Drift when measuring time using add_timer

Drift when measuring time using add_timer

[Mohan V]

Hello All,

I am testing the timers on a OMAP4 based board. I am seeing a drift or
the time difference between the timer value and when the function callback
happens. This drift is around 1000ms or 1s for a expiry value of >=5mins.
Is the time drift expected? Is this way of measuring right or is there
any other way?

OMAP4 as you may know is a multi-core processor and CONFIG_SMP is enabled
along with the following:
CONFIG_NO_HZ, CONFIG_HIGH_RES_TIMERS

My kernel configurations:
kernel: 2.6.37-rc7 (commit id: ffc96d)
config: omap2plus_defconfig

My driver does the following:

struct timeval tv;
test_timer_proc_write()
{
           del_timer_sync(&my_timer);
           period = 300000; /* 5min */
           my_timer.expires = jiffies + msecs_to_jiffies(period);
           my_timer.function = func_timeout; /* Call back function*/
           /* Save the current time in global variable */
           do_gettimeofday(&timeval);
           /* add timer with an expiry value of 5mins*/
           add_timer(&my_timer);
}

/* Get the current time and print the difference between the two values */
func_timeout()
{
           struct timeval tv1;
           do_gettimeofday(&tv);
           /* take the difference between current time and saved time */
           ms = (tv.tv_sec - tv1.tv_sec) * 1000 + \
                   (tv.tv_usec - tv1.tv_usec) / 1000;
           printk("Interval = %d\n", ms);
}

Thanks for your time.

Regards,
Mohan

Re: Drift when measuring time using add_timer

[Nikita V. Youshchenko]

>            do_gettimeofday(&timeval);

Try getnstimeofday() instead

Re: Drift when measuring time using add_timer

[Mohan V]

On Tue, Dec 28, 2010 at 5:44 PM, Nikita V. Youshchenko <yoush@cs.msu.su> wrote:
>>            do_gettimeofday(&timeval);
>
> Try getnstimeofday() instead
>
I tried with getnstimeofday(), I am getting the same result. Anyway
getnstimeofday()
gets called from do_gettimeofday().

Thanks,
Mohan

Re: Drift when measuring time using add_timer

[Yong Zhang]

On Tue, Dec 28, 2010 at 8:05 PM, Mohan V <mohanvforum@gmail.com> wrote:
> Hello All,
>
> I am testing the timers on a OMAP4 based board. I am seeing a drift or
> the time difference between the timer value and when the function callback
> happens. This drift is around 1000ms or 1s for a expiry value of >=5mins.
> Is the time drift expected? Is this way of measuring right or is there
> any other way?
>
> OMAP4 as you may know is a multi-core processor and CONFIG_SMP is enabled
> along with the following:
> CONFIG_NO_HZ, CONFIG_HIGH_RES_TIMERS
>
> My kernel configurations:
> kernel: 2.6.37-rc7 (commit id: ffc96d)
> config: omap2plus_defconfig
>
> My driver does the following:
>
> struct timeval tv;
> test_timer_proc_write()
> {
>           del_timer_sync(&my_timer);
>           period = 300000; /* 5min */
>           my_timer.expires = jiffies + msecs_to_jiffies(period);
>           my_timer.function = func_timeout; /* Call back function*/

what's the slack of your timer?
And what about setting my_timer.slack = 0; ?

Thanks,
Yong

>           /* Save the current time in global variable */
>           do_gettimeofday(&timeval);
>           /* add timer with an expiry value of 5mins*/
>           add_timer(&my_timer);
> }
>
> /* Get the current time and print the difference between the two values */
> func_timeout()
> {
>           struct timeval tv1;
>           do_gettimeofday(&tv);
>           /* take the difference between current time and saved time */
>           ms = (tv.tv_sec - tv1.tv_sec) * 1000 + \
>                   (tv.tv_usec - tv1.tv_usec) / 1000;
>           printk("Interval = %d\n", ms);
> }
>
> Thanks for your time.
>
> Regards,
> Mohan
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/
>



-- 
Only stand for myself.

Re: Drift when measuring time using add_timer

[Mohan V]

On Wed, Dec 29, 2010 at 2:15 PM, Yong Zhang <yong.zhang0@gmail.com> wrote:
> On Tue, Dec 28, 2010 at 8:05 PM, Mohan V <mohanvforum@gmail.com> wrote:
>> Hello All,
>>
>> I am testing the timers on a OMAP4 based board. I am seeing a drift or
>> the time difference between the timer value and when the function callback
>> happens. This drift is around 1000ms or 1s for a expiry value of >=5mins.
>> Is the time drift expected? Is this way of measuring right or is there
>> any other way?
>>
>> OMAP4 as you may know is a multi-core processor and CONFIG_SMP is enabled
>> along with the following:
>> CONFIG_NO_HZ, CONFIG_HIGH_RES_TIMERS
>>
>> My kernel configurations:
>> kernel: 2.6.37-rc7 (commit id: ffc96d)
>> config: omap2plus_defconfig
>>
>> My driver does the following:
>>
>> struct timeval tv;
>> test_timer_proc_write()
>> {
>>           del_timer_sync(&my_timer);
>>           period = 300000; /* 5min */
>>           my_timer.expires = jiffies + msecs_to_jiffies(period);
>>           my_timer.function = func_timeout; /* Call back function*/
>
> what's the slack of your timer?
> And what about setting my_timer.slack = 0; ?
>
I hadn't set the slack value for the timer. Setting it to "0", gives me
much better results. Thanks a lot Yong.

Regards,
Mohan

> Thanks,
> Yong
>
>>           /* Save the current time in global variable */
>>           do_gettimeofday(&timeval);
>>           /* add timer with an expiry value of 5mins*/
>>           add_timer(&my_timer);
>> }
>>
>> /* Get the current time and print the difference between the two values */
>> func_timeout()
>> {
>>           struct timeval tv1;
>>           do_gettimeofday(&tv);
>>           /* take the difference between current time and saved time */
>>           ms = (tv.tv_sec - tv1.tv_sec) * 1000 + \
>>                   (tv.tv_usec - tv1.tv_usec) / 1000;
>>           printk("Interval = %d\n", ms);
>> }
>>
>> Thanks for your time.
>>
>> Regards,
>> Mohan
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>> Please read the FAQ at  http://www.tux.org/lkml/
>>
>
>
>
> --
> Only stand for myself.
>