Help regarding python run time

[Irmen de Jong]

On 20-5-2015 17:23, Ian Kelly wrote:
> On Wed, May 20, 2015 at 7:02 AM, AKHIL RANA <AKHILR at iitk.ac.in> wrote:
>> Hi,
>>
>> I am student at IIT Kanpur and working on a Opencv based Python project. I
>> am working on program development which takes less time to execute. For that
>> i have tested my small program hello word on python to now the time taken by
>> this program. I had run many time. and every time it run it gives me a
>> different run time.
>>
>> Can we some how make the program to run at constant time so that we can work
>> on how to reduce the timing ?
> 
> Not practically. The exact run time is dependent on a lot of factors
> that are mostly out of your control: what other programs are running,
> what they're doing and what resources they're using at that moment;
> what hardware interrupts occur while your program is running; is data
> to be read from disk cached or not; is data to be loaded from RAM
> cached or not; if the disk is mechanical, what cylinder and sector
> does the read head happen to be at when it gets a read request.
> 
> Instead of trying to measure an exact time over one run, you will get
> better results by running the program several times and then taking
> the minimum measurement as representative of the program's runtime
> under ideal conditions.
> 

Or measure the actual CPU clock cycles taken instead of the wall clock run time.
Then you should get a fairly constant number, if the program does the same work every
time you run it.

phobos:~ irmen$ time python test.py
real    0m3.368s
user    0m0.214s    <--- cpu time spent in user mode actually doing work
sys     0m0.053s


Irmen