[R] system.time question
Prof Brian Ripley
ripley at stats.ox.ac.uk
Sat Oct 20 18:57:59 CEST 2012
On 20/10/2012 17:16, Mark Leeds wrote:
> Hi : I looked at the help for system.time but I still have the following
> question. Can someone explain the output following output
> of system.time :
> user system elapsed
> 12399.681 5632.352 56935.647
Yes, the help page can, via ?proc.time. As it says, it depends on the OS
> Here's my take based on the fact that I was doing ps -aux | grep R off and
> on and the total amount of CPU minutes that
> got allotted before the job ended was about 5 hours and the total actual
> time that the job took was about 15 hours.
> Does elapsed = total actual time job taken ? That seems to be the case or a
> strange coincidence.
> Does user + system = CPU time from ps -aux | grep R ? That seems to be the
> case also or a weird coincidence.
On Fedora Linux, yes. Not in general (and what ps gives is pretty
OS-specific: for example, does it include time from child processes or
not -- system.time should but the OS calls used do not always do so, I
find less reliably so in Fedora 16 than 14).
> Finally, why can't the CPU get a higher percentage ? It's seems like it's
> always around 30% which would make sense since
> 5 is ~ 30% of 15 hours.
Many, many reasons. Most likely
- other things are running, and some of them have a higher priority, or
equal or lower priority and get lots of time slices ....
- R the process is waiting for resources, such as memory, discs, network
> Also, assuming my take above is correct, when talking about timing of
> algorithms, in this case, does one say the job took 5 hours or 15 hours ?
> I'm trying to see how fast an algorithm is compared to others and I'm not
> sure what the standard is. I'm on fedora 16.0 and using R 2.15. Thanks.
It depends on the purpose. CRAN's check farm cares most about CPU
usage: someone waiting for results cares about elapsed time.
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Brian D. Ripley, ripley at stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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