Memory Management
25
1 Memory Management ted From Modern Operating Systems, Andrew S. Tanenb
description
Memory Management. Adapted From Modern Operating Systems, Andrew S. Tanenbaum. Memory Management. Ideally programmers want memory that is large fast non volatile Memory hierarchy small amount of fast, expensive memory – cache some medium-speed, medium price main memory - PowerPoint PPT Presentation
Transcript of Memory Management
1
Memory Management
Adapted From Modern Operating Systems, Andrew S. Tanenbaum
2
Memory Management
• Ideally programmers want memory that is– large– fast– non volatile
• Memory hierarchy – small amount of fast, expensive memory – cache – some medium-speed, medium price main memory– gigabytes of slow, cheap disk storage
• Memory manager handles the memory hierarchy
3
Basic Memory ManagementMonoprogramming without Swapping or Paging
Three simple ways of organizing memory(in an operating system with one user process)
4
Multiprogramming with Fixed Partitions
• Fixed memory partitions– separate input queues for each partition– single input queue
5
Relocation and Protection
• Cannot be sure where program will be loaded in memory– address locations of variables, code routines cannot be absolute
(see next slide)
– must keep a program out of other processes’ partitions
• Use base and limit values– address locations added to base value to map to physical addr
– address locations larger than limit value is an error
Seeing the problemextern int x; // placement determined by loader
Fcn (void){int y;
x=x+1;}
Compiles to:
but where is x at runtime?
6
Compiled address(in 32-bit words)
instruction
[0] define storage for x – outside program
0 [function initialization (10 words)]
10 load address of x
11 load x
12 add 1 to x
13 store result
7
Swapping (1)
Memory allocation changes as – processes come into memory– leave memory
Shaded regions are unused memory
8
Swapping (2)
a. Allocating space for growing data segmentb. Allocating space for growing stack & data segment
9
Page Replacement Algorithms (1)
• Page fault forces choice – which page must be removed (the “victim”)
• Modified page must first be saved– unmodified just overwritten
• Better not to choose an often used page– will probably need to be brought back in soon
10
Page Replacement Algorithms (2)
• Optimal Page Replacement– Replace page needed at the
farthest point in future– Optimal but unrealizable
(Why?)
• Not Recently Used (NRU)
• FIFO
• Second Chance
• Clock
• Least Recently Used (LRU)rarely implemented - why?
• Not Frequently Used (NFU)
• Aging
• Working Set
• WSClock
11
Design Issues for Paging SystemsLocal versus Global Allocation Policies
• Original configuration
• Local page replacement
• Global page replacement
12
Virtual MemoryPaging (1)
The position and function of the MMU
13
Paging (2)The relation between
virtual addressesand physical memory addres-ses given bypage table
14
Page Tables (1)
Internal operation of MMU with 16 4 KB pages
214+213=16384+ 8192=2457624576+4=24580
15 bits for addressing
16-bit “word”
Remaining 8 pages ~mapped
15
Page Tables (2)
• 32 bit address with 2 page table fields
• 8 bytes/entry
• OK for 32-bit machine
• 64-bit machine needs 252 entries >30GB
Top-level page table
Second-level page tables
16
Page Tables (3)
Typical page table entry
17
TLBs – Translation Lookaside Buffers
A TLB to speed up paging
18
3 table schemes
Inverted table
1 entry/page in memoryPID+V. Page#
Problem with IPT’s
• Virtual-real translation harder
• Cannot use virtpg# as index – must search entire table– On EVERY reference
19
20
Cleaning Policy
• Need for a background process, paging daemon– periodically inspects state of memory
• When too few page frames are free– selects pages to evict using a replacement algorithm
21
Load Control
• Despite good designs, system may still thrash when– some processes need more memory – but no processes need less
• Solution :Reduce number of processes competing for memory– swap one or more to disk, divide up pages they held– reconsider degree of multiprogramming
22
Page Size
Small page size
• Advantages– less internal fragmentation – better fit for various data structures, code sections
• Disadvantages– program needs more pages has larger page table
23
Separate Instruction and Data Spaces
• One address space• Separate I and D spaces
24
Shared Pages
Two processes sharing same program sharing its page table
25
References
• Chapters 8 and 9 :OS Concepts, Silberschatz, Galvin, Gagne
• Chapter 4: Modern Operating Systems, Andrew S. Tanenbaum
• X86 architecture– http://en.wikipedia.org/wiki/Memory_segment
• Memory segment– http://en.wikipedia.org/wiki/X86
• Memory model– http://en.wikipedia.org/wiki/Memory_model
• IA-32 Intel Architecture Software Developer’s Manual, Volume 1: Basic Architecture– http://www.intel.com/design/pentium4/manuals/index_new.htm
