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We assume that your program runs correctly. It would be rather ridiculous to time a program that’s not running right, though this doesn’t mean it doesn’t happen. Depending on what you are doing, you may be interested in knowing how much time is spent overall, or you may be looking at just a portion of the program. We show you how to time the whole program first, and then talk about timing individual loops or subroutines.

Timing a whole program

Under UNIX, you can time program execution by placing the time command before everything else you normally type on the command line. When the program finishes, a timing summary is produced. For instance, if your program is called foo , you can time its execution by typing time foo . If you are using the C shell or Korn shell, time is one of the shell’s built-in commands. With a Bourne shell, time is a separate command executable in /bin . In any case, the following information appears at the end of the run:

  • User time
  • System time
  • Elapsed time

These timing figures are easier to understand with a little background. As your program runs, it switches back and forth between two fundamentally different modes: user mode and kernel mode . The normal operating state is user mode. It is in user mode that the instructions the compiler generated on your behalf get executed, in addition to any subroutine library calls linked with your program. Cache miss time is buried in here too. It might be enough to run in user mode forever, except that programs generally need other services, such as I/O, and these require the intervention of the operating system — the kernel. A kernel service request made by your program, or perhaps an event from outside your program, causes a switch from user mode into kernel mode.

Time spent executing in the two modes is accounted for separately. The user time figure describes time spent in user mode. Similarly, system time is a measure of the time spent in kernel mode. As far as user time goes, each program on the machine is accounted for separately. That is, you won’t be charged for activity in somebody else’s application. System time accounting works the same way, for the most part; however, you can, in some instances, be charged for some system services performed on other people’s behalf, in addition to your own. Incorrect charging occurs because your program may be executing at the moment some outside activity causes an interrupt. This seems unfair, but take consolation in the fact that it works both ways: other users may be charged for your system activity too, for the same reason.

Taken together, user time and system time are called CPU time . Generally, the user time is far greater than the system time. You would expect this because most applications only occasionally ask for system services. In fact, a disproportionately large system time probably indicates some trouble. For instance, programs that are repeatedly generating exception conditions, such as page faults, misaligned memory references, or floating-point exceptions, use an inordinate amount of system time. Time spent doing things like seeking on a disk, rewinding a tape, or waiting for characters at the terminal doesn’t show up in CPU time. That’s because these activities don’t require the CPU; the CPU is free to go off and execute other programs.

Questions & Answers

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 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
While the American heart association suggests that meditation might be used in conjunction with more traditional treatments as a way to manage hypertension
Beverly Reply
Researchers demonstrated that the hippocampus functions in memory processing by creating lesions in the hippocampi of rats, which resulted in ________.
Mapo Reply
The formulation of new memories is sometimes called ________, and the process of bringing up old memories is called ________.
Mapo 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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