<< Chapter < Page Chapter >> Page >

If it were all that simple, you wouldn’t need this book. While compilers are extremely clever, there is still a lot of ways to improve the performance of your code without sacrificing its portability. Instead of converting the whole program to C and using a thread library, you can assist the compiler by adding compiler directives to our source code.

Compiler directives are typically inserted in the form of stylized FORTRAN comments. This is done so that a nonparallelizing compiler can ignore them and just look at the FORTRAN code, sans comments. This allows to you tune your code for parallel architectures without letting it run badly on a wide range of single-processor systems.

There are two categories of parallel-processing comments:

  • Assertions
  • Manual parallelization directives

Assertions tell the compiler certain things that you as the programmer know about the code that it might not guess by looking at the code. Through the assertions, you are attempting to assuage the compiler’s doubts about whether or not the loop is eligible for parallelization. When you use directives, you are taking full responsibility for the correct execution of the program. You are telling the compiler what to parallelize and how to do it. You take full responsibility for the output of the program. If the program produces meaningless results, you have no one to blame but yourself.

Assertions

In a previous example, we compiled a program and received the following output:


E6000: f77 -O3 -autopar -loopinfo -o dep dep.f dep.f:"dep.f", line 6: not parallelized, call may be unsafe "dep.f", line 8: not parallelized, unsafe dependence (a)E6000:

An uneducated programmer who has not read this book (or has not looked at the code) might exclaim, “What unsafe dependence, I never put one of those in my code!” and quickly add a no dependencies assertion. This is the essence of an assertion. Instead of telling the compiler to simply parallelize the loop, the programmer is telling the compiler that its conclusion that there is a dependence is incorrect. Usually the net result is that the compiler does indeed parallelize the loop.

We will briefly review the types of assertions that are typically supported by these compilers. An assertion is generally added to the code using a stylized comment.

No dependencies

A no dependencies or ignore dependencies directive tells the compiler that references don’t overlap. That is, it tells the compiler to generate code that may execute incorrectly if there are dependencies. You’re saying, “I know what I’m doing; it’s OK to overlap references.” A no dependencies directive might help the following loop:


DO I=1,N A(I) = A(I+K) * B(I)ENDDO

If you know that k is greater than -1 or less than -n , you can get the compiler to parallelize the loop:


C$ASSERT NO_DEPENDENCIES DO I=1,NA(I) = A(I+K) * B(I) ENDDO

Of course, blindly telling the compiler that there are no dependencies is a prescription for disaster. If k equals -1 , the example above becomes a recursive loop.

Relations

You will often see loops that contain some potential dependencies, making them bad candidates for a no dependencies directive. However, you may be able to supply some local facts about certain variables. This allows partial parallelization without compromising the results. In the code below, there are two potential dependencies because of subscripts involving k and j:

Questions & Answers

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
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
sciencedirect big data base
Ernesto
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
NANO
what is fullerene does it is used to make bukky balls
Devang Reply
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
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
NANO
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
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 ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
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
Sanket Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
Privacy Information Security Software Version 1.1a
Good
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