Re: [MLUG] Linux time command -- "kernel mode" and "user mode"

[Stephen Montgomery-Smith <EMAIL:PROTECTED>]

This is my understanding.

You want to add the user and kernel times together.

User time is the time the program spent doing its stuff.  Kernel time is 
time spent doing tasks that the program has given to the kernel.  This 
includes things like writing to files.  Any function call from your 
program whose man page is from section (2) is work done by the kernel.  So 
this would include things like "write", "open", etc.  So a "printf" 
command is not a kernel command, but on the other hand the "printf" 
function includes a lot of "write" commands in it, so while the execution 
of the printf command itself is part of user time, it will make kernel 
calls and use up kernel time.

I am guessing that your program does a lot of disc input or output. 
Somehow the C++ program, being written by hand, perhaps uses the dic 
inefficiently, whereas the python program uses some built in functions 
which have been well designed to write to the dic efficiently.

What does your program do?


On Fri, 3 Nov 2006, Mike Miller wrote:

> We have written a program in C++ and we have also written it with a slightly 
> different algorithm in Python.  We want to see how much slower the Python 
> program is than the C++ program.  We get these results:
>
> Python:
> ------
> real    10m43.357s
> user    9m51.050s
> sys     0m23.420s
>
> C++:
> ------
> real    8m16.668s
> user    5m38.360s
> sys     2m37.780s
>
> The thing I'm trying to understand is what the differences are in the three 
> kinds of timings.  "real" is pretty easy to understand, but it is the least 
> useful of the three times because it depends heavily on system load.  This 
> was extracted from "man time" on GNU/Linux:
>
>  When the -p option is given the (portable) output format
>     real %e
>     user %U
>     sys %S
>  is used.
>
>  %E  Elapsed real time (in [hours:]minutes:seconds).
>
>  %e  (Not in tcsh.) Elapsed real time (in seconds).
>
>  %S  Total number of CPU-seconds that the process spent in kernel mode.
>
>  %U  Total number of CPU-seconds that the process spent in user mode.
>
> Now I just need a more thorough understanding of "kernel mode" and "user 
> mode" -- know any references?
>
> What I'm really interested in knowing is how the programs would compare in 
> elapsed time under optimal conditions -- no other process competing for CPU, 
> disk, etc.  What would come closest to answering my question? Should I just 
> add together the %S and %U times (kernel-mode seconds plus user-mode 
> seconds)?  Does that seem right?
>
> Any tips will be appreciated.  Thanks.
>
> Mike
>
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