Threads Irfan Khan Myo Thein

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.

Threads in Process

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 – multiple threads in the same process

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 states – running, blocked, or ready. Each thread has its own stack.

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 2Task 3 CPU KERNEL Light weight process (LWP) User-level thread CPU

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