Chandan Singh | os

os

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1 - introduction

1.1 what operating systems do

  • computer system - hierarchical approach = layered approach
    1. hardware
    2. operating system
    3. application programs
    4. users
  • views
    1. user view - OS maximizes work user is performing
    2. system view
      • os allocates resources - CPU time, memory, file-storage, I/O
      • os is a control program - manages other programs to prevent errors
  • program types
    1. os is the kernel - one program always running on the computer
      • only kernel can access resources provided by hardware
    2. system programs - associated with OS but not in kernel
    3. application programs
  • middleware - set of software frameworks that provide additional services to application developers

1.2 computer-system organization

  • when computer is booted, needs bootstrap program
    • initializes things then loads OS
    • also launches system processes
      • ex. Unix launches “init”
  • events
    • hardware signals with interrupt
    • software signals with system call
    • interrupt vector holds addresses for all types of interrupts
    • have to save address of interrupted instruction
  • memory
    • von Neumman architecture - uses instruction register
    • main memory is RAM
      • volatile - lost when power off
    • secondary storage is non-volatile (ex. hard disk)
    • ROM is unwriteable so static programs like bootstrap are ROM
    • access
      1. uniform memory access (UMA)
      2. non-uniform memory access (NUMA)
  • I/O
    • device driver for I/O devices
    • direct memory access (DMA) - transfers entire blocks of data w/out CPU intervention
      • otherwise device controller must move data to its local buffer and return pointer to that
  • multiprocessor systems
    1. increased throughput
    2. economies of scale (costwise)
    3. increased reliability (fault tolerant)

1.3 computer-system architecture

  1. single-processor system - one main cpu
    • usually have special-purpose processors (e.g. keyboard controller)
  2. multi-processor system / multicore system
    • multicore means multi-processor on same chip
      • multicore is generally faster
    • multiple processors in close communication
    • advantages
      • increased throughput
      • economy of scale
      • increased reliability = graceful degradation = fault tolerant
    • types
      1. asymmetric multiprocessing - boss processor controls the system
      2. symmetric multiproccesing (SMP) - each processor performs all tasks
        • more common
      3. blade server - multiple independence multiprocessor systems in same chassis
  3. clustered system - multiple loosely coupled cpus
    • types
      1. asymmetric clustering - one machine runs while other monitors it (hot-standby mode)
      2. symmetric clustering - both run something
    • parallel clusters
      • require disributed lock manager to stop conflicting parallel operations
      • can share same data via storage-area-networks
    • beowulf cluster - use ordinary PCs to make cluster

1.4 operating-system structure

  • multiprogramming - increases CPU utilization so CPU is always doing something
    • keeps job pool ready on disk
    • time sharing / multitasking - multiple jobs switch so fast that both can be interacted with
      • requires an interactive computer system
    • process - program loaded into memory
    • scheduling
      • job scheduling - picking jobs from job pool (disk -> memory)
      • CPU scheduling - what to run first (memory -> cpu)
    • memory
      • processes are swapped from main memory to disk
      • virtual memory allows for execution of process not in memory

1.5 operating-system operations

  • trap / exception - software-generated interrupt
  • user-mode and kernel mode (also called system mode)
    • when in kernel mode, mode bit is 0
    • separate mode for virtual machine manager (VMM)
    • this is built into hardware
  • kernel can use a timer to getting stuck in user mode

1.6 process management

  • program is passive, process is active
  • process needs resources
    • process is unit of work
  • single-threaded process has one program counter

1.7 memory management

  • cpu can only directly read from main memory
  • computers must keep several programs in memory
    • hardware design is impmortant

1.8 storage management

  • defines file as logical storage unit
  • most programs stored on disk until loaded
  • in addition to secondary storage, there is tertiary storage (like DVDs)
  • caching - save frequent items on faster things
    • cache coherency - make sure cache coherency is properly updated with parallel processes

1.9 protection & security

  • process can execute only within its address space
  • protection - controlling access to resources
  • security - defends a system from attacks
  • maintain list of user IDs and group IDs
    • can temporarily escalate priveleges to an effective UID - setuid command

1.10 basic data structures

  • bitmap - string of n binary digits

1.11 computing environments

  • network computers - are essentially terminals that understand web-based computing
  • distributed system - shares resources among separate computer systems
    • network - communication path between two or more computers
    • TCP/IP is most common network protocol
  • networks
    • PAN - personal-area network (like bluetooth)
    • LAN - local-area network connects computers within a room, building, or campus
    • WAN - wide-area network
    • MAN - metropolitan-area network
    • network OS provides features like file sharing across the network
    • distributed OS provides less autonomy - makes it feel like one OS controls entire network
  • client-server computing
    1. compute-server - performs actions for user
    2. file-server - stores files
  • peer-to-peer computing
    1. all clients w/ central lookup service, ex. Napster
    2. no centralized lookup service
      • uses discovery protocol - puts out request and other peer must respond
  • virtualization - allows OS to run within another OS
    • interpretation - run programs as non-native code (ex. java runs on JVM)
    • BASIC can be compiled or interpreted
  • cloud-computing - computing, storage, and applications as a service accross a network
    • public cloud
    • private cloud
    • hybrid cloud
    • software as a service (SAAS)
    • platform as a service (PAAS)
    • infrastructure as a service (IAAS)
    • cloud is behind a firewall, can only make requests to it
  • embedded systems - like microwaves / robots
    • specific tasks
    • have real-time OS - fixed time constraints

2 - OS Structures

2.1 os services

  • for the user
    • user interface - command-line interface and graphical user interface
    • program execution - load a program and run it
    • I/O operations - file or device
    • File-system manipulation
    • communications - between processes / computer systems
    • error detection
  • for system operation
    • resource allocation
    • accounting - keeping stats on users / processes
    • protection / security

2.2 user and os interface

  1. command interpreter = shell - gets and executes next user-specified command
    • could contain the code to execute the command
    • command interpreter could have code to execute commands
    • more often, executes system programs, such as “rm”, that are executed
  2. GUI

2.3 system calls

  • system calls - provide an interface to os services
  • API usually wraps system calls (ex. java)
    • libc - provided by Linux/Mac OS for C
    • system-call interface links API calls to system calls
  • passing parameters
    1. pass parameters in registers
    2. parameters stored in block of memory and address passed in register
    3. parameters pushed onto stack

2.4 system call types

  1. process control - halting, ending
    • lock shared data - no other process can access until released
  2. file manipulation
  3. device manipulation
    • similar to file manipulation
  4. information maintenance - time, date, dump()
    • single step is CPU mode which throws trap for CPU after every instruction for a debugger
  5. communications
    1. message-passing model
      • each computer has host name and network identifier (IP address)
      • each process has process name
      • daemons - system programs for receiving connections (like servers waiting for a client)
    2. shared-memory model
  6. protection

2.5 system programs

  • system programs = system utilities
  • some provide interfaces for system calls
  • other uses
    1. file management
    2. status info
    3. file modification
    4. programming-language support
    5. program loading and execution
    6. communications
    7. background services

2.6 os design and implementation

  • mechanism - how to do something
    • want this to be general so only certain parameters change
  • policy - what will be done
  • os mostly in C, low-level kernel in assembly
    • high-level is easier to port but slower

2.7 os structure

  • want modules but current models aren’t very modularized
    • monolithic system has performance advantages - very little overhead
    • in practice everything is a hybrid
  • system can be modularized with a layered approach
    • layers: hardware, …, user interface
    • easy to construct and debug
    • hard to define layers, less efficient
  • microkernel approach - used in os Mach
    • move nonessential kernel components to system / user-level
    • smaller kernel, everything communicates with message passing
    • makes extending os easier, but slower functions due to system overhead
  • loadable kernel modules
    • more flexible - kernel modules can change
  • examples (see pics)

2.8 os debugging

  • errors are written to log file and core dump (memory snapshot) is written to file
  • if kernel crashes, must save its dump to s special area
  • performance tuning - removing bottlenecks
    • monitor trace listings - log if interesting events with times / parameters
  • SolarisDTrace is a tool to debug and tune the os
  • profiling - periodically samples instruction pointer to determine which code is being executed

2.9 generation

  • system generation - configuring os on a computer
    • usually on a CD-ROM
    • lots of things must be determined (like what CPU to use)

2.10 system boot

  • bootstrap program

3 - processes

3.1 process concept

  • process - program in execution
    • batch system executes jobs = processes
    • time-shared system has user programs or tasks
    • program is passive while process is active
  • parts
    • program code - text section
    • program counter
    • registers
    • stack
    • data section
    • heap
  • same program can have many processes
  • process can be execution environment for other code (ex. JVM)
  • process state
    • new
    • running
    • waiting
    • ready
    • terminated
  • process control block (PCB) = task control block - repository for any info that varies process to process
    • process state
    • program counter
    • CPU registers
    • CPU-scheduling information
    • memory-management information
    • accounting information
    • I/O status information
    • could include information for each thread
  • parent - process that created another process

3.2 process scheduling

  • process scheduler - selects available process for multi-tasking
    • processes begin in job queue
    • processes that are ready and waiting are in the ready queue until they are dispatched - usually stored as a linked list
    • lots of things can happen here (fig 3_6)
      • ex. make I/O request and go to I/O queue
    • I/O-bound process - spends more time doing I/O
    • CPU-bound process - spends more time doing computations
    • each device has a list of process waiting in its device queue
  • scheduler - selects processes from queues
    • long-term scheduler - selects from processes on disk to load into memory
      • controls the degree of multiprogramming = number of processes in memory
      • has much more time than short-term scheduler
      • want good mix of I/O-bound and CPU-bound processes
      • sometimes this doesn’t exist
    • short-term / CPU scheduler - selects from processes ready to execute and allocates CPU to one of them
    • sometimes medium-term scheduler
      • does swapping - remove a process from memory and later reintroduce it
  • context switch - occurs when switching processes
    • when interrupt occurs, kernel saves context of old process and loads saved context of new process
    • context is in the PCB
    • might be more or less work depending on hardware

3.3 operations on processes

  • usually each process has unique process identifier (pid)
  • linux everything starts with init process (pid=1)
  • restricting a child process to a subset of the parent’s resources prevents system overload
    • parent may pass along initialization data
  • after creating new process
    1. parent continues to execute concurrently with children
    2. parent waits until some or all of its children terminate
  • two address-space possibilities for the new process:
    1. child is duplicate of parent (it has the same program and data as the parent).
    2. child loads new program
  • forking
    • when call fork() continue operation but returns 0 for parent process and nonzero for child
      • child is a copy of the parent
    • after fork, usually one process calls exec() to load binary file into memory
      • overrides program, doesn’t return unless error occurs
    • parent can call wait() until child finishes (moves itself off ready queue until child finishes)
  • on Windows, uses CreateProcess() which requires loading a new program rather than sharing address space
    • STARTUPINFO -
    • PROCESSINFORMATION -
  • process termination
    • exit() kills process (return in main calls exit)
    • process can return status value
    • parent can terminate child if it knows its pid
    • cascading termination - if parent dies, its children die
    • zombie process - terminated but parent hasn’t called wait() yet
      • remains because parent wants to know what exit status was
      • if parent terminates without wait(), orphan child is assigned init as new parent (init periodically invokes wait())

3.4 interprocess communication

  • process cooperation
    • information sharing
    • computation speedup
    • modularity
    • convenience
  • interprocess communication (IPC) - allows exchange of data and info
    1. shared memory - shared region of memory is established
      • one process establishes region
      • other process must attach to it (OS must allow this)
      • less overhead (no system calls)
      • suffers from cache coherency
      • ex. producer consumer
        • producer fills buffer and consumer empties it
        • unbounded buffer - producer can keep producing indefinitely
        • bounded buffer - consumer waits if empty, producer waits if full
        • in points to next free position
        • out points to first full position
    2. message passing - messages between coordinating processes
      • useful for smaller data
      • easier in a distributed system
        1. direct or indirect communication
        • direct requires knowing the id of process to send / receive
          • can be asymmetrical - need to know id of process to send to, but not receive from
          • results in hard-coding
        • indirect - messages are sent / received from mailboxes
          • more flexible, can send message to whoever shares mailbox
          • mailbox owned by process - owner receives those messages
          • mailbox owned by os - unclear
            1. synchronous or asynchronous communication
        • synchronous = blocking
        • when both send and recieve are blocking = rendezvous 3. automatic or explicit buffering
        • queue for messages can have 3 implementations
          1. zero capacity (must be blocking)
          2. bounded capacity
          3. unbounded capacity

3.5 examples of IPC systems

  1. POSIX - shared memory
  2. Mach - message passing
  3. Windows - shared memory for message passing

3.6 communication in client-server systems

  1. sockets - endpoint for communication
    • IP address + port number
    • connecting
      1. server listens on a port
      2. client creates socket and requests connection to server’s port
      3. server accepts connection (then usually writes data to socket)
    • all ports below 1024 are well known
    • connection-oriented=TCP
    • connectionless = UDP
    • special IP address 127.0.0.1 - loopback - refers to itself
    • sockets are low-level - can only send unstructured bytes
  2. remote procedure calls (RPCs) - remote message-based communication
    • like IPC, but between different computers
    • message addressed to an RPC daemon listening to a port
    • messages are well-structured
    • specifies a port - a number included at the start of a message packet
      • system has many ports to differentiate different services
    • uses stubs to hide details
      • they marshal the parameters
      • might have to convert data into external data representation (XDR) (to avoid issues like big-endian vs. little-endian)
    • must make sure each message is acted on exactly once
    • client must know port
      1. binding info (port numbers) may be predetermined and unchangeable
      2. binding can be dynamic with rendezvous deaemon (matchmaker) on a fixed RPC port
  3. pipes - conduit allowing 2 processes to communicate
    • four issues
      1. bidirectional?
      2. full duplex (data can travel in both directions at same time?) or half duplex (only one way)?
      3. parent-child relationship?
      4. communicate over a network?
    • ordinary pipe - write at one end, read at the other
      • unix function pipe(int fd[])
        • fd[0] is read-end and fd[1] is write-end
      • only exists while a child and parent process are communicating
        • therefore only on same machine
      • parent and child should both close unused ends of the pipe
      • on windows, called anonymous pipes
        • requires security attributes
    • named pipe - can be bidirectional
      • called FIFOs in Unix
        • only half-duplex, requires same machine
      • Windows - fulll-duplex and can be different machines
      • many processes can use them

4 - threads

  • thread - basic unit of CPU utilization
    1. program counter
    2. register set
    3. stack
  • making a thread is quicker and less resource-intensive than making a process
  • used in RPC and kernels
  • benefits
    1. responsiveness
    2. resource sharing
    3. economy
    4. scalability

4.2 - multicore programming (skipped)

  • amdahl’s law: $speedup \leq \frac{1}{S+(1-S)/N_{cores}}$
    • S is serial portion
  • parallelism
    • data parallelism - distributing subsets of data across cores and performing same operation on each core
    • task parallelism - distribution tasks across cores

4.3 - multithreading models

  • need relationship between user threads and kernel threads
    1. many-to-one model - maps user-level threads to one kernel thread
      • can’t be parallel on multicore systems
      • ex. used by Green threads
    2. one-to-one model
      • small overhead for creating each thread
      • used by Linux and Windows
    3. many-to-many model
      • less than or equal number of kernel threads
      • two-level model mixes a one-to-one model and a many-to-many model

4.4 - thread libraries

  • thread library - provides programmer with an API for creating/managing threads
  • asynchronous v. synchronous threading

1 - POSIX Pthreads

/* get the default attributes */
pthread attr init(&attr);
/* create the thread */
pthread create(&tid,&attr,runner,argv[1]);  // runner is a func to call
/* wait for the thread to exit */
pthread join(tid,NULL);
  • shared data is declared globally

2 - Windows

3 - Java - uses Runnable interface

4.5 - implicit threading (skipped)

  • implicit threading - handle threading in run-time libraries and compilers
    1. thread pool - number of threads at startup that sit in a pool and wait for work
    2. OpenMP - set of compiler directives / API for parallel programming
      • identifies parallel regions
      • uses #pragma
    3. Grand central dispatch - extends C
      • uses dispatch queue

4.6 - threading issues

  • fork/exec need to know if should fork all threads / when to replace program
  • signal notifies a process that a particular event has occurred
    1. has a default signal handler
    2. user-defined signal handler
      • delivering a signal to a process: kill(pid_t pid, int signal)
      • delivering a signal to a thread: pthread_kill(pthread_t tid, int signal)
  • thread cancellation - terminating target thread before it has completed
    1. asynchronous cancellation - one thread immediately terminates target thread
    2. deferred cancellation - target thread periodically checks whether it should terminate
      • pthread_cancel(tid)
      • uses deferred cancellation
      • cancellation occurs only when thread reaches cancellation point
  • thread-local storage - when threads need separate copies of data
  • lightweight process = LWP - between user thread and kernel thread
  • scheduler activation - kernel provides application with LWPs
    • upcall - kernel informs application about certain events

4.7 - linux (skipped)

  • linux process / thread are same = task
  • uses clone() system call

5 - process synchronization

  • cooperating process can effect or be affected by other executing processes
  • ex. consumer/producer
    • if counter++ and counter– execute concurrently, don’t know what will happen
    • this is a race condition

5.2 - critical-section problem

  • each process has critical section where it updates common variables
  • 3 requirements
    1. mutual exclusion - 2 processes can’t concurrently do critical section
    2. progress - things should be in critical selection
    3. bounded waiting - every process should eventually get to critical selection
  • kernels
    1. preemptive kernels
      • more responsive
    2. nonpreemptive kernels
      • no race conditions

5.3 - peterson’s solution

  • peterson’s solution
    • not guaranteed to work

5.4 - synchronization hardware

  • locking - protecting critical regions using locks
  • single-processor solution
    • prevent interrupts while shared variable is being modified
    • ex. test_and_set()
  • instructions do things like swapping atomically - as one uninterruptable unit
    • these are basically locked instructions
    • ex. compare_and_swap()

5.5 - mutex locks

  • mutex
    • simplest synchronization tool
    • this type of mutex lock is called spinlock because requires busy waiting - processes not in critical section are continuously looping
    • good when locks are short

5.6 - semaphores

  • semaphore S - integer variable accessed through wait() (like trying to execute) and signal() (like releasing)
    • counting semaphore - unrestricted domain
    • binary sempahore - 0 and 1
wait(S) {
	while(S<=0)
		// busy wait
	S--;
}
signal(S) {
	S++;
}
  • to improve performace, replace busy wait by process blocking itself
    • places itself into a waiting queue
    • restarted when other process executes a signal() operation
typedef struct{ 
	int value;
	struct process *list;
} semaphore;
wait(semaphore *S) { 
	S->value--;
	if (S->value < 0)
		add this process to S->list;
}
signal(semaphore *S) { 
	S->value++;
	if (S->value <= 0){
		remove a process P from S->list; 
		wakeup(P); // resumes execution
	}
}
  • deadlocked - 2 processes are in waiting queues, can’t wakeup unless other process signals them
  • indefinite blocking=starving - could happen if we remove processes from waiting queue in LIFO order
    • bottom never gets out
  • priority inversion
    • only occurs when processes have > 2 priorities
    • usually solved with a priority-inheritance protocol
      • when a process accesses resources needed by a higher-priority process, it inherits the higher priority until they are finished with the resources in question

5.7 - classic synchronization problems

  1. bounded-buffer problem
  2. readers-writers problem
    • writers must have exclusive access
    • readers can read concurrently
  3. dining-philosophers problem

5.8 - monitors

  • monitor - highl-level synchronization construct
    • only 1 process can run at a time
    • abstract data type which includes a set of programmer-defined operations with mutual exlusion
    • has condition variables
      • these can only call wait() or signal()
      • when a signal is encountered, 2 options
        1. signal and wait
        2. signal and continue
  • can implement with a semaphore
    • 1st semaphore: mutex - process must wait before entering and signal after leaving the monitor
    • 2nd semaphore: next - signaling processes use next to suspend themselves
    • 3rd semaphore: next_count = number of suspended processes
 wait(mutex);
// body of F

if (next count > 0) 
	signal(next);
else
	signal(mutex);
  • conditional-wait construct can help with resuming
    • x.wait(c);
    • priority number c stored with name of process that is suspended
    • when x.signal() is executed, process with smallest priority number is resumed next

5.9.4 - pthreads synchronization

#include <pthread.h> 
pthread mutex t mutex;

/* create the mutex lock */ 
pthread mutex init(&mutex,NULL) // null specifies default attributes

pthread mutex lock(&mutex); // acquire the mutex lock
/* critical section */
pthread mutex unlock(&mutex); // release the mutex lock
  • these functions return 0 w/ correct operation otherwise error code
  • POSIX specifies named and unnamed semaphores
    • name has name and can be shared by different processes
#include <semaphore.h> sem t sem;
/* Create the semaphore and initialize it to 1 */ sem init(&sem, 0, 1);

/* acquire the semaphore */ 
sem wait(&sem);

/* critical section */

/* release the semaphore */ 
sem post(&sem);

5.10 - alternative approaches (skip)

5.11 - deadlocks

  • resource utilization
    1. request
    2. use
    3. release
  • deadlock requires 4 simultaneous conditions
    1. mutual exclusion
    2. hold and wait
    3. no preemption
    4. circular wait
  • deadlocks can be described by system resource-allocation graph
    • request edge - directed edge from process P to resource R means P has requested instance of resource type R
    • assignment edge - R-> P
    • if the graph has no cycles, not deadlocked
    • if cycle, possible deadlock
  • three ways to handle
    1. use protocol to never enter deadlock
    2. enter, detect, recover
    3. ignore the problem
      • developers must write code that avoids deadlocks

7 - main memory

7.1 - background

  • CPU can only directly access main memory and registers
  • accessing memory is slower than registers

    • processor must stall or use cache
  • processes need separate memory spaces
    1. base register - holds smallest usable address
    2. limit register - specifies size of range
      • os / hardware check these, throw a trap if there was error
  • input queue holds processes waiting to be be brought into memory
  • compiler binds symbolic addresses to relocatable addresses

    • linkage editor binds relocatable addresses to absolute addresses
  • CPU uses virtual address=logical address
    • memory-management unit (MMU) maps from virtual to physical address
      • simple ex. add virtual address to a process’s base register = relocation register
  • dynamic loading - don’t load whole process, only load things when called
  • dynamically linked libraries - system libraries linked to user programs when the programs are run

    • stub - tells how to load / locate library routine
  • shared libraries - all use same library

7.2 (skipped)

7.3 - contiguous memory allocation

  • contiguous memory allocation - each process has a section
    • put OS in low memory and process memory in higher
  • transient OS code - not often used
    • ex. drivers
    • can remove this and change OS memory usage by decreasing val in OS limit register
  • split mem into partitions
    • each partition can only have 1 process
    • multiple-partition method - free partitions take a new process
    • variable-partition scheme - OS keeps table of free mem
      • all available mem = hole
      • holes are divided between processes
        1. first-fit - allocate first hole big enough
        2. best-fit - allocate smallest hole that is big enough
        3. worst-fit - allocate largest hole (largest leftover hole)
          • worst
  • external fragmentation - there is enough free mem, but it isn’t contiguous
    • 50-percent rule - 1/3 of mem is unusable
    • solved with compaction - shuffle mem to put free mem together
      • can be expensive to move mem around
  • internal fragmentation - extra mem a proc is allocated but not using (because given in block sizes)
  • 2 types of non-contiguous solutions
    1. segmentation
    2. paging

7.4 - segmentation (skip)

  • segments make up logical address space
    • name (or number)
    • length
  • logical address is a tuple
    • (segment-number, offset)
  • segment table
    • each entry has segment base and segment limit
  • doesn’t avoid external fragmentation

7.5 - paging (skip)

  • break physical mem into fixed-size frames and logical mem into corresponding pages
  • CPU address = [page number page offset]
    • page table contains base address of each page in physical mem
    • usually, each process gets a page table
  • frame table keeps track of which frames are available / who owns them
  • paging is prevalent
  • avoids external fragmentation, but has internal fragmentation
  • small page tables can be stored in registers
    • usually page-table base register points to page table in mem
    • has translation look-aside buffer - stores some page-table entries
      • some entries are wired down - cannot be removed from TLB
      • some TLBS store address-space identifiers (ADIDs)
        • identify a process
        • otherwise hard to contain entries for several processes
      • want high hit ratio
  • page-table often stores a bit for read-write or read-only
    • valid-invalid bit sets whether page is in a process’s logical address space
    • OR page-table length register - says how long page table is
  • can share reentrant code = pure code
    • non-self-modifying code

7.6 - page table structure (skip)

  • page tables can get quite large (total mem / page size)
    1. hierarchical paging - ex. two-level page table
    • also called forward-mapped page table
    • unused things aren’t filled in
    • for 64-bit, would generally require too many levels 2. hashed page tables
      • virtual page number is hash key -> physical page number
      • clustered page tables - each entry stores everal pages, can be faster
        1. inverted page tables
      • only one page table in system
      • one entry for each page/frame of memory
      • takes more time to lookup
        • hash table can speed this up
      • difficulty with shared memory

7.7-9 (skipped)

6 - cpu scheduling

  • preemptive - can stop and switch a process that is currently running

6.3 - algorithms

  1. first-come, first-served
  2. shortest-job-first
    • can be preemptive or non preemptive
  3. priority-scheduling
    • indefinite blocking / starvation
  4. round-robin
    • every process gets some time
  5. multilevel queue scheduling
    • ex. foreground and background
  6. multilevel feedback queues
    • allows processes to move between queues

6.4 - thread scheduling

  • process contention scope - competition for CPU takes place among threads belonging to same process
    • PTHREAD_SCOPE_PROCESS - user-level threads onto available LWPs
    • PTHREAD_SCOPE_SYSTEM - binds LWP for each user-level thread

6.5 - multiple-processor scheduling

  • asymmetric vs. symmetric
    • almost everything is symmetric (SMP)
    • processor affinity - try to not switch too much
    • load balancing - try to make sure all processes share work
    • multithreading
      1. coarse-grained - thread executes until long-latency event, such as memory stall
      2. fine-grained - switches between instruction cycle

6.6 - real-time systems

  • event latency - amount of time that elapses from when an event occurs to when it is serviced
    1. interrupt latency - period of time from the arrival of an interrupt at the CPU to the start of the routine that services the interrupt
    2. dispatch latency
    1. Preemption of any process running in the kernel
    2. Release by low-priority processes of resources needed by a high-priority process
  • rate-monotonic scheduling - schedules periodic tasks using a static priority policy with preemption

6.7 - SKIP

8 - virtual memory

8.1 - background

  • lots of code is seldom used
  • virtual mem allows the execution of processes that are not completely in
  • benefits
    • programs can be larger than physical mem
    • more processes in mem at same time
    • less swapping programs into mem
  • sparse address space - virtual address spaces with hole (betwen heap and stack)

8.2 - demand paging

  • demand paging - load pages only when they are needed
    • lazy pager - only swaps a page into memory when it is needed
    • can use valid-indvalid bit in page table to signal whether a page is in memory
  • memory resident - residing in memory
  • accessing page marked invalid causes page fault
    • must restart after fetching page
      1. don’t let anything change while fetching
      2. use registers to store state before fetching
  • pure demand paging - never bring a page into memory until it is required

    • programs tend to have locality of reference, so we bring in chunks at a time
  • extra time when there is a page fault
    1. service the page-fault interrupt
    2. read in the page
    3. restart the process
      • effective access time is directly proportional to page-fault rate
  • anonymous memory - pages not associated with a file

8.3 - copy-on-write

  • copy-on-write - allows parent and child processes intially to share the same pages
    • if either process writes, copy of shared page is created
    • new pages can come from a set pool
  • zero-fill-on-demand - zeroed out before being allocated
  • virtual memory fork - not copy-on-write
    • child uses adress space of parent
    • parent suspended
    • meant for when child calls exec() immediately

8.4 - page replacement - select which frames to replace

  • multiprogramming might over-allocate memory

    • all programs might need all their mem at once
  • buffers for I/O also use a bunch of mem
  • when over-allocated, 3 options
    1. terminate user process
    2. swap out a process
    3. page replacement
  • want lowest page-fault rate
  • test with reference string, which is just a list of memory references
  • if no frame is free, find one not being used and free it
  • write its contents to swap space
  • modify bit=dirty bit reduces overhead

    • if hasn’t been modified then don’t have to rewrite it to disk
  • page replacement examples
    1. FIFO

      • Belady’s anomaly - for some algorithms, page-fault rate may increase as number of allocate frames increases
    2. optimal (OPT / MIN)

      • replace the page that will not be used for the longest period of time
    3. LRU - least recently used (last used)
      1. implement with counters since each use
      2. stack of page numbers (whenever something is used, put it on top)
        • stack algorithms - set of pages in memory for n frames is always a subset of the set of pages that would be in memory with n + 1 frames
        • don’t suffer from Belady’s anomaly
    4. LRU-approximation
      • reference bit - set whenever a page is used
      • can keep additional reference bits by recording reference bits at regular intervals1
      • second-chance algorithm - FIFO, but if ref bit is 1, set ref bit to 0 and move on to next FIFO page
      • can have clock algorithm
      • enhanced second-chance - uses reference bit and modify bit
        • give preference to pages that have been modified
    5. counting-based - count and implement LFU (least frequently used) or MFU (most frequently used)
  • page-buffering algorithms
    • pool of free frames - makes things faster
    • list of modified pages - written to disk whenever paging device is idle
    • som algorithms, like databases perform better when they get their own memory capability called raw disk instead of being managed by OS

8.5 frame-allocation algorithms - how many frames to allocate to teach process in memory (skipped)

8.6 - thrashing

  • if low-priority process gets too few frames, swap it out
    • thrashing - process spends more time paging than executing
      • CPU utilization stops increasing
  • local replacement algorithm = priority replacement algorithm - if one process starts thrashing, cannot steal frames from another
    • locality model - each locality is a set of pages actively used together
    • give process enough for its current locality
  • working-set model - still based on locality
    • defines working-set window $\delta$
    • defines working set as pages in most recent $\delta$ refs
    • OS adds / suspends processes according to working set sizes
    • approximate with fixed-interval timer
  • page-fault frequency - add / decrease pages based on targe page-fault rate

8.8.1 - buddy system

  • memory allocated with power-of-2 allocator - requests are given powers of 2
    • each page is split into 2 buddies and each of those splits again recursively
    • coalescing - buddies can be combined quickly

9 - mass-storage structure

9.4 - disk scheduling

  • bandwidth - total number of bytes transferred, divided by time
  • first-come first-served
  • shortest-seek-time-frist
  • SCAN algorithm - disk swings side to side servicing requests on the way
    • also called elevator algorithm
    • also has circular-scan

9.5 - disk management

  • low-level formatting - dividing disk into sectors that controller can read/write
    • blocks have header / trailer with error-correcting codes
  • bad blocks are corrupted - need to replace them with others = sector sparing = forwarding
    • sector slipping - just renumbers to not index bad blocks

10 - file-system interface

10.1

  • os maintains open-file table
  • might require file locking
  • must support different file types

10.2 - access methods

  • simplest - sequential
  • direct access = relative access
    • uses relative block numbers

10.3

  • disk can be partitioned
  • two-level directory
    • users are first level
    • directory is 2nd level
  • extend this into a tree
    • acyclic makes it faster to search
    • cycles require very slow garbage collection
  • link - pointer to another thing

11 - file-system implementation

11.1

  • file-control block (FCB) contains info about file ownership, etc.

11.4

  • contiguous allocation
  • linked allocation
    • FAT
  • indexed allocation - all the pointers in 1 block

11.5

  • keep track of free-space list
    • implemented as bit map
  • keep track of linked list of free space
  • grouping - block stores n-1 free blocks and 1 pointer to next block
  • counting - keep track of ptr to next block and the number of free blocks after that

12 - i/o systems

  • bus - shared set of wires
  • registers
    • data-in - read by the host
    • data-out
    • status
    • control
  • interrupt chaining - each element in the interrupt vector points to the had of a list of interrupt handlers
  • system calls use software interrupt
  • direct memory access - read large chunks instead of one byte at a time
  • device-status table
  • spool - buffer for device (ex. printer) that can’t hold interleaved data