FCFS Scheduling (Cont.)

First-Come, First-Served (FCFS) Scheduling

CPU Scheduling

CPU Scheduler

Ø  Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them.

Ø  CPU scheduling decisions may take place when a process:

1.            Switches from running to waiting state.

2.            Switches from running to ready state.

3.            Switches from waiting to ready.

4.            Terminates.

Ø  Scheduling under 1 and 4 is nonpreemptive.

Ø  All other scheduling is preemptive.

Dispatcher

Ø  Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves:

·         switching context

·         switching to user mode

·         jumping to the proper location in the user program to restart that program

Dispatch latency – time it takes for the dispatcher to stop one process and start another running.

Scheduling Criteria

Ø  CPU utilization – keep the CPU as busy as possible

Ø  Throughput – # of processes that complete their execution per time unit

Ø  Turnaround time – amount of time to execute a particular process

Ø  Waiting time – amount of time a process has been waiting in the ready queue

Ø  Response time – amount of time it takes from when a request was submitted until the first response is produced, not output  (for time-sharing environment)

Optimization Criteria

Ø  Max CPU utilization

Ø  Max throughput

Ø  Min turnaround time

Ø  Min waiting time

Ø  Min response time

Threads

Single and Multithreaded Processes


Benefits
------------
Responsiveness
Resource Sharing
Economy
Utilization of MP Architectures

User Threads
-------------
Thread management done by user-level threads library
Examples
    - POSIX Pthreads
    - Mach C-threads
    - Solaris threads

Kernel Threads
---------------
Supported by the Kernel
Examples
    - Windows 95/98/NT/2000
     - Solaris
    - Tru64 UNIX
    - BeOS
    - Linux


Multithreading Models
----------------------
Many-to-One
One-to-One
Many-to-Many

Many-to-One
------------
Many user-level threads mapped to single kernel thread.
Used on systems that do not support kernel threads.
Many-to-One Model


One-to-One
-----------
Each user-level thread maps to kernel thread.
Examples
    - Windows 95/98/NT/2000
    - OS/2
One-to-one Model


Many-to-Many Model
-------------------
Allows many user level threads to be mapped to many kernel threads.
Allows the  operating system to create a sufficient number of kernel threads.
Solaris 2
Windows NT/2000 with the ThreadFiber package
Many-to-Many Model



Threading Issues
-----------------
Semantics of fork() and exec() system calls.
Thread cancellation.
Signal handling
Thread pools
Thread specific data

Pthreads
---------
a POSIX standard (IEEE 1003.1c) API for thread creation and synchronization.
API specifies behavior of the thread library, implementation is up to development of the library.
Common in UNIX operating systems.
Solaris 2 Threads
 
Solaris Process

Windows 2000 Threads
---------------------
Implements the one-to-one mapping.
Each thread contains
    - a thread id
    - register set
    - separate user and kernel stacks
    - private data storage area

Linux Threads
--------------
Linux refers to them as tasks rather than threads.
Thread creation is done through clone() system call.
Clone() allows a child task to share the address space of the parent task (process)

Java Threads
-------------
Java threads may be created by:
 - Extending Thread class
 - Implementing the Runnable interface
Java threads are managed by the JVM.