Topic #3: Threads and Processes Flashcards
It is an abstraction of a running program, an activity of some kind
Process
It is a series of instructions for a computer to perform that may be stored on a disk
Program
It is the rapid switching back and forth of pseudo parallel sequential processes
Multiprogramming
Events which cause Processes Creation
- System Initialization
- Execution of a process creation system call by a running process
- a user request to create a new process
- initiation of a batch job
Events which cause Termination
Normal Exit (Voluntary)
Error Exit (Voluntary)
Fatal Error (Involuntary)
Killed by another process (Involuntary)
Three States a process may be in
Running
Ready
Blocked
A state that a process can be in that refer to actually using the CPU at that instant
Running
A state that a process can be in that refers to a runnable or a temporarily stopped process to let others run
Ready
A state that a process can be in wherein it is unable to run until some external event happpens
Blocked
It is where information that must be saved when the process is switched from running to ready or blocked state so that it can be restarted later
Process table
Other name for Processs Table
Process Control Blocks
How many entries are there per process
One entry per process
It contains the address of the interrupt service procedure
Interrupt Vector
It is a signal emitted by a device attached to a computerr or a from a program that requires the OS to stop and figure out what to do next
Interrupt
These are sometimes referred to as lightweight processes
Threads
It is the situation that allows multiple threads to run within the same process
Multithreading
Advantages of Threads
- threads are more responsive as it executes an output and can be immediately returned
- context switch time between threads is lower
- multiple threads can be scheduled on multiple processor
- resources can be shared among all threads in a process
- communication between threads is eeasier as they share an address space
- multiple threads = multiple jobs = multiple completion = increased throughput
It is the thread package defined by the IEEE to make it possible to write protable threaded programs
Pthreads
It is a thread call that creates a new thread
Pthread_create
It is a thread call that terminates the calling thread
Pthread_exit
It is a thread call that waits for a specific thread to exit
Pthread_join
It is a thread call that releases the CPU to let another thread run
Pthread_yield
It is a thread call that creates and initializes a thread’s attribute structure
Pthread_attr_init
It is a thread call that removes a thread’s attribute structure
Pthread_attr_destroy
All Pthreads have certain properties
- an identifier
- a set of registers
- a set of attributes
Main places to implement threads
- user space
- the kernel
Implementing Threads in User Space
- kernel knows nothing about them; it is managing single-threaded proccesses
- each process needs its own thread table
- implemented by a library
Advantages of Implementing Threads in User Space
- Can be implemented in an OS that does not support threads
- customized scheduling algorithm for each process
- it scales better
Disadvantages of Implementing Threads in User Space
- problem of howw blocking system calls are implemented
- problem of page faults
- one thread can only use the CPU
- programmers want theads where threads are blocked often
Implementing Threads in a Kernel
- No run-time system is needed
- no thread table for each process; one thread table for the entire system
- information about the threads is now ketp in the kernel
- no need for nonblocking system calls
Pros and Cons of Nonblocking system calls
- a system call is susbtantial and requires more overhead but has issues with signals
Hybrid Implementation of Threads
- kernel is aware of only kernel-level threads and schedules those
- some of the kernel threads may have user-level threads multiplexed on top
- user level threads are created, destroyed, scheduled like other user-level threads even if OS cant multihread
- each kernel-level thread takes turns using user-level threads in the kernel
IPC
Inter-process Communication
Mechanisms for IPC
- Atomic Read/Write
- Locks
- Semaphores
- Monitors
- Message Passing
Need to communicate with other processes
- to pass info form process to process
- to enable proper sequencing when dependencies are present
- to ensure that no two process are the in the critical region at the same time
Race Conditions
Two or more processes are reading and writing shared data
Conditions in order to avoid race conditions
- Mutual Exclusion: No two processes may be simultaneously in the critical regions
- No Assumption: no assumptions may be made about speeds or the number of CPUs
- Progress: no process running outside its critical region may be blocked
- No Starvation: no progress should have to Wait forever to enter the critical region
it is a condition required for the avoidance of a racce condition wherein no two proccesses may be simultaneously be inside their critical regions
Mutual Exclusion
it is a condition required for the avoidance of a racce condition wherein no assumptions may be made about the speeds or the number of CPUs
No assumptions
it is a condition required for the avoidance of a racce condition wherein no process running outside its critical region may block other processes
Progress
it is a condition required for the avoidance of a racce condition wherein no process should have to wait forever to enter its critical region
No Starvation
It is the part of the program where shared variables are accessed
Critical Region
Disabling Interrupts
Each process disables all interrupts just after entering the critical region and re-enable them before leaving
It is a single shared variable where the process sets it to 1 and enters the critical region, if it is 1 the process has to wait for it to become 0
Lock Variables
Classsical IPC Problem
- Producer-Consumer Problem
- Dining Philosopher Problem
- Reader Writers Problem
Explain the Dining Philosophers Problem
It models processes competing for exclusive access to limited resources
Explain the Producer-Consumer Problem
- It models acess to a bounded buffer
- Producers wont try to add data into the buffer if its full and consumers wont try to remove data if buffer is empty
Explain the Readers-Writers Problem
- Models access to a database
- two readers can read at once: a writer should not wait longer than needed
What are the units of execution
Processes
How are the units of execution represented
Process Control Blocks
How is work scheduled in the CPU
- Process States
- Process Queues
- Context Switches
How does a process move from one state to another
- Scheduling
- I/O
- Creation
- Termination
How are processes created
- CreateProccess (Windows)
- Fork/exec (Unix)
Comparea and Contrast Kernel-Level Threads ot User-level threads
Kernel-level threads are better but requires significant overhead; User-level threads are better but are not well integrated in the OS
