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
access ....
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, rip...@stats.ox.ac.uk
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