do_gettimeofday( ) precision?

do_gettimeofday( ) precision?

[abhijit]

hello,

the comment on top of do_gettimeofday( ) [arch/i386/kernel/time.c] says:

/*
 * This version of gettimeofday has microsecond resolution
 * and better than microsecond precision on fast x86 machines with TSC.
 */
void do_gettimeofday(struct timeval *tv)
{
...

but the code below limits usec to <= 1000000. so isn't the resolution
not limited to microsec even on TSC capable boxes? 

if one wants higher resolution (for timestamping etc.) whats the preferred
way?

thanks
abhijit

Re: do_gettimeofday( ) precision?

[Lukasz Michal Rak]

On Wed, 23 Jun 2004, abhijit wrote:

> hello,
>
> the comment on top of do_gettimeofday( ) [arch/i386/kernel/time.c] says:
>
> /*
>  * This version of gettimeofday has microsecond resolution
>  * and better than microsecond precision on fast x86 machines with TSC.
>  */
> void do_gettimeofday(struct timeval *tv)
> {
> ...
>
> but the code below limits usec to <= 1000000. so isn't the resolution
> not limited to microsec even on TSC capable boxes?
>
> if one wants higher resolution (for timestamping etc.) whats the preferred
> way?
>
> thanks
> abhijit

	Hi!

 I think you are right that do_gettimeofday() is limited to microsec even
on boxes with TSC register. This function returns only seconds and
microseconds so you won't get better resolution with it. I don't know the
way how to do it, but I wonder about precision of such mechanism (if any
exists).
 Regards,

		Lukasz

Re: do_gettimeofday( ) precision?

[abhijit]

On Wed, 23 Jun 2004 17:54:13 +0200 (CEST), Lukasz Michal Rak
<l.rak@elka.pw.edu.pl> wrote:

> I don't know the way how to do it, but I wonder about 
> precision of such mechanism (if any exists).

basically i want some counter w/ microsend resolution that fits into 32 bits.
couple of possible solutions come to my mind:

[1]  use TSC and convert it to microsec
[2] use get_timeofday( ) and convert to microsec
[3] use xtime variable directly and convert to microsec
(any other ways?)

conversion in [1] will be costly (a div involved). 
conversion in [2]/[3] can be done using bit operators. (usec|sec << 20)
but [2] will incur function call overhead which i'd like to avoid.

so is using xtime directly ok and reliable?

thanks
abhijit

Re: do_gettimeofday( ) precision?

[abhijit]

On Thu, 24 Jun 2004 12:13:11 +0530, abhijit <slashdev@gmail.com> wrote:
>
> basically i want some counter w/ microsend resolution that fits into 32 bits.
> couple of possible solutions come to my mind:
> 
> [1]  use TSC and convert it to microsec
> [2] use get_timeofday( ) and convert to microsec
> [3] use xtime variable directly and convert to microsec
> (any other ways?)
> 
> conversion in [1] will be costly (a div involved).
> conversion in [2]/[3] can be done using bit operators. (usec|sec << 20)
> but [2] will incur function call overhead which i'd like to avoid.
> 
> so is using xtime directly ok and reliable?

ok. after reading up the source, i think xtime isn;t updated frequently and will
not have the required resolution :( so calling do_gettimeofday() seems the only
way out (unless somebody has any other idea).

btw why isn't do_gettimeofday( ) declared as "inline". so its bit
cheaper to call
especially where you are timestamping (say packets) often.

thanks
abhijit