Threads
Despite of the fact that a thread must execute in process, the process and
its associated threads are different concept. Processes are used to group
resources together and threads are the entities scheduled for execution on the
CPU.
A thread is a single sequence stream
within in a process. Because threads have some of the properties of processes,
they are sometimes called lightweight processes. In a process, threads allow
multiple executions of streams. In many respect, threads are popular way to improve
application through parallelism. The CPU switches rapidly back and forth among
the threads giving illusion that the threads are running in parallel. Like a
traditional process i.e., process with one thread, a thread can be in any of
several states (Running, Blocked, Ready or Terminated). Each thread has its own
stack. Since thread will generally call different procedures and thus a
different execution history. This is why thread needs its own stack. An operating system that has thread facility,
the basic unit of CPU utilization is a thread. A thread has or consists of a program counter (PC), a register set, and
a stack space. Threads are not independent of one other like processes as a
result threads shares with other threads their code section, data section, OS
resources also known as task, such as open files and signals.
Processes Vs Threads
As we mentioned earlier that in many respect threads operate in the same way
as that of processes. Some of the similarities and differences are:
Similarities
- Like processes threads share CPU and only one thread active (running) at a
time.
- Like processes, threads within a processes, threads within a processes
execute sequentially.
- Like processes, thread can create children.
- And like process, if one thread is blocked, another thread can run.
Differences
- Unlike processes, threads are not independent of one another.
- Unlike processes, all threads can access every address in the task .
- Unlike processes, thread are design to assist one other. Note that
processes might or might not assist one another because processes may
originate from different users.
Why Threads?
Following are some reasons why we use threads in designing operating systems.
- A process with multiple threads make a great server for example printer
server.
- Because threads can share common data, they do not need to use
interprocess communication.
- Because of the very nature, threads can take advantage of multiprocessors.
Threads are cheap in the sense that
- They only need a stack and storage for registers therefore, threads are
cheap to create.
- Threads use very little resources of an operating system in which they are
working. That is, threads do not need new address space, global data, program
code or operating system resources.
- Context switching are fast when working with threads. The reason is that
we only have to save and/or restore PC, SP and registers.
But this cheapness does not come free - the biggest drawback is that there is
no protection between threads.
User-Level Threads
User-level threads implement in user-level libraries, rather than via systems
calls, so thread switching does not need to call operating system and to cause
interrupt to the kernel. In fact, the kernel knows nothing about user-level
threads and manages them as if they were single-threaded processes.
Advantages:
The most obvious advantage of this technique is that a user-level threads
package can be implemented on an Operating System that does not support threads.
Some other advantages are
- User-level threads does not require modification to operating systems.
- Simple Representation:
Each thread is represented simply by a PC, registers, stack and a small
control block, all stored in the user process address space.
- Simple Management:
This simply means that creating a thread, switching between threads and
synchronization between threads can all be done without intervention of the
kernel.
- Fast and Efficient:
Thread switching is not much more expensive than a procedure call.
Disadvantages:
- There is a lack of coordination between threads and operating system
kernel. Therefore, process as whole gets one time slice irrespect of whether
process has one thread or 1000 threads within. It is up to each thread to
relinquish control to other threads.
- User-level threads requires non-blocking systems call i.e., a
multithreaded kernel. Otherwise, entire process will blocked in the kernel,
even if there are runable threads left in the processes. For example, if one
thread causes a page fault, the process blocks.
Kernel-Level Threads
In this method, the kernel knows about and manages the threads. No runtime
system is needed in this case. Instead of thread table in each process, the
kernel has a thread table that keeps track of all threads in the system. In
addition, the kernel also maintains the traditional process table to keep track
of processes. Operating Systems kernel provides system call to create and manage threads.
The implementation of general structure of kernel-level thread is
<DIAGRAM>
Advantages:
- Because kernel has full knowledge of all threads, Scheduler may decide to
give more time to a process having large number of threads than process having
small number of threads.
- Kernel-level threads are especially good for applications that frequently
block.
Disadvantages:
- The kernel-level threads are slow and inefficient. For instance, threads
operations are hundreds of times slower than that of user-level threads.
- Since kernel must manage and schedule threads as well as processes. It
require a full thread control block (TCB) for each thread to maintain
information about threads. As a result there is significant overhead and
increased in kernel complexity.
Advantages of Threads over Multiple Processes
- Context Switching Threads are very inexpensive to
create and destroy, and they are inexpensive to represent. For example, they
require space to store, the PC, the SP, and the general-purpose registers, but
they do not require space to share memory information, Information about open
files of I/O devices in use, etc. With so little context, it is much faster to
switch between threads. In other words, it is relatively easier for a context
switch using threads.
- Sharing Treads allow the sharing of a lot resources
that cannot be shared in process, for example, sharing code section, data
section, Operating System resources like open file etc.
Disadvantages of Threads over Multiprocesses
- Blocking
The major disadvantage if that if the kernel is single threaded, a system call
of one thread will block the whole process and CPU may be idle during the
blocking period.
- Security
Since there is, an extensive sharing among threads there is a potential problem
of security. It is quite possible that one thread over writes the stack of
another thread (or damaged shared data) although it is very unlikely since
threads are meant to cooperate on a single task.
Application that Benefits from Threads
A proxy server satisfying the requests for a number of computers on a LAN would
be benefited by a multi-threaded process. In general, any program that has to do
more than one task at a time could benefit from multitasking. For example, a
program that reads input, process it, and outputs could have three threads, one
for each task.
Application that cannot Benefit from Threads
Any sequential process that cannot be divided into parallel task will not
benefit from thread, as they would block until the previous one completes. For
example, a program that displays the time of the day would not benefit from
multiple threads.
Resources used in Thread Creation and Process Creation
When a new thread is created it shares its code section, data section and
operating system resources like open files with other threads. But it is
allocated its own stack, register set and a program counter.
The creation of a new process differs from that of a thread mainly in the
fact that all the shared resources of a thread are needed explicitly for each
process. So though two processes may be running the same piece of code they need
to have their own copy of the code in the main memory to be able to run. Two
processes also do not share other resources with each other. This makes the
creation of a new process very costly compared to that of a new thread.
Context Switch
To give each process on a multiprogrammed machine a fair share of the CPU, a
hardware clock generates interrupts periodically. This allows the operating
system to schedule all processes in main memory (using scheduling algorithm) to
run on the CPU at equal intervals. Each time a clock interrupt occurs, the
interrupt handler checks how much time the current running process has used. If
it has used up its entire time slice, then the CPU scheduling algorithm (in
kernel) picks a different process to run. Each switch of the CPU from one
process to another is called a context switch.
Major Steps of Context Switching
- The values of the CPU registers are saved in the process table of the process
that was running just before the clock interrupt occurred.
- The registers are loaded from the process picked by the CPU scheduler to run
next.
In a multiprogrammed uniprocessor computing system, context switches occur
frequently enough that all processes appear to be running concurrently.
If a process has more than one thread, the Operating System can use the context
switching technique to schedule the threads so they appear to execute in
parallel. This is the case if threads are implemented at the kernel level.
Threads can also be implemented entirely at the user level in run-time
libraries. Since in this case no thread scheduling is provided by the Operating
System, it is the responsibility of the programmer to yield the CPU frequently
enough in each thread so all threads in the process can make progress.
Action of Kernel to Context Switch Among Threads
The threads share a lot of resources with other peer threads belonging to the
same process. So a context switch among threads for the same process is easy. It
involves switch of register set, the program counter and the stack. It is
relatively easy for the kernel to accomplished this task.
Action of kernel to Context Switch Among Processes
Context switches among processes are expensive. Before a process can be
switched its process control block (PCB) must be saved by the operating system.
The PCB consists of the following information:
- The process state.
- The program counter, PC.
- The values of the different registers.
- The CPU scheduling information for the process.
- Memory management information regarding the process.
- Possible accounting information for this process.
- I/O status information of the process.
When the PCB of the currently executing process is saved the operating system
loads the PCB of the next process that has to be run on CPU. This is a heavy
task and it takes a lot of time.
