Overview of Threads

A thread is an encapsulation of the flow of control in a program. Most people are used to writing single-threaded programs - that is, programs that only execute one path through their code "at a time". Multi-threaded programs may have several threads running through
different code paths "simultaneously".
A thread is similar to the sequential programs: a single thread also has a beginning, an end, a sequence, and at any given time during the run time of the thread there is a single point of execution. However, a thread itself is not a program--it cannot run on its own--but runs within a program.

Why use threads?
Threads should not alter the semantics of a program. They simply change the timing of operations. As a result, they are almost always used as an elegant solution to performance related problems. Here are some examples of situations where you might use threads :
* Doing lengthy processing: When a windows application is calculating it cannot process any more messages. As a result, the display cannot be updated.
* Doing background processing: Some tasks may not be time critical, but need to execute continuously.
* Doing I/O work: I/O to disk or to network can have unpredictable delays. Threads allow you to ensure that I/O latency does not delay unrelated parts of your application.

In the program, some operations incur a potentially large delay or CPU hogging, but this delay or CPU usage is unacceptable for other operations; they need to be serviced now.
* Making use of multiprocessor systems: You can't expect one application with only one thread to make use of two or more processors! Chapter 3 explains this in more detail.
* Efficient time sharing: Using thread and process priorities, you can ensure that everyone gets a fair allocation of CPU time.

Similarities between process and threads :
* 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 between process and threads :
* 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.

Overview Of Process

A process is an activity which takes place over time and which has a precise aim regarding the result to be achieved. The concept of a process is hierarchical which means that a process may consist of a partially ordered set of sub-processes. A process is rather abstract. It describes the essentials of the purpose, structure, rationale, roles and timing, leaving plenty of implementation freedom. The power of a process is its abstraction, which enables its application in a wide range of applications, by tailoring its implementation to the specific application. A process can be tailored and elaborated in one or more procedures, which describe cookbook-like what need to be done when and by whom.
The process has been given many definitions for instance :
* A program in Execution.
* An asynchronous activity.
* The 'animated sprit' of a procedure in execution.
* The entity to which processors are assigned.
* The 'dispatchable' unit.
In Process model, all software on the computer is organized into a number of sequential processes. A process includes PC, registers, and variables. Conceptually, each process has its own virtual CPU. In reality, the CPU switches back and forth among processes.
A process goes through a series of discrete process states :
* New State: The process being created.
* Running State: A process is said to be running if it has the CPU, that is, process actually using the CPU at that particular instant.
* Blocked (or waiting) State: A process is said to be blocked if it is waiting for some event to happen such that as an I/O completion before it can proceed. Note that a process is unable to run until some external event happens.
* Ready State: A process is said to be ready if it use a CPU if one were available. A ready state process is runable but temporarily stopped running to let another process run.
* Terminated state: The process has finished execution.

A process in an operating system is represented by a data structure known as a process control block (PCB) or process descriptor. The PCB contains important information about the specific process including :
* The current state of the process i.e., whether it is ready, running, waiting, or whatever.
* Unique identification of the process in order to track "which is which" information. * A pointer to parent process.
* Similarly, a pointer to child process (if it exists).
* The priority of process (a part of CPU scheduling information).
* Pointers to locate memory of processes.
* A register save area.
* The processor it is running on.

Cancer research: Virus that causes cervical cancer also causes penile cancer

Not too many people have heard of penile cancer, that is, cancer that affects the penis; and it is rare, with only around 1% of total cancer cases in the United States and Europe being those of penile cancer. The cancer can cause mutilations, and affects a higher number (upto 10% of all cancer patients) in certain parts of Africa and Asia. Penile cancer starts at the tip (also called the glans) of the penis, and spreads from there. Some of the common symptons of penile cancer include:
* A wart-like growth or lesion
* An open sore that won't heal
* A reddish rash
* Persistent, smelly discharge under the foreskin
The good news is that research has establised that the virus that causes cervical cancer also causes at least half of penile cancer cases, and hence, the human papillomavirus (HPV) vaccine, used against cervical cancer, can also be used against penile cancers (link to article):

Dr. Silvia de Sanjose and colleagues reviewed cases of penile cancer reported in clinical studies between 1986 and 2008 and found 46.9 percent of tumors were associated with HPV.
Nearly all of these were linked to HPV strains 16 and 18, the two types that most commonly cause cervical cancer and which are targeted by Gardasil and Cervarix, they wrote in the Journal of Clinical Pathology.
Merck reported results of a clinical trial last November showing that Gardasil was effective in preventing lesions caused by the virus in men.

Finding that an existing medicine can be used in the fight against cancer is always great news.

First-Come-First-Served (FCFS) Scheduling

First-Come-First-Served algorithm is the simplest scheduling algorithm is the simplest scheduling algorithm. Processes are dispatched according to their arrival time on the ready queue. Being a non-preemptive discipline, once a process has a CPU, it runs to completion. The FCFS scheduling is fair in the formal sense or human sense of fairness but it is unfair in the sense that long jobs make short jobs wait and unimportant jobs make important jobs wait.
FCFS is more predictable than most of other schemes since it offers time. FCFS scheme is not useful in scheduling interactive users because it cannot guarantee good response time. The code for FCFS scheduling is simple to write and understand. One of the major drawback of this scheme is that the average time is often quite long.

CHARACTERISTICS :
o Non-preemptive.
o Ready queue is a FIFO queue.
o Jobs arriving are placed at the end of queue.
o Dispatcher selects first job in queue and this job runs to completion of CPU burst.

Advantages:
- Simple.
- Low overhead.
Disadvantages:
- Inappropriate for interactive systems.
- Large fluctuations in average turnaround time are possible.

Example :
Process Burst Time
P1 24
P2 3
P3 3
Suppose that the processes arrive in the order: P1 , P2 , P3.
Waiting time for P1 = 0; P2 = 24; P3 = 27
Average waiting time: (0 + 24 + 27)/3 = 17

Suppose that the processes arrive in the order P2 , P3 , P1.
Waiting time for P1 = 6; P2 = 0; P3 = 3
Average waiting time: (6 + 0 + 3)/3 = 3
Much better than previous case.

Overview Of Mutual Exclusions in Operating Systems

A way of making sure that if one process is using a shared modifiable data, the other processes will be excluded from doing the same thing.
Formally, while one process executes the shared variable, all other processes desiring to do so at the same time moment should be kept waiting; when that process has finished executing the shared variable, one of the processes waiting; while that process has finished executing the shared variable, one of the processes waiting to do so should be allowed to proceed. In this fashion, each process executing the shared data (variables) excludes all others from doing so simultaneously. This is called Mutual Exclusion.
Mutual exclusion (often abbreviated to mutex) algorithms are used in concurrent programming to avoid the simultaneous use of a common resource, such as a global variable, by pieces of computer code called critical sections. A critical section is a piece of code where a process or thread accesses a common resource. The critical section by itself is not a mechanism or algorithm for mutual exclusion. A program, process, or thread can have critical section in it without any mechanism or algorithm, which implements mutual exclusion.
Examples of such resources are fine-grained flags, counters or queues, used to communicate between code that runs concurrently, such as an application and its interrupt handlers. The problem is acute because a thread can be stopped or started at any time.

How Mutual Exclusion is Done ?
We need to stop the two threads from working on the same data at the same time. The most common way to do this today is by using locks. A lock can be either locked or unlocked. As long as you do not forget to lock or unlock the door, this algorithms guarantees mutual exclusion and protects the so called critical region.
C:
1. omp_set_lock (&my_lock);
2. i++;
3. omp_unset_lock (&my_lock);
Actually, the lock needs to be initialized beforehand and destroyed sometime afterwards as well, but that's not too difficult either.
C:
1. #pragma omp critical
2. {
3. i++;
4. }
Or even simpler, like this:
C:
1. #pragma omp atomic
2. i++;
This basically does the same thing, except you do not need to worry about initialization and destruction of the lock and you cannot forget to unlock the mutex accidentally.

Overview of Race Conditions in Operating Systems

A race condition happens when a system depends on something being done outside of its control before the system reaches a point where it needs to use the results of that something, and there's no way to guarantee that that something will actually be finished when the system needs it.
For example, suppose there's a person who runs a program every morning that prints letters that have been queued throughout the previous day. There's another person in another department who runs a program that queues a letter, and then offers to let the person modify it while it's sitting in the printing queue. If the person runs this program too early in the day (before the printing program gets run), they're essentially in a "race" to finish their work before the printing program runs.

Symptoms Of Race Condition :

The most common symptom of a race condition is unpredictable values of variables that are shared between multiple threads. This results from the unpredictability of the order in which the threads execute. Sometime one thread wins, and sometime the other thread wins. At other times, execution works correctly. Also, if each thread is executed separately, the variable value behaves correctly.
While many different devices are configured to allow multitasking, there is still an internal process that creates a hierarchy of functions. In order for certain functions to take place, other functions must occur beforehand. While the end user perceives that all the functions may appear to be taking place at the same time, this is not necessarily the case.
One common example of a race condition has to do with the processing of data. If a system receives commands to read existing data while writing new data, this can lead to a conflict that causes the system to shut down in some manner. The system may display some type of error message if the amount of data being processed placed an undue strain on available resources, or the system may simply shut down. When this happens, it is usually a good idea to reboot the system and begin the sequence again. If the amount of data being processed is considerable, it may be better to allow the assimilation of the new data to be completed before attempting to read any of the currently stored data.

Nikon Coolpix S220 10MP Digital Camera with 3x Optical Zoom and 2.5 inch LCD (Aqua Green)

Nikon’s ultra slim, stylish Coolpix S220 combines 10.0 effective megapixels with a 3x optical Zoom-Nikkor glass lens for stunning prints as large as 16x20 inches. The sleek, all-metal design slips right in your shirt pocket or purse, and the incredible, bright 2.5-inch LCD makes it easy to compose and share pictures even in bright sunlight. The Coolpix S220, with Nikon’s EXPEED image processor and new 4-Way Image Stabilization, enables you to take incredible pictures, and Nikon’s new Smart Portrait System combines automatic in-camera features that enable users to get great portraits of smiling faces without red-eye and blink.

Technical Details :
* 10.0-megapixel resolution for photo-quality prints up to 16 x 20 inches.
* 3x optical Zoom-Nikkor glass lens; 4-way VR image stabilization.
* 2.5-inch high-resolution LCD screen.
* Nikon's Smart Portrait System; Red-eye Fix, Face Priority AE and more.
* Capture images to SD/SDHC memory cards (not included).



Coolpix S220 Highlights :
- 10.0-megapixel resolution.
- 3x optical Zoom-Nikkor glass lens.
- New EXPEED image processor.
- 4-Way VR Image stabilization.
- Bright 2.5-inch high-resolution LCD.
- Scene Auto Selector.

FEEDBACK :
My Nikon S220 is a good camera.
It is simply a very easy to use, very resaonably priced, point & shoot camera. Simple controls are very nice.
Overall - The physical size, megapixel rating, functionality, ease of use, and quality of images obtained are worth giving it a 5 Star rating.