CMPT 300: Final Review Chapters 8 12

1
CMPT 300 Final Review Chapters 8 12
2
Memory Management Ch. 8, 9

• Address spaces
• Logical (virtual) generated by the CPU
• Physical seen by the memory unit
• User program deals with logical addresses never
  sees physical addresses
• Mapping from logical to physical addresses
• Static done at compile-time
• Dynamic done at load or execution time

3
Memory Allocation

• Memory is usually divided into two partitions
  for
• Kernel usually held in low memory
• User processes usually held in high memory
• Typically, the two partitions are managed
  differently
• Memory allocation
• Contiguous
• First-, Best-, Worst-fit
• Fragmentation external and internal
• Paging, segmentation, virtual memory

4
Paging
5
Segmentation
1
2
3
4
user space
physical memory space
6
Segmentation Hardware
7
Paging and Segmentation

• Paging issues
• Page size
• Internal fragmentation and page table size
• Page table (one table for each process)
• Access time use cache (TLB)
• Size use two- or three-levels page tables
  use hashed or inverted page tables for gt 32 bits
  architectures
• Segmentation variable size
• External fragmentation
• Facilitates sharing of code and data (share the
  whole segment)
• Combined Segmentation paging
• Example Intel Pentium

8
Virtual Memory

• Logical address space can be much larger than
  physical address space
• Only the needed parts of the program are brought
  to memory for execution
• Allows address spaces to be shared by several
  processes
• Allows for more efficient process creation
• Virtual memory can be implemented via
• Demand paging
• Demand segmentation

9
Virtual Memory and Page Fault
10
Page Replacement

• When we need to bring a page from disk and there
  is no free frame
• Find a victim page to evict from memory
• Objective minimize number of page faults (they
  are very costly)
• Replacement algorithms
• FIFO, OPT, LRU, second-chance,

11
Thrashing

• Thrashing ? a process is busy swapping pages in
  and out more than executing (because OS allocated
  too few frames to it)

12
Locality Model

• As a process executes, it moves from locality to
  locality, where a locality is a set of pages that
  are actively used together
• Locality could be function, module, segment of
  code,
• Locality model helps us to estimate the working
  set of a process
• Working set pages that are actively being used
  (within a window)

13
Allocating Kernel Memory

• Treated differently from user memory because
• Kernel requests memory for structures of varying
  sizes
• Process descriptors (PCB), semaphores, file
  descriptors,
• Some of them are less than a page
• Some kernel memory needs to be contiguous
• some hardware devices interact directly with
  physical memory without using virtual memory
• Virtual memory may just be too expensive for the
  kernel (cannot afford a page fault)

14
Kernel Memory Buddy System
15
Kernel Memory Slab Allocation
16
File Systems Ch. 10, 11, 12
Application Programs
Logical File System

• Interface file and directory structure
• Maintains pointers to logical block addresses

• Implementation block allocation,
• Maps logical into physical addresses

File-organization Module
Device Drivers

• Implementation device-specific instructions
• Writes specific bit patterns to device controller

Storage Devices
17
File System Interface

• Files
• The smallest storage unit from the users
  perspective
• A collection of blocks on the disk from OS
  perspective
• Directory structure
• Tree-structured with links (general graphs)
• Each file has an entry in the directory

18
File System Implementation

• To implement a file system, we need
• On-disk structures, e.g.,
• directory structure, number of blocks, location
  of free blocks, boot information,
• (In addition to data blocks, of course)
• In-memory structures to
• improve performance (caching)
• manage file system

19
On-disk Structures
Free block
Boot block
Superblock
Directory structure
File control block
Data block
20
Opening and Reading from a File
21
Block Allocation

• Allocate free blocks to files to achieve
  efficient disk space utilization, and fast file
  access
• Three common allocation methods
• Contiguous
• Linked
• Indexed
• OS keeps track of free blocks using
• Bitmaps
• Linked lists

22
Combined Scheme UNIX inode
23
Disk Structure and Scheduling

• Disk structure
• cylinders, tracks, sectors, logical blocks
• Disk operation
• Move the head to desired track (seek time)
• Wait for desired sector to rotate under the head
  (rotational latency time)
• Transfer the block to a local buffer, then to
  main memory (transfer time)
• Disk scheduling
• To minimize seek time (head movements)
• Algorithms FCFC, SSTF, SCAN, LOOK

24
Warp Up What We Have Covered

• How to manage resources in a single computer

25
Warp Up What We Have NOT Covered

• How to manage resources distributed over multiple
  computers
• CMPT 371 Computer Networks
• Protocols and algorithms to make computers talk
  to each other
• CMPT 401 Operating Systems II
• Distributed file systems, distributed
  synchronization, inter-process communications,
  consistency and reliability, security
• Special-purpose operating systems, e.g.,
• Real-time OS
• OS on small devices (PDAs, sensors,)

26
That is it with CMPT 300!

• Good Luck on Your Final