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A trace is the sequence of operations (and their data) performed in a particular run of a concurrent program.Equivalently, it is the sequence of states during a run i.e. , the collection of variable and program counter values.
An atomic operation is indivisible. A context switch to another process cannot happen during this operation.

Both the syntax and semantics of the concurrency primitives can vary among programming languages and concurrency libraries.We will use a particular language, Promela , which implements the most commonly-used features ofconcurrent programs. In Promela,assignment statements are defined to be atomic. We'll later contrast atomic with non-atomic assignment statements.

Problems with concurrency

Programmers are comfortable with debugging single-threaded code, where most questions involve ``doesthis sequence of instructions compute the correct answer?''Concurrency, however, introduces the added complications of communication and synchronization;a single task may be spread across several sequences of instructions running asynchronously,and multiple threads can interact in unintended ways. The following are five common categories of problems with concurrency.

(Informal) Deadlock is when two or more threads stop and wait for each other.
(Informal) Livelock is when two or more threads continue to execute,but make no progress toward the ultimate goal.

Deadlock vs. livelock

As an analogy, consider two people walking in a hallway towards each other. The hallway is wide enough for twopeople to pass. Of interest is what happens when the two people meet.If on the same side of the hallway, a polite strategy is to step to the other side.A more belligerent strategy is to wait for the other person to move. Two belligerent people will suffer in deadlock, glaringface to face in front of each other. Two polite people could suffer from livelockif they repeatedly side-step simultaneously. (No conclusions on morality are to be inferred fromthe fact that one polite and one belligerent person don't have any problems.)

Two belligerent hallway-walkers can deadlock.
(Informal) Fairness is the idea that each thread gets a turn to make progress.

There are more specific notions of fairness that describe when and how often threads are guaranteed to get a turn.For example, do we expect that each thread should be executed as often as any other, or is it acceptable if one runs a hundred stepsfor each step of any other thread? When considering fairness, one must also consider the system codeimplementing the threads. The implementation includes a scheduler that determines how to interleave the threads, and the schedulermight or might not provide any fairness guarantees.

(Informal) Starvation is when some thread gets deferred forever.

Fairness and starvation

Assume we are modeling a store. Consumers buy products, reducing the shelf inventory. Meanwhile, employees restockthe shelves, increasing the shelf inventory. More simply, let thread C be a loop that repeatedly decrements a counter, and thread P be a loop that repeatedly increments that same counter.

Unless our scheduler (which may be nature) otherwise gives some guarantees,this system does not ensure that the two threads execute fairly.In particular, it is possible for one thread to starve the other.

Questions & Answers

can someone help me with some logarithmic and exponential equations.
Jeffrey Reply
sure. what is your question?
okay, so you have 6 raised to the power of 2. what is that part of your answer
I don't understand what the A with approx sign and the boxed x mean
it think it's written 20/(X-6)^2 so it's 20 divided by X-6 squared
I'm not sure why it wrote it the other way
I got X =-6
ok. so take the square root of both sides, now you have plus or minus the square root of 20= x-6
oops. ignore that.
so you not have an equal sign anywhere in the original equation?
Commplementary angles
Idrissa Reply
im all ears I need to learn
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what is a good calculator for all algebra; would a Casio fx 260 work with all algebra equations? please name the cheapest, thanks.
Kevin Reply
a perfect square v²+2v+_
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Abdirahman Reply
algebra 2 Inequalities:If equation 2 = 0 it is an open set?
Kim Reply
or infinite solutions?
The answer is neither. The function, 2 = 0 cannot exist. Hence, the function is undefined.
Embra Reply
if |A| not equal to 0 and order of A is n prove that adj (adj A = |A|
Nancy Reply
rolling four fair dice and getting an even number an all four dice
ramon Reply
Kristine 2*2*2=8
Bridget Reply
Differences Between Laspeyres and Paasche Indices
Emedobi Reply
No. 7x -4y is simplified from 4x + (3y + 3x) -7y
Mary Reply
is it 3×y ?
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J, combine like terms 7x-4y
Bridget Reply
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Rachael Reply
I'm not good at math so would you help me
what is the problem that i will help you to self with?
how do you translate this in Algebraic Expressions
linda Reply
Need to simplify the expresin. 3/7 (x+y)-1/7 (x-1)=
Crystal Reply
. After 3 months on a diet, Lisa had lost 12% of her original weight. She lost 21 pounds. What was Lisa's original weight?
Chris Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
many many of nanotubes
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what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
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what is system testing?
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
what is system testing
what is the application of nanotechnology?
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
anybody can imagine what will be happen after 100 years from now in nano tech world
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
silver nanoparticles could handle the job?
not now but maybe in future only AgNP maybe any other nanomaterials
can nanotechnology change the direction of the face of the world
Prasenjit Reply
At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
Ali Reply
the Beer law works very well for dilute solutions but fails for very high concentrations. why?
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how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Model checking concurrent programs. OpenStax CNX. Oct 27, 2005 Download for free at http://cnx.org/content/col10294/1.3
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