OPERATING SYSTEM ENGINEERING NOTES 1

OPERATING SYSTEM ENGINEERING NOTES 1




1. Explain the concept of Reentrancy?

It is a useful, memory-saving technique for multiprogrammed timesharing systems.

A Reentrant Procedure is one in which multiple users can share a single copy of a

program during the same period. Reentrancy has 2 key aspects: The program code

cannot modify itself, and the local data for each user process must be stored

separately. Thus, the permanent part is the code, and the temporary part is the

pointer back to the calling program and local variables used by that program. Each

execution instance is called activation. It executes the code in the permanent part,

but has its own copy of local variables/parameters. The temporary part associated

with each activation is the activation record. Generally, the activation record is

kept on the stack.

Note

: A reentrant procedure can be interrupted and called by an interrupting

program, and still execute correctly on returning to the procedure.

2. Explain Belady's Anomaly?

Also called FIFO anomaly. Usually, on increasing the number of frames allocated

to a process virtual memory, the process execution is faster, because fewer page

faults occur. Sometimes, the reverse happens, i.e., the execution time increases

even when more frames are allocated to the process. This is Belady's Anomaly.

This is true for certain page reference patterns.

3. What is a binary semaphore? What is its use?

A binary semaphore is one, which takes only 0 and 1 as values. They are used to

implement mutual exclusion and synchronize concurrent processes.

4. What is thrashing?

It is a phenomenon in virtual memory schemes when the processor spends most of

its time swapping pages, rather than executing instructions. This is due to an

inordinate number of page faults.

5. List the Coffman's conditions that lead to a deadlock.

1.

Mutual Exclusion: Only one process may use a critical resource at a time.

2.

Hold & Wait: A process may be allocated some resources while waiting for

others.

3.

No Pre-emption: No resource can be forcible removed from a process

holding it.

4.

Circular Wait: A closed chain of processes exist such that each process

holds at least one resource needed by another process in the chain.

6. What are short, long and medium-term scheduling?

Long term scheduler

determines which programs are admitted to the system for

processing. It controls the degree of multiprogramming. Once admitted, a job

becomes a process.

Medium term scheduling

is part of the swapping function. This relates to

processes that are in a blocked or suspended state. They are swapped out of realmemory

until they are ready to execute. The swapping-in decision is based on

memory-management criteria.

Short term scheduler

, also know as a dispatcher executes most frequently, and

makes the finest-grained decision of which process should execute next. This

scheduler is invoked whenever an event occurs. It may lead to interruption of one

process by preemption.

7. What are turnaround time and response time?

Turnaround time is the interval between the submission of a job and its

completion. Response time is the interval between submission of a request, and the

first response to that request.

8. What are the typical elements of a process image?

User data

: Modifiable part of user space. May include program data, user stack

area, and programs that may be modified.

User program

: The instructions to be executed.

System Stack

: Each process has one or more LIFO stacks associated with it. Used

to store parameters and calling addresses for procedure and system calls.

Process control Block

(PCB): Info needed by the OS to control processes.

9. What is the Translation Lookaside Buffer (TLB)?

In a cached system, the base addresses of the last few referenced pages is

maintained in registers called the TLB that aids in faster lookup. TLB contains

those page-table entries that have been most recently used. Normally, each virtual

memory reference causes 2 physical memory accesses- one to fetch appropriate

page-table entry, and one to fetch the desired data. Using TLB in-between, this is

reduced to just one physical memory access in cases of TLB-hit.

10. What is the resident set and working set of a process?

Resident set is that portion of the process image that is actually in real-memory at a

particular instant. Working set is that subset of resident set that is actually needed

for execution. (Relate this to the variable-window size method for swapping

techniques.)