Re: time system call expensive?

[Vadiraj <vadiraj.cs@gmail.com>]

Great!! thanks for the tar.

I'm not sure if we can time() sys call is quite expensive all time.

Simple code I tried

#include <stdio.h>
#include <time.h>


int main()
{
    int i,k = 0;
    time_t t,j;
    for(i =0 ; i < 100; i++)
    {
        usleep(100);
        //t = time(NULL);
        k++;
        //j = time(NULL);
    }
}

Time without the time() calls.


time ./time_performance

real	0m2.091s
user	0m0.000s
sys	0m0.001s


output with time calls.

time ./time_performance

real	0m2.007s
user	0m0.000s
sys	0m0.001s


Almost same.

timing k++ is not the righ thing but the point is, time as such is not
so expensive IMHO. I did notice considerable difference when I loop to
a 100000 times without the usleep().

just k++ in the loop.
time ./time_performance

real	0m0.003s
user	0m0.000s
sys	0m0.003s


with time() sys call before and after k++ in the loop.
time ./time_performance

real	0m0.168s
user	0m0.026s
sys	0m0.139s




Cheers,
Vadi


On Thu, Jul 21, 2011 at 5:16 PM, Akos Marton <makos999@gmail.com> wrote:
> I advice you `man 3 clock`.
> If you want to get much more precision the attached project can help you.
> Does it help?
>
> Regards,
> mAkos
>
>
> Michal Nazarewicz wrote:
>> On Thu, 21 Jul 2011 06:41:14 +0200, Vadiraj <vadiraj.cs@gmail.com> wrote:
>>> I'm evaluating time consumed by method. I'm using time(NULL) system
>>> call to capture time before and after the call to the function.
>>> Just wanted to know if this has a considerable performance hit? For
>>> all I believe time syscall is quite optimized and should not really be
>>> matter of concern.
>>>
>>> Please let me know if someone have evaluated time(NULL) system over head..
>>>
>>> Assuming the method I'm evaluating is a frequently called method.
>>
>> Why do you worry about it?  What do you need the time for?  If are
>> really using time(2) it means that the function you're calling run time
>> is counted in seconds.  If that's the case, two call to time(2) are
>> by all means negligible.
>>
>> If you need in for benchmark, you would probably do something like:
>>
>> start = time(NULL)
>> for (vary big number)
>>     call function you benchmark
>> end = time(NULL)
>>
>> In either case, you should check gettimeofday(2).
>>
>
>
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