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What Are Threads ? a light, fine, string like length of material
made up of two or more fibers or strands of spun cotton, flax, silk, etc. twisted together and used in sewing Webster’s New World Dictionary
Overview of Process An abstraction of a running program
A process includes the code, current value of the program counter, registers, and variables
CPU switches from process to process
Process Image User program User data Stack(s)
For function calls and parameter passing Process control block (execution context, or state)
Pointers to all of the above Attributes needed by the OS to control the
process Process identification information Processor state information Process control information
Process Identification in PCB A few numeric identifiers may be used
Unique process identifier Indexes (directly or indirectly) into the primary
process table User identifier
The user who initiated the process Effective user: the user whose permissions the
process inherits Identifier for the process that created this
process I.E. A pointer to the process’ parent
Processor State Information in PCB
Contents of processor registers User-visible registers Control and status registers Stack pointers
Program status word (PSW) Contains status information Example: the EFLAGS register on
Pentium machines
Process Control Information in PCB
Scheduling and state information Process state (i.E.: Running, ready, blocked...) Priority of the process Event for which the process is waiting (if blocked)
Process privileges Access to certain memory locations OS resources
Memory management Pointers to segment/page tables assigned to this
process Resource ownership and utilization
Resource in use: open files, I/O devices... History of usage: accounting (of CPU time, I/O...)
Process Creation
Assign a unique process identifier Allocate space for the process
image Initialize process control block
Many default values (example: state is new, no I/O devices or files...)
Set up appropriate linkages Example: add new process to linked list
used for the scheduling queue
Context Switching A context switch may occur whenever the OS
is invoked System call
Explicit request by the program, such as open file The process will likely be blocked
OS will dispatch a new process Trap
An error resulted from the last instruction May cause the process to be moved to the terminate state
Interrupt The cause is external to the execution of the current
instruction Control is transferred to the interrupt handler
Hence the OS is event driven
Steps in Context Switching
Save context of processor including program counter and other registers
Update the PCB of the running process with its new state and other associate info
Move PCB to appropriate queue - ready, blocked
Select another process for execution Update PCB of the selected process Restore CPU context from the PCB
Introduction to Threads A process can be considered as based on
Resource grouping Execution Its own address space
A thread represents the execution part of a process.
A thread has a program counter, register states, stack pointer.
All threads of a process share its address space.
All threads of a process share its resources.
Why Use Threads Over Processes
Both thread and process models provide concurrent program execution
Creating new process can be expensive It takes time: calling into the OS kernel is
needed Can trigger process rescheduling activity:
context switch It takes up memory resource: entire process is
replicated Communication and synchronization is expensive
Requiring calling into the OS kernel
Why Use Threads Over Processes, contd
Threads can be created without replicating an entire process
Most of the work of creating a thread is done in user space rather than the OS kernel
Thread can synchronize by monitoring a variable, as opposed to processes that require calling into the OS kernel
The benefits of the thread model results from staying inside the user address space of the program
Multithreading OS MS-DOS – single process, single thread UNIX – multiple processes, single thread per
process JVM – single process with multiple threads Windows 2000, Linux, OS/2, Solaris – multiple
processes with multiple threads
Thread Usage Example: word processor
Displays contents; Edits (e.G. Typing); Reformats; Auto saves; Printing etc.
Why Threads Become Popular Now?
SMPs (symmetric multiprocessors) 2 to 64 processors sharing
Buss I/O system Same memory
One operating system for all processors Examples:
SGI PowerChallenge (8 MIPS 1000 CPUs) -- CYC807 Sun ultra enterprise 6000 (8 CPUs) -- CYC807 ALR SMP server (4 Pentium pro) - CYC414
Three Types of Thread System
Kernel-supported threads (mach, OS/2, NT)
User-level threads; Supported above the kernel, via a set of library calls at the user level (linux via clone)
Hybrid approach implements both user-level and kernel-supported threads (Solaris 2)
Kernel-level Versus User-level Threads
User-level thread User-level activities; No kernel involvement Basic scheduling unit in OS is process Threads of the same process can not run on
different CPUs in SMP in parallel
Kernel-level thread Each process consists of several threads Basic scheduling unit is thread Can run on different CPUs in SMP in parallel
Advantages of Kernel Threads
Higher application throughput If there were no kernel thread support
Need I/O; It means the process goes into waiting state and wait until the I/O is complete
With multiple kernel threads per task Block the I/O requesting thread and
continue to work on another thread Increases the overall throughput of the
application
Advantages of User Level Threads
Threads are cheap Can be implemented at user levels,
no kernel resources Threads are fast
No system calls, switching modes involved
Sun Solaris 2 Mixed approach
OS schedules light-weight process (LWP) User-level library schedules user-level threads
User threads are cheap, can be thousands per task
Each LWP supports one or more user threads LWPs are what we’ve been calling kernel threads Solaris has entities called kernel threads; They are
scheduling artifacts contained in the OS
User/ Kernel Threads in Sun Solaris 2
Kernel thread
Task 1 Task 2 Task 3
CPU
KERNEL
Light weightprocess (LWP)
User-level thread
CPU CPU CPU