Threads

G.Anuradha(Reference : William Stallings)

Characteristics of a Process

• Resource Ownership: Process image is in a virtual address space and time to time it has been allocated resources . OS provides protection to the process (Process or Task)

• Scheduling/execution: Process has an execution state and priority and OS does the scheduling and execution (Thread or Light Weight Process)

Multithreading

• Ability of and OS to support multiple threads of execution within a single process

• Single thread of execution per process is called as single-threaded approach

• Process in a multithreaded environment is the unit of resource allocation and protection– Virtual address space that holds the process

image– Protected access to processor(s),resources

Threads and ProcessesMS-DOS

UNIX

Java

Windows 2000Solaris Linux

Thread

• Within a process there may be thread(s)– Has an execution state– Saved thread context when not running– An execution stack– Some per-thread static storage for local variables– Access to memory and resources of its process

shared with all other threads in that process

Single threaded and multithreaded process models

Share the state and resources of the process and reside in the same address space and access the same data

Benefits of threads

1. Takes Less time to create a thread than a process

2. Takes less time to terminate a thread than a process

3. Takes less time to switch between threads within the same process

4. Enhance efficiency in communication between different executing programs.

Examples of users of threads in single –user multiprocessing system

• Foreground and background work• Asynchronous processing• Speed execution• Modular program structure

Note:- 1. Suspension of a process suspends all the threads in that process as well

2. Termination of a process terminates all the threads in that process as well

Thread Functionality

• Thread States• Thread Synchronization• User level threads• Kernel level threads

Thread states

• Running • Ready• Blocked• Note:- There is no suspended state becos its

only in the process level. If a process is suspended all threads associated with it are also suspended

Operations associated with threads

• Associated with change in thread state we have four basic thread operations

• Spawn: When a new process is spawned a thread for that process is also spawned.

• Block: When a thread need to wait for an event it will block. The processor may now turn to the execution of another ready thread.

• Unblock: After the event for a blocked thread occurs its moved into the ready queue

• Finish: After a thread completes, its register context and stacks are deallocated.

Performance benefits of threads that do not block and entire process

Multithreading example on a uniprocessor

Thread Synchronization

• All threads of a process share the same address space and other resources of a process

• Any alternation in the resources by one thread affects the environment of other threads in the same process

• Activities of various threads need to be properly synchronised

Types of threads

• User-level threads• Kernel-level threads

User-level threads

• Application does the work of thread management

• Thread libraries are used for this purpose

• Kernel is unaware of the thread creations and schedules the process as a unit assigning a single execution state

• Thread library• Data structure for the new

thread• Passes control to one of the

threads within this process that is in the ready state

Relationship between ULT states and Process states

Relationship between ULT states and Process states

In figures b and c kernel switches control back to process B so execution resumes in thread 2

Advantages of ULT

1. Thread switching requires only user mode privileges. So its saves the overhead of two mode switches

2. Scheduling is application specific without disturbing the OS scheduler

3. ULTs run on any operating system. Thread library utilities are shared by all applications

Disadvantage of ULT

• Many system calls are blocking system calls. This not only blocks that thread but all threads in a process

• Multithreaded application cannot take advantage of multiprocessing. One single thread within a process can execute at a time.

How to overcome the disadvantages?

• Write an application as multiple processes rather than multiple threads. This eliminates the advantage of threads. Overhead increases

• Jacketing:- convert a blocking system call into nonblocking system call. In this case instead of calling a I/O routine a I/o jacket routine is called which checks whether the I/O is buzy or not. If I/O is buzy it enters the Ready state and passes control to another thread.

Kernel level threads

• No thread management code in the application area

• Just an API• All threads are

supported by a single process.

• Scheduling by kernel is done by thread basis

Advantages and disadvantages of KLT

• Advantages– Kernel can simultaneously schedule multiple

threads from the same process on multiple processors

– If one thread is blocked the kernel can schedule another thread

• Disadvantage– Transfer of control is mode switch

Combined Approaches• Thread creation, scheduling,

synchronization of threads is done in the user space.

• Multiple ULTs can be mapped onto some number of KLTs

• Multiple threads within the same application can run in parallel on multiple processors

• Blocking system call need not block the entire process

Relationship Between Threads and Processes

TRIX• Follows a many-to-many relationship between

threads and processes• There is a concept of domain and thread– Domain is a static entity consisting of an address

space and ports through which messages may be send and received

– Thread is a single execution path with an execution stack, processor state and scheduling information

• Multiple threads may execute in a single domain or a single user activity can be performed in multiple domains as well

One-To-Many Relationship

• Used in distributed operating systems. Eg Cloud operating systems

• Activity• CASE STUDIES ON trix • CASE STUDIES ON CLOUD OS

UNIX SVR4 Process Management

• In UNIX most of the OS executes within the environment of a user process.

• So it has user and kernel mode of execution

Categories of process in unix

• System processes– Run in kernel mode– Executes OS code to

perform administrative and housekeeping function

• User Processes– Run in user mode– Executes user programs

and utilities in kernel mode to execute instructions which belong to the kernel

– Enters the kernel mode by issuing a system call when an exception is generated or when an interrupt occurs

UNIX process state transition Diagram

Process states in Unix

Processes which are unique in UNIX

• Process 0 is a special process that is created when the system boots. (Swapper process)

• Process 0 spawns process 1 (init process)• All processes in UNIX has process 1 as its

ancestor

Process DescriptionProcess Image

User –level context

Register Context System level context

User-level context

Register context

System level context

Process Control

• When a parent process is forked(kernel mode)– A slot is allocated in the process table for the new

process– An unique process ID is assigned to child process– Copy of process image of the parent is made– Increments counters for any files owned by the

parent– Assigns the child process to a ready to run state– Returns the ID number of the child to the parent

process and 0 to child process

Dispatcher routine

• Stay in parent process. Control returns to user mode at the point of fork call of parent

• Transfer control to child process• Transfer control to another process. Both

parent and child are left in Ready to Run state.

Linux process and thread mgt• Task- task struct• Task-struct– State:Execution state of a process– Scheduling information– Identifiers- process has UID and group has GID– Interprocess communicaiton– Links– Times and timers– File system– Virtual memory– Processor-specific context

Linux process/Thread model