Review

Virtual Memory That is Larger Than Physical Memory 

Virtual-address Space

Demand Paging Bring a page into memory only when it is needed Less I/O needed Less memory needed Faster response More users Page is needed  reference to it invalid reference  abort not-in-memory  bring to memory Lazy swapper – never swaps a page into memory unless page will be needed Swapper that deals with pages is a pager

Valid-Invalid Bit With each page table entry a valid–invalid bit is associated (v  in-memory, i  not-in-memory) Initially valid–invalid bit is set to i on all entries Example of a page table snapshot: During address translation, if valid–invalid bit in page table entry is I  page fault Frame # valid-invalid bit v v v v i …. i i page table

Page Table When Some Pages Are Not in Main Memory

Steps in Handling a Page Fault

Performance of Demand Paging Page Fault Rate 0  p  1.0 if p = 0 no page faults if p = 1, every reference is a fault Effective Access Time (EAT) EAT = (1 – p) x memory access + p (page fault overhead + swap page out + swap page in + restart overhead )

Page Replacement Algorithms Want lowest page-fault rate Evaluate algorithm by running it on a particular string of memory references (reference string) and computing the number of page faults on that string In all our examples, the reference string is 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5

Graph of Page Faults Versus The Number of Frames

FIFO Illustrating Belady’s Anomaly

Optimal Algorithm Replace page that will not be used for longest period of time 4 frames example 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 How do you know this? Used for measuring how well your algorithm performs 1 4 6 page faults 2 3 4 5

Least Recently Used (LRU) Algorithm Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 Counter implementation Every page entry has a counter; every time page is referenced through this entry, copy the clock into the counter When a page needs to be changed, look at the counters to determine which are to change 1 1 1 1 5 2 2 2 2 2 3 5 5 4 4 4 4 3 3 3

Use Of A Stack to Record The Most Recent Page References

LRU Approximation Algorithms Reference bit With each page associate a bit, initially = 0 When page is referenced bit set to 1 Replace the one which is 0 (if one exists) We do not know the order, however Second chance Need reference bit Clock replacement If page to be replaced (in clock order) has reference bit = 1 then: set reference bit 0 leave page in memory replace next page (in clock order), subject to same rules

Second-Chance (clock) Page-Replacement Algorithm

Counting Algorithms Keep a counter of the number of references that have been made to each page LFU Algorithm: replaces page with smallest count MFU Algorithm: based on the argument that the page with the smallest count was probably just brought in and has yet to be used

Thrashing (Cont.)

Buddy System Allocates memory from fixed-size segment consisting of physically-contiguous pages Memory allocated using power-of-2 allocator Satisfies requests in units sized as power of 2 Request rounded up to next highest power of 2 When smaller allocation needed than is available, current chunk split into two buddies of next-lower power of 2 Continue until appropriate sized chunk available

Buddy System Allocator

Chapter 10: File-System Interface

File Attributes Name – only information kept in human-readable form Identifier – unique tag (number) identifies file within file system Type – needed for systems that support different types Location – pointer to file location on device Size – current file size Protection – controls who can do reading, writing, executing Time, date, and user identification – data for protection, security, and usage monitoring Information about files are kept in the directory structure, which is maintained on the disk

Tree-Structured Directories

Acyclic-Graph Directories Have shared subdirectories and files

(a) Existing. (b) Unmounted Partition

Access Lists and Groups Mode of access: read, write, execute Three classes of users RWX a) owner access 7  1 1 1 RWX b) group access 6  1 1 0 c) public access 1  0 0 1 Ask manager to create a group (unique name), say G, and add some users to the group. For a particular file (say game) or subdirectory, define an appropriate access. owner group public chmod 761 game Attach a group to a file chgrp G game

Chapter 11: File System Implementation

A Typical File Control Block

In-Memory File System Structures

Allocation Methods An allocation method refers to how disk blocks are allocated for files: Contiguous allocation Linked allocation Indexed allocation

Free-Space Management Bit vector (n blocks) 1 2 n-1 … 0  block[i] free 1  block[i] occupied bit[i] =  Block number calculation (number of bits per word) * (number of 0-value words) + offset of first 1 bit

Free-Space Management (Cont.) Bit map requires extra space Example: block size = 212 bytes disk size = 230 bytes (1 gigabyte) n = 230/212 = 218 bits (or 32K bytes) Easy to get contiguous files Linked list (free list) Cannot get contiguous space easily No waste of space Grouping Counting

Free-Space Management (Cont.) Need to protect: Pointer to free list Bit map Must be kept on disk Copy in memory and disk may differ Cannot allow for block[i] to have a situation where bit[i] = 1 in memory and bit[i] = 0 on disk Solution: Set bit[i] = 1 in disk Allocate block[i] Set bit[i] = 1 in memory

Log Structured File Systems Log structured (or journaling) file systems record each update to the file system as a transaction All transactions are written to a log A transaction is considered committed once it is written to the log However, the file system may not yet be updated The transactions in the log are asynchronously written to the file system When the file system is modified, the transaction is removed from the log If the file system crashes, all remaining transactions in the log must still be performed

Snapshots in WAFL

Chapter 12: Mass-Storage Systems

Illustration shows total head movement of 640 cylinders. FCFS Illustration shows total head movement of 640 cylinders.

SSTF (Cont.)

SCAN (Cont.)

C-SCAN (Cont.)

C-LOOK (Cont.)

RAID Levels

RAID levels block vs. byte parity distribution

Chapter 13: I/O Systems

Interrupt-Driven I/O Cycle

Block and Character Devices Block devices include disk drives Commands include read, write, seek Raw I/O or file-system access Memory-mapped file access possible Character devices include keyboards, mice, serial ports Commands include get, put Libraries layered on top allow line editing

Two I/O Methods Synchronous Asynchronous

Life Cycle of An I/O Request

Chapter 14: Protection

Domain Structure Access-right = <object-name, rights-set> where rights-set is a subset of all valid operations that can be performed on the object. Domain = set of access-rights

Domain Implementation (MULTICS) Let Di and Dj be any two domain rings. If j < I  Di  Dj

Modified Access Matrix of Figure B