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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

how do they get the third part x = (32)5/4
kinnecy Reply
can someone help me with some logarithmic and exponential equations.
Jeffrey Reply
sure. what is your question?
ninjadapaul
20/(×-6^2)
Salomon
okay, so you have 6 raised to the power of 2. what is that part of your answer
ninjadapaul
I don't understand what the A with approx sign and the boxed x mean
ninjadapaul
it think it's written 20/(X-6)^2 so it's 20 divided by X-6 squared
Salomon
I'm not sure why it wrote it the other way
Salomon
I got X =-6
Salomon
ok. so take the square root of both sides, now you have plus or minus the square root of 20= x-6
ninjadapaul
oops. ignore that.
ninjadapaul
so you not have an equal sign anywhere in the original equation?
ninjadapaul
Commplementary angles
Idrissa Reply
hello
Sherica
im all ears I need to learn
Sherica
right! what he said ⤴⤴⤴
Tamia
hii
Uday
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+_
Dearan Reply
kkk nice
Abdirahman Reply
algebra 2 Inequalities:If equation 2 = 0 it is an open set?
Kim Reply
or infinite solutions?
Kim
The answer is neither. The function, 2 = 0 cannot exist. Hence, the function is undefined.
Al
y=10×
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 ?
Joan Reply
J, combine like terms 7x-4y
Bridget Reply
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
China
Cied
types of nano material
abeetha Reply
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.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
AMJAD
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
AMJAD
what is system testing
AMJAD
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
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?
bamidele Reply
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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