Run time environment

1. RUN-TIME
ENVIRONMENTSHEAP MANAGAGEMENT

2. HEAP
- portion of the store used for data that lives
indefinitely or until explicitly deleted by the
program
Nevermore by Paul
Gaugin

3. MEMORY MANAGER

4.  the subsystem that allocates and
deallocates space within the heap
 keeps track of all the free space in heap
storage at all times
 serves as an interface between
application programs and the operating
system
MEMORY MANAGER

5.  produces a chunk of contiguous heap
memory of the requested size
 if no chunk of the needed size is
available, it increases the heap storage
by getting consecutive bytes of virtual
memory from the OS
ALLOCATION
MEMORY
ALLOCATION

6.  returns deallocated space to the pool of
free space (so it can reuse the space)
 typically does not return memory to the
OS, even if heap usage drops
DEALLOCATION
MEMORY
ALLOCATION

7.  Space Efficiency
 Program Efficiency
 Low Overhead
MEMORY MANAGER

8. WINTERTemplate
MEMORY
HIERARCHY
01

9. 02
Register
1st-Level
Cache
2nd-Level Cache
Physical Memory
Virtual Memory (Disk)
Memory Diagram
Typical Sizes
32 Words
16 - 64KB
128KB - 4MB
256MB - 2GB
> 2GB
Typical Access Times
1 ns
5 - 10 ns
40 - 60 ns
100 - 150 ns
3 - 15 ms

10. 02Memory Diagram
Register
1st-Level
Cache
2nd-Level Cache
Physical Memory
Virtual Memory (Disk)
The goal is to obtain
the best average
access speed while
minimizing the total
cost of the entire
memory system.

11. 03Principle of Locality
Also known as Locality of Reference
The phenomenon of the same value or
related storage locations being frequently
accessed.
90/10 Rule comes
from the empirical
observation:
“A program spends
90% of its time in
10% of its code.”

12. 03
Two Types of Access Locality
1. Temporal Locality
2. Spatial Locality
Principle of Locality

13. 03
1. Temporal Locality
the memory locations the program
accesses are likely to be accessed again
within a short period of time
Principle of Locality
Example: Instruction in the
body of inner loops

14. 03
2. Spatial Locality
memory locations close to the location
accessed are likely also to be accessed
within a short period of time
Principle of Locality
Example: Traversing the
elements in a one-dimentional
array

15. Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition,
Reducing Fragmentation

16. 8.16 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
CONTIGUOUS MEMORY

17. 8.17 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Contiguous Allocation
 Main memory usually into two partitions:
 Resident operating system, usually held in low memory
with interrupt vector
 User processes then held in high memory

18. 8.18 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Contiguous Allocation
 Hole – block of available memory; holes of various size are
scattered 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)
OS
process 5
process 8
process 2
OS
process 5
process 2
OS
process 5
process 2
OS
process 5
process 9
process 2
process 9
process 10

19. 8.19 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Dynamic Storage-Allocation Problem
 First-fit: Allocate the first hole that is big enough
 Best-fit: Allocate the smallest hole that is big enough; must
search entire list, unless ordered by size
 Produces the smallest leftover hole
 Worst-fit: Allocate the largest hole; must also search entire
list
 Produces the largest leftover hole
How to satisfy a request of size n from a list of free holes

20.  Coalescing
REDUCING FRAGMENTATION

21. COALESCING
- combining adjacent chunks of the heap to
form a larger chunk

22.  Boundary Tags
- keeping a free/used bit at both ends of each
chunk
- adjacent to the free/used bit is a count of
the # of bytes
 Doubly Linked, Imbedded Free List
- free chunks are linked in a doubly linked list
- chunks must accommodate two boundary
tags and two pointers
COALESCING
REDUCING
FRAGMENTATION

23. MANUAL DEALLOCATION
REQUEST

24.  Memory-leak Error
- failing to EVER delete data that cannot be
referenced
 Dangling-pointer-dereference
[note: dangling pointers - pointers to storage that
has
been deallocated]
- referencing deleted data
MANUAL DEALLOCATION
REQUESTS

25.  Object Ownership
- associate an owner with each object (e.g.
functions and their variables)
 Reference Counting
- associate a count with each dynamically
allocated object
PROGRAMMING CONVENTIONS &
TOOLS
MANUAL DEALLOCATION
REQUESTS

26.  Region-based Allocation
- allocate all objects in the same region; then
delete the entire region
PROGRAMMING CONVENTIONS &
TOOLS
MANUAL DEALLOCATION
REQUESTS

27. Fin.