Memory Management edx - Yimg

jaspersugarlandSoftware and s/w Development

Dec 14, 2013 (3 years and 7 months ago)

106 views

Memory Management

-

Memory Basic Concepts:

1
-

Main memory and registers are only storage
.
CPU can access directly

2
-

Register access in one CPU clock (or less) but Main memory can take many cycles

3
-

Cache

sits between main memory and CPU registers

4
-

Protection of
memory required to ensure correct operation


-

What is the input queue?

-

C
ollection of processes on the disk that are waiting to be brought into memory to run the
program.


-

How to define the Logical Address Space?

-

Using a pair of
base

and

limit

registers. (N.
B A Process Logical Space is from base to
base +limit).


-

Size=base+limit.






-

What is the difference between logical and physical address?

Logical (virtual )
Address

Physical Address

-

generated by the CPU

-

Logical and physical addresses are
the same in compile
-
time and loa
d
-
time address
-
binding schemes

-

logical (virtual) and physical
addresses differ in execution
-
time
-

address seen by the
memory unit


address
-
binding scheme

-

Logical and physical addresses are the
same in compile
-
time and load
-
time

address
-
binding schemes

-


logical (virtual) and physical addresses
differ in execution
-
time

address
-
binding
scheme




-

What is the Memory
-
Management Unit (MMU)?

-

It is a
h
ardware device that maps virtual to physical address

-

In MMU scheme, the value in the relocation register is added to every address generated
by a user process at the time it is sent to memory

-

The user program deals with
logical
addresses; it never sees the
real physical

addresses




(
Dynamic relocation using a re
-
location registers
)

-

What is the swapping?

-

A process can be swapped temporarily out of memory to a backing store, and then
brought back into memory for continu
ed execution
.

-

Modified versions of swapping are found on many systems (i.e., UNIX, Linux, and
Windows)

-

System maintains a
ready queue

of ready
-
to
-
run processes which have memory images
on disk


-

What is a backing store?

-


F
ast disk large enough to accommodate copies of
all memory images for all users.

-

It

must provide

direct access to these memory i
mages



-

What is the
Roll out, roll in

policy?

-


It is a
swapping variant used for
priority
-
based

scheduling algorithms

-


lower
-
pr
iority process is swapped out so higher
-
priority process can be loaded and
executed


-

What is Contiguous Allocation
?

-

Main memory usually
divided
into two partitions:

-

1
-

Resident operating system, usually held in low memory with interrupt vector

-

2
-

User
processes

held in high memory


-

What are the two types of partition allocation?

Single
-
partition allocation

Multiple
-
partition allocation

-

Relocation
-
register scheme used to
protect user processes from each
other, and from changing operating
-
system code and

data.

-

Relocation

(Base)

register
contains
value of smallest physical address

-

Limit register
contains range of
logical addresses


each logical
address must be less than the limit
register.


-

Hole


block of available memory;
holes of various size are scat
tered
throughout memory

-

When a process arrives, it is
allocated memory from a hole large
enough to accommodate it

-

Operating system maintains
information about:

a) allocated partitions



b) free partitions (hole)




-

How to solve the Dynamic Storage
-
Allocation Problem? Or How to satisfy a
request of size
n

from a list of free holes?

First Fit

Best Fit

Worst Fit

-

Allocate the
first

hole that is big
enough

-

Allocate the
smallest

hole that is big
enough; must search
entire list, unless
ordered by size

-

Produces the
smallest leftover
hole


-

Allocate the
largest

hole; must also
search entire list

-

Produces the largest
leftover hole


-

First
-
fit and best
-
fit better than worst
-
fit in
terms of
speed

and

storage
utilizatio
n.


-

What are the two types of fragmentation?

External Fragmentation

Internal Fragmentation

-

T
otal memory space exists to
satisfy a request, but it is not
contiguous

-

External fragmentation

is the
phenomenon in which free storage
becomes divided into many small
pieces over time.

-

A
llocated memory may be slightly
larger than requested memory; this
size difference is memory internal
to a partition, but not being used

-

Internal fragmentation

occu
rs
when storage is allocated without
ever intending to use it.

-


This space is wasted. While this
seems foolish, it is often accepted in
return for increased efficiency or
simplicity. The term "internal"
refers to the fact that the unusable
storage is insid
e the allocated
region but is not being used.


-

How to reduce external fragmentation?

-

Reduce external fragmentation by
compaction

-

Shuffle memory contents to place all free memory together in one large block

-

Compaction is possible
only

if relocation is dynamic, and is done at execution time


-

What is paging?

-

Paging is a memory
-
management scheme that permits the physical
-
address space o
f a
process to be noncontiguous,
process is allocated physical memory whenever the latter is
available
.


-

How to apply paging?

-

Divide physical memory into fixed
-
sized blocks called
frames

(size is power of 2,
between 512 bytes and 8,192 bytes)

-

Divide logical memory into blocks of same size called
pages


-

Keep track of all free frames

-

To run a program of size
n

pages, need to find
n

free frames and load program

-

Set up a page table to translate logical to physical addresses


-

What is the effect of increasing frame size on internal and external
fragmentation?

-

Paging itself is a form of dynamic relocation.

-

When we u
se a paging scheme, we have no external fragmentation

-

However, we may have some internal fragmentation. Notice that frames are allocated as
units. If the memory requirements of a process do not happen to fall on page boundaries,
the last frame allocated ma
y not be completely full. For example, if pages are 2,048
bytes, a process of 72,766 bytes would need 35 pages plus 1,086 bytes. It would be
allocated 36 frames, resulting in an internal fragmentation of 2048
-

1086 = 962 bytes. In
the worst case, a proces
s would need n pages plus one byte. It would be allocated n + 1
frames, resulting in an internal fragmentation of almost an entire frame.

-

If process size is independent of page size, we expect internal fragmentation to average
one
-
half page per process.

-

Th
is consideration suggests that small page sizes are desirable.


-

What is the Address Translation Scheme?

-

Address generated by CPU is divided into:

Page Number (P)

Page Offset (d)

-

used as an index into a
page

table

which contains base address of
each page in physical memory

-

combined with base address to
define the physical memory address
that is sent to the memory unit







-

For given logical address space 2
m
and page size

2
n

(To understand the
Address Translation Architecture
, paging, and allocating free
frames

Check Slides 213:215)


-

How to implement the page table?

-

Page table is kept in main memory

-

Page
-
table base register (PTBR)

points to the page table

-

Page
-
table length reg
ister (PRLR)

indicates size of the page table

-

In this scheme every data/instruction access requires two memory accesses. One for the
page table and one for the data/instruction.



-

What are the different Page Table Structures
?

(you don’t have to study the
hashed & the inverted)

Hierarchical Paging

Hashed Page Tables


Inverted Page Tables


-

Break up the logical
address space into
multiple page tables

-

A simple technique is
a two
-
level page table

-


-

Common in address
spaces > 32 bits

-

The virtual page number
is
hashed into a page
table. This page table
contains a chain of
elements hashing to the
same location.

-

Virtual page numbers are
compared in this chain
searching for a match. If
a match is found, the
corresponding physical
-

One entry for e
ach real
page of memory

-

Entry consists of the virtual
address of the page stored
in that real memory
location, with information
about the process that owns
that page

-

Decreases memory needed
to store each page table,
but increases time needed
to search the
table when a
page reference occurs

frame is extracted.

-


-

Use hash table to limit the
search to one


or at most
a few


page
-
table entries

-



-

What are the Shared Pages?

-

An advantage of paging providing the possibility of sharing common code.

-

This is particularly important in a
time
-
sharing environment.

-

Systems that use inverted page tables have difficulty implementing shared memory


-

How do Shared Pages work?

-

Shared code:

-

One copy of read
-
only (reentrant) code shared among processes (i.e., text editors,
compilers, window
systems).

-

Shared code must appear in same location in the logical address space of all processes

-

Private code and data
:

-

Each process keeps a separate copy of the code and data

-

The pages for the private code and data can appear anywhere in the logical addre
ss space

-

(Text Editor Example: Only one copy of the editor needs to be kept in physical memory.
Each user's page table maps onto the same physical copy of the editor, but data pages are
mapped onto different frames).