processes continue to run until they either exit or block, whereas a round-robin process will be preempted after a while and will be moved to the end of the scheduling queue.
It is called light weight process to emphasize the fact that a thread is like a process but is more efficient and uses fewer resources( n hence "lighter")and they also share the address space.
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.
Demand Paging: Demand paging is the paging policy that a page is not read into memory until it is requested, that is, until there is a page fault on the page.
Page fault interrupt: A page fault interrupt occurs when a memory reference is made to a page that is not in memory. The present bit in the page table entry will be found to be off by the virtual memory hardware and it will signal an interrupt.
Trashing: The problem of many page faults occurring in a short time, called page thrashing
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