<< Chapter < Page Chapter >> Page >

Given that we have multithreaded capabilities and multiprocessors, we must still convince the threads to work together to accomplish some overall goal. Often we need some ways to coordinate and cooperate between the threads. There are several important techniques that are used while the program is running with multiple threads, including:

  • Fork-join (or create-join) programming
  • Synchronization using a critical section with a lock, semaphore, or mutex
  • Barriers

Each of these techniques has an overhead associated with it. Because these overheads are necessary to go parallel, we must make sure that we have sufficient work to make the benefit of parallel operation worth the cost.

Fork-join programming

This approach is the simplest method of coordinating your threads. As in the earlier examples in this chapter, a master thread sets up some global data structures that describe the tasks each thread is to perform and then use the pthread_create( ) function to activate the proper number of threads. Each thread checks the global data structure using its thread-id as an index to find its task. The thread then performs the task and completes. The master thread waits at a pthread_join( ) point, and when a thread has completed, it updates the global data structure and creates a new thread. These steps are repeated for each major iteration (such as a time-step) for the duration of the program:


for(ts=0;ts<10000;ts++) { /* Time Step Loop */ /* Setup tasks */for (ith=0;ith<NUM_THREADS;ith++) pthread_create(..,work_routine,..) for (ith=0;ith<NUM_THREADS;ith++) pthread_join(...) }work_routine() { /* Perform Task */return; }

The shortcoming of this approach is the overhead cost associated with creating and destroying an operating system thread for a potentially very short task.

The other approach is to have the threads created at the beginning of the program and to have them communicate amongst themselves throughout the duration of the application. To do this, they use such techniques as critical sections or barriers.

Synchronization

Synchronization is needed when there is a particular operation to a shared variable that can only be performed by one processor at a time. For example, in previous SpinFunc( ) examples, consider the line:

globvar++;

In assembly language, this takes at least three instructions:


LOAD R1,globvar ADD R1,1STORE R1,globvar

What if globvar contained 0, Thread 1 was running, and, at the precise moment it completed the LOAD into Register R1 and before it had completed the ADD or STORE instructions, the operating system interrupted the thread and switched to Thread 2? Thread 2 catches up and executes all three instructions using its registers: loading 0, adding 1 and storing the 1 back into globvar . Now Thread 2 goes to sleep and Thread 1 is restarted at the ADD instruction. Register R1 for Thread 1 contains the previously loaded value of 0; Thread 1 adds 1 and then stores 1 into globvar . What is wrong with this picture? We meant to use this code to count the number of threads that have passed this point. Two threads passed the point, but because of a bad case of bad timing, our variable indicates only that one thread passed. This is because the increment of a variable in memory is not atomic . That is, halfway through the increment, something else can happen.

Questions & Answers

where we get a research paper on Nano chemistry....?
Maira Reply
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
learn
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
learn
Google
da
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Bhagvanji
hey
Giriraj
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
revolt
da
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
Damian
yes that's correct
Professor
I think
Professor
Nasa has use it in the 60's, copper as water purification in the moon travel.
Alexandre
nanocopper obvius
Alexandre
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
Rafiq
if virus is killing to make ARTIFICIAL DNA OF GRAPHENE FOR KILLED THE VIRUS .THIS IS OUR ASSUMPTION
Anam
analytical skills graphene is prepared to kill any type viruses .
Anam
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
Hafiz
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply

Get the best Algebra and trigonometry course in your pocket!





Source:  OpenStax, High performance computing. OpenStax CNX. Aug 25, 2010 Download for free at http://cnx.org/content/col11136/1.5
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'High performance computing' conversation and receive update notifications?

Ask