Download Threads

Threads
Irfan Khan
Myo Thein
What Are Threads ?
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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
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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
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User program
User data
Stack(s)
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For function calls and parameter passing
Process control block
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(execution context, or state)
Pointers to all of the above
Attributes needed by the OS to control the process
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Process identification information
Processor state information
Process control information
Process Identification in PCB
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A few numeric identifiers may be used
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Unique process identifier
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User identifier
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Indexes (directly or indirectly) into the primary process
table
The user who initiated the process
Effective user: the user whose permissions the process
inherits
Identifier for the process that created this process
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I.E. A pointer to the process’ parent
Processor State Information in PCB
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Contents of processor registers
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User-visible registers
Control and status registers
Stack pointers
Program status word (PSW)
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Contains status information
Example: the EFLAGS register on Pentium
machines
Process Control Information in PCB
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Scheduling and state information
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Process privileges
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Access to certain memory locations
OS resources
Memory management
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Process state (i.E.: Running, ready, blocked...)
Priority of the process
Event for which the process is waiting (if blocked)
Pointers to segment/page tables assigned to this process
Resource ownership and utilization
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Resource in use: open files, I/O devices...
History of usage: accounting (of CPU time, I/O...)
Process Creation
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Assign a unique process identifier
Allocate space for the process image
Initialize process control block
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Many default values (example: state is
new, no I/O devices or files...)
Set up appropriate linkages
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Example: add new process to linked list
used for the scheduling queue
Context Switching
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A context switch may occur whenever the OS
is invoked
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System call
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Explicit request by the program, such as open file
The process will likely be blocked
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Trap
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An error resulted from the last instruction
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May cause the process to be moved to the terminate state
Interrupt
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OS will dispatch a new process
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
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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
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A process can be considered as based on
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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.
Threads in Process
Why Use Threads Over Processes
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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
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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
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Multithreading – multiple threads in the same
process
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Multithreading OS
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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
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Thread states – running, blocked, or ready.
Each thread has its own stack.
Thread Usage
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Example: word processor
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Displays contents;
Edits (e.G. Typing);
Reformats;
Auto saves;
Printing etc.
Why Threads Become Popular Now?
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SMPs (symmetric multiprocessors)
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2 to 64 processors sharing
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Buss
I/O system
Same memory
One operating system for all processors
Examples:
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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
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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
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User-level thread
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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
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Each process consists of several threads
Basic scheduling unit is thread
Can run on different CPUs in SMP in parallel
Advantages of Kernel Threads
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Higher application throughput
 If there were no kernel thread support
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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
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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
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Threads are cheap
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Can be implemented at user levels, no
kernel resources
Threads are fast
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No system calls, switching modes involved
Sun Solaris 2
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Mixed approach
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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
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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
Light weight
process (LWP)
Task 1
Task 2
User-level thread
Task 3
Kernel thread
KERNEL
CPU
CPU
CPU
CPU
Thank You