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In addition to the negative performance impact due to cache misses, the virtual memory system can also slow your program down if it is too large to fit in the memory of the system or is competing with other large jobs for scarce memory resources.

Under most UNIX implementations, the operating system automatically pages pieces of a program that are too large for the available memory out to the swap area. The program won’t be tossed out completely; that only happens when memory gets extremely tight, or when your program has been inactive for a while. Rather, individual pages are placed in the swap area for later retrieval. First of all, you need to be aware that this is happening if you don’t already know about it. Second, if it is happening, the memory access patterns are critical. When references are too widely scattered, your runtime will be completely dominated by disk I/O.

If you plan in advance, you can make a virtual memory system work for you when your program is too large for the physical memory on the machine. The techniques are exactly the same as those for tuning a software-managed out-of-core solution, or loop nests. The process of “blocking” memory references so that data consumed in neighborhoods uses a bigger portion of each virtual memory page before rotating it out to disk to make room for another. We examine the techniques for blocking in [link] Chapter 8.

Gauging the size of your program and the machine’s memory

How can you tell if you are running out-of-core? There are ways to check for pag- ing on the machine, but perhaps the most straightforward check is to compare the size of your program against the amount of available memory. You do this with the size command:


% size myprogram

On a System V UNIX machine, the output looks something like this:


53872 + 53460 + 10010772 = 10118104

On a Berkeley UNIX derivative you see something like this:


text data bss hex decimal 53872 53460 10010772 9a63d8 10118104

The first three fields describe the amount of memory required for three different portions of your program. The first, text, accounts for the machine instructions that make up your program. The second, data, includes initialized values in your pro- gram such as the contents of data statements, common blocks, externals, character strings, etc. The third component, bss, (block started by symbol), is usually the largest. It describes an uninitialized data area in your program. This area would be made of common blocks that are not set by a block data. The last field is a total for all three sections added together, in bytes. Warning: The size command won’t give you the full picture if your program allocates memory dynamically, or keeps data on the stack. This area is especially important for C programs and FORTRAN programs that create large arrays that are not in COMMON.

Next, you need to know how much memory you have in your system. Unfortunately, there isn’t a standard UNIX command for this. On the RS/6000, /etc/lscfg tells you. On an SGI machine, /etc/hinv does it. Many System V UNIX implementations have an /etc/memsize command. On any Berkeley derivative, you can type:

Questions & Answers

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
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
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
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 did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, High performance computing. OpenStax CNX. Aug 25, 2010 Download for free at http://cnx.org/content/col11136/1.5
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