Memory allocation

Question 1

What Is Memory Allocation?

Answer 1

Memory allocation is how the operating system gives parts of memory (RAM) to processes so they can run programs.
It decides:
Where a program goes in memory
How much memory it gets
How to manage that memory while the program runs

Question 2

Two Types of Memory Allocation

Answer 2

Static Allocation:
Memory is assigned at compile time.

Once set, it doesn’t change.

Example: global variables, fixed arrays

Dynamic Allocation:
Memory is assigned at runtime, as needed.

Can grow or shrink using functions like malloc() or free() in C.

Used for structures like linked lists, trees, etc

Question 3

Problem with Variable Size Allocation

Answer 3

Obvious approach, allocate just memory requested
* Placing things in memory (or on disk, etc.) with such
approach rapidly becomes big problem…
* If we free a large area of memory, disk, …– Tend to place something smaller in space, leaving a gap,
which prevents us from then storing something larger which leads to fragmentation

Question 4

Fragmentation

Answer 4

Can become severe problem– Sum of un-allocated space can be large– Requires complex de-fragmentation
* Shuffling storage contents– Must be invisible to running processes

Question 5

Making best use of
Variable Size Regions

Answer 5

Dynamic storage allocation schemes…
– First fit and Next fit
* Always allocate first region of sufficient size found
– Best fit
* Always allocate smallest memory hole of sufficient size
– Worst fit
* Always allocate largest memory hole (of sufficient size)
* Aims to increase chance of later allocation

Question 6

Variable vs. Fixed Sized Schemes

Answer 6

Variable/ Dynamic allocation schemes:
Allocate requested amount of memory– External fragmentation
* Space left outside/ between allocated memory

Fixed size schemes: Always allocate memory in fixed-sized blocks– Internal fragmentation
* Processes typically get more memory than requested,
rounded up to n * block size, which generally remains unused

Question 7

Fragmentation Issues

Answer 7

Internal Fragmentation: Wasted space inside an allocated block.

External Fragmentation: Wasted space between allocated blocks.

Question 8

Buddy Allocation Scheme

Answer 8

Idea is that a memory area can be split in two–Two buddies
- Likewise, two buddies can be combined– Reforming original pair into larger block
- Balances speed and memory use

Question 9

Slab Allocation

Answer 9

Used in kernels.
Preallocates commonly used memory blocks.
Fast and efficient for objects like process control blocks.