4: Chapter Threads
Operating System Concepts 8th Edition,
Silberschatz, Galvin and Gagne 2009
Chapter 4: Threads
s Overview
s Multithreading Models
s Thread Libraries
s Threading Issues
s Operating System Examples
q Windows
XP Threads
q Solaris
q Linux
Threads
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Objectives
s To introduce the notion of a thread a
fundamental unit of CPU utilization that forms
the basis of multithreaded computer systems
s To discuss the APIs for the Pthreads, Win32,
and Java thread libraries
s To examine issues related to multithreaded
programming
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Processes and Threads
s Processes have two characteristics:
q Resource
ownership - process includes a
virtual address space to hold the process
image
q Scheduling/execution
- follows an execution
path that may be interleaved with other
processes
s These two characteristics are treated
independently by an operating system
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Process
s The resource
ownership is
referred to as the
process or task
process
s Has program, data,
PCB, shared
variables, process
image, etc
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Threads
s Scheduling is
referred to as a
thread or
lightweight
process
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Threads in Process
s Each thread has
q An
execution state (running, ready, etc.)
q Saved
q An
thread context when not running
execution stack
q Some
per-thread static storage for local
variables
q Access
to the memory and resources of its
process (all threads of a process share this)
s One way to view a thread is as an independent
program counter operating within a process.
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Traditional approach
s Single thread of
execution per
process
s Concept of
thread not
recognized
independently
s This is the
single-threaded
approach
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Multithreading
s The ability of
an OS to
support
multiple,
concurrent
paths of
execution
within a
single
process.
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Benefits of Threads
s Takes less time to create a new thread than a
process
s Less time to terminate a thread than a process
s Switching between two threads takes less time
that switching processes
s Threads can communicate with each other
q without
invoking the kernel
q No
need for shared memory or message
passing
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Example: Remote Procedure Call
s Consider:
qA
program that performs two remote
procedure calls (RPCs)
q
to two different hosts
q to
obtain a combined result
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RPC Using Single Thread
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RPC Using One Thread per Server
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Example: web server
s Web server accepts concurrent client requests
s Either serve one client at a time or create several processes
s Rather have one thread to listen client requests and create
one independent thread to serve each request
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Multicore systems
s Multiple computing cores on a single cheap
s In a single core, multithreading only implement
concurrency where thread interleave over time but only
one thread is executed at a time
s Multithreading helps to efficiently used multiple core
architectures, threads may run in parallel
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Thread Libraries
s Threads are implemented with thread libraries
which provide programmer with API for creating
and managing threads
s Two primary ways of implementing
q Library
entirely in user space: Invoking a
function in the library results in a local function
call, not a system call
q Kernel-level
library supported by the OS: Code
and data structures in kernel space, invoking a
function results in a system call
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Types of Threads
s Threads which are supported entirely by libraries in the user
space are called user threads
s Threads which are supported entirely by libraries in the
kernel are called kernel threads
s Whether it is in user mode or kernel mode, thread libraries
must exist to support the management of threads
s There is a complex relationship between user application,
user threads and kernel threads
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User-Level Threads (UTL)
s All thread
management is
done by userlevel threads
library
s The kernel is not
aware of the
existence of
threads
s POSIX Pthreads
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User Level Threads
s Advantages
q
Thread switching does not require kernel privileges, all
thread management done in the user address space
q
Scheduling can be application specific
q
ULTs can run on any OS
s Disadvantages:
q
When a ULT execute a system call, all threads of the
corresponding process are blocked
q
Cannot take advantage of multiprocessing, one
process to one processor at a time
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Kernel-Level Threads (KLT)
s Kernel maintains context
information for the
process and the threads
q No
thread
management done by
application
s Scheduling is done on a
thread basis
s Windows, Solaris, Linux,
Mac OS X are examples
of this approach
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Advantages/Disadvantages of KLT
s Advantages:
q The
kernel can simultaneously schedule
multiple threads from the same process on
multiple processors.
q If one thread in a process is blocked, the
kernel can schedule another thread of the
same process.
s Disadvantages:
q The
transfer of control from one thread to
another within the same process requires a
mode switch to the kernel
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Combined Approaches
s Thread creation
done in the user
space
s Bulk of scheduling
and synchronization
of threads by the
application
s Example is Solaris
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Combined Multithreading Models
s Relationships existing between user level
threads and kernel level threads:
q Many-to-One
q One-to-One
q Many-to-Many
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Many-to-One
s Many user-level
threads mapped to
single kernel thread
s Examples:
q Solaris
Green
Threads
q GNU
Portable
Threads
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Relationships between Thread and Process(1)
s Process B is
running
s Thread one is
ready
s Thread 2 is
running
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Relationships between Thread and Process(3)
s OS place B in
Ready and
switches to
another process
s According to data
structure of the
user level threads
library, thread 2
still running
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Relationships between Thread and Process(2)
s Thread 2 made a
system call
s Which blocks
process B
s Thread 2 still
seen as running
by the user level
threads library
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Relationships between Thread and Process(4)
s Thread 2 needs
actions performed
by thread 1
s Thread 2 enters a
blocked state
s Thread 1 transits
from Ready to
Running
s Process B remains
Running
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One-to-One
s Each user-level thread maps to kernel thread
s Examples: Windows NT/XP/2000; Linux; Solaris 9
and later
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Many-to-Many
s Allows many user
level threads to be
mapped to many
kernel threads
s Allows the
operating system to
create a sufficient
number of kernel
threads
s Solaris before 9
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Implementation: Combined Approach
LWP is a data structure
appearing as a virtual
processor to the ULT
Each LWP attached to a
kernel thread
Application schedules
threads to run on the
LWP
If kernel thread blocked
than user threads
blocked also
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Thread basics
s Thread operations include
q thread
creation, termination,
synchronization (joins,blocking),
scheduling, data management and
process interaction.
s A thread does not maintain a list of created
threads, nor does it know the thread that
created it.
s All threads within a process share the same
address space.
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Thread basics
s Threads in the same process share:
q process
instructions
q most
data
q open
files (descriptors)
q signals
and signal handlers
q current
working directory
q User
and group id
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Thread basics
s Each thread has a unique:
q thread
q set
ID
of registers, stack pointer
q stack
for local variables, return
addresses
q signal
mask
q priority
q return
Operating System Concepts 8th Edition
value: errno
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Thread Libraries
s Three primary thread libraries:
q
POSIX Pthreads:
API
threads extension of POSIX standard.
Provided at either user or kernel level.
q Win32
threads:
Kernel
q
level threads under Windows.
Java threads:
Created
Operating System Concepts 8th Edition
and managed in Java programs.
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Pthreads
s A POSIX standard (IEEE 1003.1c) API for
thread creation and synchronization
s API specifies behavior of the thread library,
implementation is up to development of the
library
s Common in UNIX operating systems
(Solaris, Linux, Mac OS X)
s https://computing.llnl.gov/tutorials/pthreads/
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Pthreads example
1. #include<pthread.h>
2. #include<stdio.h>
3. int sum; /*This data is shared by the thread(s) */
4. void *runner(void *param); /* the thread */
5. main(int argc, char *argv[]) {
6. pthread_t tid; pthread_attr_t attr; /* thread attributes */
7. if(argc != 2)
{
8. fprintf(stderr,"usage: a.out <integer value>\n");
9. exit(); }
10.if(atoi(argv[1]) < 0) { fprintf(stderr, "%d must be >= 0 \n",
atoi(argv[1])); exit(); }
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Thread creation and join
1. pthread_attr_init(&attr); /* get the default
attributes */
2. /*create the thread */
3. pthread_create(&tid,&attr,runner,argv[1]);
4. /* Now wait for the thread to exit */
5. pthread_join(tid,NULL);
6. printf("sum = %d\n",sum);
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Thread execution
s /*Thread begin execution in this function */
s void *runner(void *param) {
q
int upper = atoi(param); int i; sum=0;
q
if(upper > 0) {
for(i=1; i <= upper;i++)
sum += i;
q
}
q
pthread_exit(0);
s}
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Two other examples
s See pdf files PthreadEx1 and PthreadEx2
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Java Threads
s Java threads are managed by the JVM, they are
user level threads
s Java threads may be created by:
q Extending
Thread class
q Implementing
the Runnable interface
s Typically implemented using the threads model
provided by underlying OS
q In
Windows XP, one-to-one model (each Java
thread maps to a kernel thread
q Similarly
with Solaris 9 and +
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Java Threads
s When a thread is created, it must be
permanently bound to an object with a run()
method.
s When the thread is started, it will invoke the
object's run() method.
s Create the thread and supply it an object with
a run() method
s The object's run() method invoked when
thread starts. Useful if want many threads
sharing an object.
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Java Threads - Example Program
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Java Threads - Example Program
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Threading Issues
s Semantics of fork() and exec() system calls
s Thread execution
s Thread cancellation of target thread
q
Asynchronous or deferred
s Signal handling
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Semantics of fork() and exec()
s Does fork() duplicate only the calling thread or all
threads?
q
Some Unix have two versions of fork():
One
duplicate all threads: forkall()
One
duplicate only the thread invoking fork1()
s The execution of exec() destroy completely the calling
program, including all the threads belonging to the
corresponding process
q
Should not use the fork that duplicate all threads if
exec() is called
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Relations Threads/Processes
s Actions that affect all of the threads in a process:
q The
OS must manage these at the process
level
s Examples:
q Suspending
a process involves suspending all
threads of the process
q Termination
of a process, terminates all
threads within the process
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Thread Execution States
s Key thread states are Running, Ready and Waiting
s No suspend states as it is a per-process concept
q If
a process is swapped out, so all the threads
since they shared the same address space with
the swapped process
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Thread Execution States (cont)
s A thread is blocked when waiting for an event
(saving its registers, program counter and stack
pointers)
s The occurrence of the event on which the thread
was blocked triggers the thread to be placed in
the ready queue
s A thread within the process may spawn other
threads
s May synchronize with other threads
q Similar
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Thread Cancellation
s Terminating a thread before it has
finished
s Two general approaches:
q Asynchronous
cancellation
terminates the target thread
immediately
q Deferred
cancellation allows the
target thread to periodically check if
it should be cancelled
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Signals
s Signals are used in UNIX systems to notify a process that a
particular event has occurred
s Signals is an inter-process communication (Windows does
not have signals)
s A signal is not an exception although some exceptions may
trigger a signal
q
For example, if a process attempted to divide by zero, a
divide error exception would be generated and the kernel
exception handler will to send the SIGFPE signal to the
process
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Examples of signals
s Examples of signals
q
SIGHUP 1 Hangup ;
q
SIGINT 2 Interrupt ;
q
SIGKILL 9 Killed (by a control C key stroke);
q
SIGBUS 10 Bus Error ;
q
SIGSEGV 11 Segmentation Fault;
q
SIGSYS 12 Bad Argument to System Call ;
q
SIGCHLD 18 Child Status;
s http://pubs.opengroup.org/onlinepubs/009695399/basedefs/signal.h.html
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Signal Handling
s When a process is de-scheduled, it looks for pending signals
or it may be interrupted by OS on signal reception
s A signal handler (default or user defined) is used to process
signals
1.
Signal is generated by particular event
2.
Signal is delivered to a process
3.
Signal is handled
Every signal has default handler provided by the kernel
However, users can install signal handlers with the signal()
system call
Processes may choose to ignore some signals except
SIGKILL and SIGSTOP
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Signal Handling and Threads
s Delivering signals to multithreaded processes is complicated
s Options:
q
Deliver the signal to the thread to which the signal applies
q
Deliver the signal to every thread in the process
q
Deliver the signal to certain threads in the process
q
Assign a specific threat to receive all signals for the
process
s If a thread generates a signal then it is delivered to the thread
causing the signal
s Threads may block some signals, signals deliver to the other
threads which do not block it
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Operating System Examples
s Windows XP Threads
s Solaris Threads
s Linux Threads
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Window XP Thread Structure
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Windows Threads
s One-to-one model (each user-level thread associated with a
kernel thread)
s Thread has a user stack and a kernel stack
s Data structures:
q
ETHREAD
Pointer
to owner process; address of the routine where
thread start
q
KTHREAD
Scheduling
and synchronization information, kernel
stack
q
TEB
User-mode
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Windows TEB
Windows thread information (environment) block
s http://en.wikipedia.org/wiki/Win32_Thread_Information_Block
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Windows Thread States
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Windows Threads Multiprocessing
s A Windows process must contain at least one thread.
That thread may then creates other threads
s Threads from different processes may run in parallel
s In a multiprocessor system, thread on a same process
may execute in parallel
s Threads from a same process shared information
through the common address space they share
s Threads from different processes shared information
through the shared memory segments
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Solaris Process
s Solaris makes use of four separate thread-
related concepts:
q Process:
includes the users address space,
stack, and process control block.
q User-level
threads: a user-created unit of
execution within a process.
q Lightweight
processes: a mapping between
ULTs and kernel threads.
q Kernel
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Relationship between Processes and Threads
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ULTs/LWP/KLTs
s One kernel thread for each LWP
s A process may consist of a single ULT bound to
a single LWP. There is a single thread which
corresponds to traditional Unix process
s A process may consist of several ULTs each
bound to a single LWP, each LWP is bound to a
single kernel thread
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Linux Process/Thread Model
s Linux does not recognize a distinction between
processes and threads
s Use a mechanism similar to LWP of Solaris
where ULTs are mapped into kernel-level
processes that share the same group ID
s Processes in a group share resources such as
files and memory, no need for context switch
when scheduler swaps processes in the same
group
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Linux Threads
s Linux refers to them as tasks rather than
threads
s Thread creation is done through clone()
system call
s clone() allows a child task to share the address
space of the parent task (process)
s When clone() is invoked, it is passed a set of
flags that determine how much sharing take
place
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Linux Threads
s Multiple user-level threads that constitute a
single user-level process are mapped into
kernel-level processes that share the same ID
group using the clone() system call
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End of Chapter 4
Operating System Concepts 8th Edition,
Silberschatz, Galvin and Gagne 2009