Lecture 12
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Code examples on csserver in directory
/home/hwang/cs470/lecture12
Reminder: Homework 3 and Process
Management Project due on Wednesday.
Questions?
Monday, February 6
CS 470 Operating Systems - Lecture 12
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Outline
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Threads
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CS 470 Operating Systems - Lecture 12
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Threads
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So far, we have assumed that each process
has a single thread of control. This is often
called a heavyweight process.
Most modern OS's allow a process to have
multiple threads of control. Each one is often
called a lightweight process.
A thread is a flow of control within a process. It
is the basic unit of CPU utilization. Like a
process it has a thread ID (TID), program
counter, registers, and a stack.
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Threads
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Unlike a process, a thread shares code, data,
and OS resources like open files with other
threads in its process.
code
code
code
code
data
data
data
data
stack
stack
stack
stack stack stack
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Three processes
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Three threads
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Multi-threaded Applications
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Many applications are now multi-threaded.
Some are for convenience. E.g. web browser
might have threads to display images, handle
network traffic, …
For others, multiple threads is almost
necessary. E.g. web server, since do not want
to handle requests serially. Traditional
implementations fork a new process to handle a
request. Since each request usually is
independent of each other, this works well.
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Multi-threaded Applications
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For applications with lots of shared data, forking
a new process is a very heavyweight technique
with lots of overhead. Creating a thread is
much easier.
Modern OS kernels are often multi-threaded
with each service being handled by a separate
thread. Examples: Solaris interrupt handling,
Linux free memory management.
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Benefits of Multi-threading
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Responsiveness: an application can continue to
run even if parts are blocked. This is especially
good for networked applications
Resource sharing: decreases contention.
Economy: easier to create and switch between
threads. Example: in Solaris 2, process
creation 30x slower; context switch 5x slower.
Multi-core/processor architectures: easier to
utilize.
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Multi-core Programming
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Multiple cores appear as separate processors
to the OS.
Multi-threaded programming is a mechanism to
take advantage of this.
E.g., 4 threads on a single core will alternate:
123412341234...
On a dual-core machine, might assign two
threads to each core: 131313... and 242424...
On a quad-core, perhaps one thread per core.
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Multi-core Programming
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Challenges to effective utilization
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How to divide activities? Is this a programmer
issue?
How to balance activities? Equal work or equal
value from each core.
How to manage data? How is it divided among the
cores?
How to synchronize threads? One task may
depend on the result of another.
How to test and debug? Much more difficult.
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Thread Models
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Generally, two types of threads
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Kernel-level: running as part of the OS. Creation is
similar to process creation. Most OS's support this.
User-level: running on top of the OS and
implemented as a library API.
Programmers usually deal with user-level
threads. These are mapped onto kernel-level
threads by the OS.
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Many-To-One
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Multiple user-level threads
are mapped to one kernel
level thread. This is not true
concurrency.
user
user
kernel
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Fast, since all management is in user space.
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But system calls block the entire process.
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user
Examples: Solaris Green threads, GNU
Portable Threads
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One-To-One
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One user thread maps to one
kernel thread.
More parallelism, can allocate
on separate processors.
user
user
user
kernel
kernel
kernel
Can run another thread when one blocks.
Kernel thread creation is relatively expensive,
so may limit the number of threads.
Examples: Linux, Win95 & later, Solaris 9 &
later
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Many-To-Many
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Overcomes limitations of
previous models.
Use as many user-level
threads as want to.
user
user
user
kernel
kernel
kernel
Create as many kernel-level threads as
needed.
User-level threads are multiplexed among the
kernel threads.
Examples: IRIX, HP-UX, pre-Solaris 9
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Thread Libraries
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Libraries provide programmer API for creating
and managing threads. They can be user-level
(regular library) or kernel-level (system calls).
Three main ones in use:
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P(osix) threads: standard specification of
behavior, not an implementation. Can be either
user-level or kernel-level. Most systems have or
can get one.
Win32 threads: kernel-level
Programming language API, e.g. Java thread
objects.
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Library Examples
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File thrd-posix.c shows use of the pthreads
library.
To compile, need to explicitly link the pthread
library
$gcc ­o th_xmpl thrd­posix.c ­lpthread
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See "man pthread.h" for overview
documentation. Also see man pages for
individual functions, e.g. pthread_create.
File thrd-win32.c shows use of Win32 threads.
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Library Examples
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File Driver.java show use of Java thread API.
Objects either extend Thread class directly or
implement the Runnable interface. The
Runnable interface requires a method named
run that is executed on creation.
Since thread object is part of JVM, usually
implemented on top of native thread support.
To compile and run Java programs
$ javac Driver.java
$ java Driver
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Threading Issues
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How does fork( ) work? fork( ) creates a copy
of a process. Should it duplicate all threads, or
just the calling thread? Perhaps it depends on
whether exec( ) is being called?
How does thread cancellation work?
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Asynchronous cancellation: immediately terminate
thread. What problems may arise? Most common.
Deferred cancellation: schedule a cancellation, but
wait for target to notice and terminate self. E.g.
Pthreads have cancellation points.
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Threading Issues
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How does signal handling work? Signals get
delivered to a process. Which thread handles
the signal?
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Thread to which signal applies. E.g., synchronous
signals in response to an illegal operation.
All threads. E.g., asynchronous signal like SIGINT
that terminates a process.
Subset of threads. Perhaps handled by the first
non-blocked thread in the set.
A dedicated signal handling thread. E.g., Solaris 2.
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Thread Creation
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Threads may be created more often than
processes, so want creation to be fast.
Common technique is to have a pool of threads
that are waiting to be loaded with code and run.
This is also done with processes, e.g. web
servers.
Pools can also be used to limit the number of
threads/processes running at a given time.
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Examples
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Example: Win XP implementation of Win32 API
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OS runs each application as a process
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Process can have more than one thread
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Default model is one-to-one
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Can use Fiber library to get many-to-many
Example: Linux threads
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Uses clone( ) to create. Works like fork( ).
Separate flags to control sharing. If share all, a true
thread. If share none, almost a process.
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