# 0.1 Concurrent processes: basic issues  (Page 2/3)

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Trace
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.
Atomic
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.
Livelock
(Informal) Livelock is when two or more threads continue to execute,but make no progress toward the ultimate goal.

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.)

Fairness
(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.

Starvation
(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.

how to know photocatalytic properties of tio2 nanoparticles...what to do now
it is a goid question and i want to know the answer as well
Maciej
Do somebody tell me a best nano engineering book for beginners?
what is fullerene does it is used to make bukky balls
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
so some one know about replacing silicon atom with phosphorous in semiconductors device?
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
for screen printed electrodes ?
SUYASH
What is lattice structure?
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
what's the easiest and fastest way to the synthesize AgNP?
China
Cied
types of nano material
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
Porter
what is the k.e before it land
Yasmin
what is the function of carbon nanotubes?
Cesar
I'm interested in nanotube
Uday
what is nanomaterials​ and their applications of sensors.
what is nano technology
what is system testing?
preparation of nanomaterial
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
what is system testing
what is the application of nanotechnology?
Stotaw
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
Azam
anybody can imagine what will be happen after 100 years from now in nano tech world
Prasenjit
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
Azam
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
Prasenjit
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
Damian
silver nanoparticles could handle the job?
Damian
not now but maybe in future only AgNP maybe any other nanomaterials
Azam
Hello
Uday
I'm interested in Nanotube
Uday
this technology will not going on for the long time , so I'm thinking about femtotechnology 10^-15
Prasenjit
can nanotechnology change the direction of the face of the world
how did you get the value of 2000N.What calculations are needed to arrive at it
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