MEMORY MANAGEMENT- Asiri Rathnayake

LECTURE CONTENT

Memory management (Basics) Introduction Relocation / Addressing Protection / Sharing Placement / Replacement

Memory management techniques Partitioning

Fixed partitioning Dynamic partitioning Buddy system

Simple paging Simple segmentation

Virtual memory

FROM THE TEXT (CHAPTER 07/08)

7.1 Memory management requirements 7.2 Memory partitioning 7.3 Paging 7.4 Segmentation 8.1 Hardware and control structures (read)

Inverted page tables Page size

8.2 Operating system software (read) Basic page replacement algorithms

8.3 Unix and Solaris memory management (optional) 8.4 Linux memory management (optional) 8.5 Windows memory management (optional)

MEMORY MANAGEMENT (BASICS)

Introduction Relocation / Addressing Protection / Sharing Placement / Replacement

INTRODUCTION

A program needs to be brought into memory for execution

In a single-tasking operating system memory management is trivial. But in a multi-tasking operating system it is a major concern (why?)

In memory management we are concerned about subdividing main memory to accommodate multiple processes

It is important to manage memory in such a way that there is a steady supply of ready processes for the processor (why?)

INTRODUCTION…

Usually the task of memory management involves both software and hardware components

Note: Until the discussion of virtual memory, we

assume that the whole process image should be loaded in memory for execution

RELOCATION / ADDRESSING

A program consists of a sequence of instructions and some of these instructions refer other memory locations A function within the same program. A variable pointing to a data structure in heap. A global variable More…

RELOCATION / ADDRESSING…

RELOCATION / ADDRESSING…

What if these memory references contain actual physical memory addresses (locations in RAM)? It would be required that the process image be

loaded into a specific area of main memory to be executed.

There for, memory references within a process use logical addresses as opposed to physical addresses

These logical addresses generated by the process at runtime (addresses emitted by CPU) are translated into physical addresses by MMU

This scheme allows process images to be relocated into any area of main memory as required

PROTECTION / SHARING

A memory management scheme should be capable of providing protection between processes

A process should not be allowed to read/write memory locations of some other process without permission

Still, the memory management scheme should be flexible enough to provide sharing of memory between processes when required – there are advantages of sharing memory between processes

PLACEMENT / REPLACEMENT

A memory management scheme usually keeps track of free memory slots currently available in the system

The algorithm / policy used in allocating a free memory slot to a newly admitted process is known as the placement scheme

The algorithm / policy used in replacing (swapping out) an existing process in order to provide room for a higher priority process is known as the replacement scheme

MEMORY MANAGEMENT TECHNIQUES

Partitioning Fixed partitioning Dynamic partitioning Buddy system

Simple paging Simple segmentation

PARTITIONING – FIXED PARTITIONING

A very simplistic memory management scheme

Main memory is divided into partitions at the system generation time (Hardware)

Either equal-size partitions or unequal-size partitions may be used

PARTITIONING – FIXED PARTITIONING…

PARTITIONING – FIXED PARTITIONING…

Having equal-size partitions poses two major problems: A program may be too big to fit in a partition. Main memory utilization is extremely inefficient

due to internal fragmentation – no matter how small the process image is, it will still occupy a whole partition (internally un-used space).

Unequal-size partitions can lessen both of these issues (how?)

PARTITIONING – FIXED PARTITIONING…

Placement algorithm: With equal-size partitions it’s a trivial task;

simply allocate a free partition. With unequal-size partitions the smallest

available partition that can hold the process image is selected.

Replacement algorithm: With equal-size partitions it’s a scheduling

decision among all partitions. (provided: relocation is possible)

With unequal-size partitions it’s a scheduling decision among all large-enough partitions. (provided: relocation is possible)

PARTITIONING – DYNAMIC PARTITIONING

When a process is brought into memory, it is allocated a contiguous chunk of memory to suit its need

The operating system keeps track of free memory slots available for new processes

Dynamic partitioning suffers from external fragmentation – as time goes on, memory external to all partitions (free memory) becomes too much fragmented

A point will arrive where a new process can’t be admitted even if there is enough total free memory in the system

There for it is necessary to perform a costly compaction operation from time to time to make sure that the available free memory is in a contiguous chunk

PARTITIONING – DYNAMIC PARTITIONING…

PARTITIONING – DYNAMIC PARTITIONING…

Placement algorithm: placement algorithm has a huge impact on the level of external fragmentation. Best fit: Select the smallest free memory slot

that is large enough. External fragmentation is high (why?) – worst

performer First-fit: Select the first free memory slot that is

large enough. Litters up the front of the free memory list which is a

good thing (why?) – best performer Next-fit: Start from the previous allocation point

and select the next free memory slot that is large enough. Litters up the end of the free memory slot – slightly

behind first-fit

Replacement algorithm: not discussed

PARTITIONING – BUDDY SYSTEM

A reasonable compromise to both fixed and dynamic partitioning schemes: Memory is allocated in powers of 2 2L = smallest size block that can be allocated 2U = largest block that can be allocated (size of

the entire memory available for processes) When a process of size S needs to be allocated:

If 2U > S ≥ 2U-1 the entire memory block is allocated Else memory is partitioned into two buddies of size 2U-1

If 2U-1 > S ≥ 2U-2 first buddy of above two buddies is allocated

The process continues until a suitable memory block is found

PARTITIONING – BUDDY SYSTEM…

PARTITIONING – BUDDY SYSTEM…

Buddy system is a reasonable compromise to both fixed and dynamic partitioning schemes It reduces internal fragmentation – we are

allocating the smallest block of memory which is a power of 2

Less external fragmentation – free memory tend to line up to the end of main memory

PARTITIONING (RELOCATION)

In all three schemes we discussed above (fixed, dynamic, buddy) relocation can be achieved by using a relative addressing scheme

A relative address is a form of logical address which is expressed as a relative quantity with respect to a pivot point Ex. 0x0001 memory location relative to the

beginning of code segment means the actual physical memory location is 1 + physical memory location of the code segment.

PARTITIONING (RELOCATION)…

SIMPLE PAGING

Main memory is partitioned into relatively small equal-sized chunks known as frames

Process images are also partitioned into equal-sized chunks of the same size known as pages

The operating system maintains a list of free frames available for allocation

With paging, internal fragmentation is minimal (on average half the page size) and external fragmentation is not possible

SIMPLE PAGING…

SIMPLE PAGING…

SIMPLE PAGING…

Unlike previous memory management schemes paging does not require the whole process image to be continuous

Instead, a pages to frames mapping table (page table) is maintained by the operating system for each process

SIMPLE PAGING…

A logical memory address consists of two parts; the page number and the offset within that page

SIMPLE PAGING…

By setting page size to a power of 2, address translation (which is done in hardware) can be simplified as shown below:

SIMPLE SEGMENTATION

The process image is divided into unequal-sized memory chunks known as segments

These segments are loaded into memory independent of one another

Somewhat similar to dynamic partitioning - not the whole process image but individual segments are allocated continuous chunks of memory

While paging is transparent for the programmer, segmentation is a visible mechanism; it allows programmers to organize programs and data into individual modules that can be treated differently Ex. Share a particular library code between two

processes by loading it into a separate shared segment (read only)

SIMPLE SEGMENTATION…

A logical address consists of two parts; the segment number and the offset within the segment

SIMPLE SEGMENTATION…

Since different segments can be of different lengths the segment table (hence the address translation process) is bit more complex:

VIRTUAL MEMORY

Both paging and segmentation has two properties that make virtual memory possible: Use of logical addresses within a process –

relocation is made possible A process image is divided into multiple pieces

The basic observation behind virtual memory is very simple: It is not necessary that all of the pages or all of the

segments of a process be in main memory during execution

Also, memory references within a program tends to be local; they use to cluster with respect to time (principle of locality) – so it is only necessary to have those pages or segments in memory for that duration

VIRTUAL MEMORY…

Operation: Operating system begins by bringing in one or

few pages / segments to include the initial program and data sections

Execution begins and things proceed smoothly as long as all referenced memory locations are in working set / resident set (current set of pages in memory)

When the processor encounters a logical address that is not in main memory, an interrupt is generated indicating a memory access fault

The process is put in a blocking state until the required page / segment is brought into memory (I/O)

When the requested page / segment is brought into memory the process becomes ready to execute again

VIRTUAL MEMORY…

It might be necessary that when a new page / segment is brought into memory an existing page be swapped out (why?)

If the swapped out page / segment is re-referenced shortly after, it has to be brought back again! The operating system should be clever enough

not to swap out such a page / segment A badly designed operating system can lead to a

situation known as thrashing where the operating system spends most of its time swapping in and out pages rather than doing useful processing

This is the subject of replacement algorithms, these algorithms try to exploit principle of locality

VIRTUAL MEMORY…

Despite all the technical issues that need to be dealt with virtual memory, there are two distinct advantages that make virtual memory an attractive concept: More processes may be maintained in main

memory – high degree of multiprogramming A process can be larger than all of main memory

available

THE END