Paper 2 CS System Software & Operating System Part 6

Contiguous memory allocation is a classical memory allocation model that assigns a process consecutive memory blocks (that is, memory blocks having consecutive addresses).

“→ Internal fragmentation is the wasted space within each allocated block because of rounding up from the actual requested allocation to the allocation granularity.
→ External fragmentation is the various free spaced holes that are generated in either your memory or disk space.
→ Both the first-fit and best-fit strategies for memory allocation suffer from external fragmentation. As the processes are loaded and removed from memory, the free memory space is broken into little pieces.
→ External fragmentation exists when there is enough total memory space to satisfy a request, but the available spaces are not contiguous.
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“● Mirroring copies identical data onto more than one drive.
● Striping partitions each drive’s storage space into units ranging from a sector (512 bytes) up to several megabytes.
●The stripes of all the disks are interleaved and addressed in order.
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An operation (or set of operations) is atomic or uninterruptible if it appears to the rest of the system to occur instantaneously.

“● Fuzzy logic is an approach to computing based on “”degrees of truth”” rather than the usual “”true or false”” (1 or 0) Boolean logic on which the modern computer is based.
● An embedded system is a controller programmed and controlled by a real-time operating system (RTOS) with a dedicated function within a larger mechanical or electrical system, often with real-time computing constraints.
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“● An access method is a program or a hardware mechanism that moves data between the computer​ and an outlying device such as a hard disk (or other form of storage) or a display terminal.
● Memory can be accessed Sequential, direct, random and associative manner.
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“Monolithic kernel means that the whole operating system runs in kernel mode (i.e. highly privileged by the hardware). That is, no part of the OS runs in user mode (lower privilege). Only applications on top of the OS run in user mode.
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Scheduler activations are a threading mechanism that, when implemented in an operating system’s process scheduler, provide kernel-level thread functionality with user-level thread flexibility and performance

Peterson’s solution is a concurrent programming algorithm for mutual exclusion that allows two or more processes to share a single-use resource without conflict, using only shared memory for communication

“→ In shortest job first scheduling algorithm, the processor selects the process with the smallest execution time to execute next.
→ SJF scheduling algorithm will result in the maximum throughput
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→ Object Linking & Embedding (OLE) is a proprietary technology developed by Microsoft that allows embedding and linking to documents and other objects.
→ For developers, it brought OLE Control Extension (OCX), a way to develop and use custom user interface elements. On a technical level, an OLE object is any object that implements the IOleObject interface, possibly along with a wide range of other interfaces, depending on the object’s needs.

“→ Ageing is a scheduling technique used to avoid starvation. Fixed priority scheduling is a scheduling discipline, in which tasks queued for utilizing a system resource are assigned a priority each.
→ A task with a high priority is allowed to access a specific system resource before a task with a lower priority is allowed to do the same.
→ A disadvantage of this approach is that tasks assigned with a lower priority may be starved when a large number of high priority tasks are queued. Aging is used to gradually increase the priority of a task, based on its waiting time in the ready queue.
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“→ A light-weight process (LWP) is a means of achieving multitasking.
→ A LWP runs in user space on top of a single kernel thread and shares its address space and system resources with other LWPs within the same process. Multiple user level threads, managed by a thread library, can be placed on top of one or many LWPs – allowing multitasking to be done at the user level, which can have some performance benefits.
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“→ Actually they are given single job, so when we are using round robin algorithm it won’t perform any context switch because we are executing same job.
→ But according to their intention will cause total 6 switches. 0-2, 2-4, 4-6, 6-8, 8-10, 10-12, 12-14. We are initially started with 0. So, It will take 6 connected switches
Note: We thought they made this question wrong.”

Non-volatile memory (NVM) or non-volatile storage is a type of computer memory that can retrieve stored information even after having been power cycled. In contrast, volatile memory needs constant power in order to retain data. Examples of non-volatile memory include read-only memory, flash memory, ferroelectric RAM, most types of magnetic computer storage devices (e.g. hard disk drives, solid state drives, floppy disks, and magnetic tape), optical discs, and early computer storage methods such as paper tape and punched cards.

Process Control Block (PCB, also called Task Controlling Block, Entry of the Process Table, Task Struct, or Switchframe) is a data structure in the operating system kernel containing the information needed to manage the scheduling of a particular process.

“There are three processes, so there is a possibility to occur deadlock when we are taking 3 resources. If we are taking 3+1 resources means it will never occur deadlock.
Note: If there are ‘n’ processes then ‘n+1’ resources allocated, will never occur deadlock.”

“• Segmentation and paging avoids external fragmentation but still it suffers from internal fragmentation.
• Internal fragmentation is the wasted space within each allocated block because of rounding up from the actual requested allocation to the allocation granularity.
• External fragmentation is the various free spaced holes that are generated in either your memory or disk space. External fragmented blocks are available for allocation, but may be too small to be of any use.
• Resource starvation is a problem encountered in concurrent computing where a process is perpetually denied necessary resources to process its work. Starvation may be caused by errors in a scheduling or mutual exclusion algorithm.
• Ageing is a scheduling technique used to avoid starvation.”

Virtual memory technique allows execution of programs larger than the size of physical memory. Demand paging we are using for virtual memory technique.

In FIFO page replacement policy sometimes behaves differently. The abnormal behaviour is nothing but Belady’s anomaly. Belady’s anomaly is nothing but adding more frames may cause more page faults.

POSIX Threads, usually referred to as pthreads, is an execution model that exists independently from a language, as well as a parallel execution model. It allows a program to control multiple different flows of work that overlap in time. Each flow of work is referred to as a thread, and creation and control over these flows is achieved by making calls to the POSIX Threads API. POSIX Threads is an API defined by the standard POSIX.1c, Threads extensions (IEEE Std 1003.1c-1995).

“Banker’s Algorithm :
Banker’s algo can be applied even when Resource-system have multiple instances of each resource type. It can also be applied when Resource-system have single instance of each resource type But in that case, It would be less efficient than Resource-Allocation Graph Algorithm.”

“The term dispatch latency describes the amount of time it takes for a system to respond to a request for a process to begin operation. With a scheduler written specifically to honor application priorities, real-time applications can be developed with a bounded dispatch latency.
Note: The time taken by the dispatcher to stop one process and start another running is known as dispatch latency.”

A physical address (also real address, or binary address), is a memory address that is represented in the form of a binary number on the address bus circuitry in order to enable the data bus to access a particular storage cell of main memory, or a register of memory mapped I/O device.

“New→ The process is being created
Running→ Instructions are being executed
Ready→ The process is waiting to be assigned to a processor
Waiting→ The process is waiting for some event to occur(such as IO exception or reception of a signal)
Terminated: The process has finished execution.
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“→ Demand paging follows that pages should only be brought into memory if the executing process demands them. This is often referred to as lazy evaluation as only those pages demanded by the process are swapped from secondary storage to main memory. Contrast this to pure swapping, where all memory for a process is swapped from secondary storage to main memory during the process startup.
→ Demand paging is a method of virtual memory management. In a system that uses demand paging, the operating system copies a disk page into physical memory only if an attempt is made to access it and that page is not already in memory (i.e., if a page fault occurs).
→ It follows that a process begins execution with none of its pages in physical memory, and many page faults will occur until most of a process working set of pages are located in physical memory. This is an example of a lazy loading technique.
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“Semaphores are used to solve the problem of process synchronization.
There are two types of semaphores
1. Counting Semaphore
2. Binary semaphore
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“S1: True:A small page size causes large page tables
S2: True:Internal fragmentation is increase with small pages. Actually Internal fragmentation is decrease with small pages
S3:True:I/O transfers are more efficient with large pages”

Circular scanning works just like the elevator to some extent. It begins its scan toward the nearest end and works it way all the way to the end of the system. Once it hits the bottom or top it jumps to the other end and moves in the same direction. Keep in mind that the huge jump doesn’t count as a head movement.