OS notes 4 CS3 batch

Fragmentation: As processes are loaded and removed from memory, the free memory space is broken into little pieces. It happens after sometimes that processes cannot be allocated to memory blocks considering their small size and memory blocks remains unused. This problem is known as Fragmentation.

Fragmentation is of two types

1	External fragmentation	Total memory space is enough to satisfy a request or to reside a process in it, but it is not contiguous so it can not be used.
2	Internal fragmentation	Memory block assigned to process is bigger. Some portion of memory is left unused as it can not be used by another process.

External fragmentation can be reduced by compaction or shuffle memory contents to place all free memory together in one large block. To make compaction feasible, relocation should be dynamic.

Segmentation : Segmentation is a technique to break memory into logical pieces where each piece represents a group of related information. For example ,data segments or code segment for each process, data segment for operating system and so on. Segmentation can be implemented using or without using paging. Unlike paging, segment are having varying sizes and thus eliminates internal fragmentation. External fragmentation still exists but to lesser extent.

Address generated by CPU is divided into . . . .

• Segment number (s) -- segment number is used as an index into a segment table which contains base address of each segment in physical memory and a limit of segment.
• Segment offset (o) -- segment offset is first checked against limit and then is combined with base address to define the physical memory address.

The Critical-Section Problem:

n processes all competing to use some shared data. Each process has a code segment, called critical section, in which the shared data is accessed.

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Problem – ensure that when one process is executing in its critical section, no other process is allowed to execute in its critical section.

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Structure of process Pi

repeat

entry section

critical section

exit section

reminder section

until false

Solution to Critical-Section Problem

1. Mutual Exclusion: If process Pi is executing in its critical section, then no other processes can be executing in their critical sections.

2. Progress: If no process is executing in its critical section and there exist some processes that wish to enter their critical section, then the selection of the processes that will enter the critical section next cannot

be postponed indefinitely.

3. Bounded Waiting: A bound must exist on the number of times that their processes are allowed to enter their critical sections after a process has made a request to enter its critical section and before that request is granted.

Semaphores

Semaphore is a synchronization tool which can be used to deal with the critical-section problem. A semaphore is a protected variable whose value can be accessed and altered only by the operations P (wait) and V (signal) and an initialization operation we shall call semaphore-initialize.

Two Types of Semaphores

·       Counting semaphore – integer value can range over an unrestricted domain.

·       Binary semaphore – integer value can range only between 0 and 1; can be simpler to implement.

Starvation – indefinite blocking. A process may never be removed from the semaphore queue in which it is suspended.

THRASHING:

Working set: collection of memory currently being used by a process .If all working sets do not fit in memory, thrashing occurs.

–  one page replaces another

–  Percentage of accesses that generate page faults skyrockets

 Typical solution: swap out the entire processes

Thrashing ≡ a process is busy swapping pages in and out.

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?

The thrashing is caused by mainly by the file indexing in vista and not with your page file. Indexing usually happens in the first 15mins after you boot up - it’s normal. Your page file seems very large - try to make it a fixed size of about 1024Mb on a second HD if possible. Have you inserted a spare USB memory stick for ready boost - it helps reduce thrashing later on.

How does the operating system detect thrashing?

An operating system would have no idea that thrashing is happening. The OS would be responsible for making use of virtual memory to make up for the lack of physical RAM during a particular computing session, but would have no concept that this is causing issues with hardware.

Once thrashing is detected, what can the operating system do to eliminate it?

Thrashing is usually a symptom of low memory in your system. To minimize it, try adding more memory to your computer or try to cut down on use interface elements that use extra memory like complicated background images and detailed icons.