linux-kernel - Re: Drift when measuring time using add

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Message-ID: <AANLkTinq4MTrLek8DE5SAR=-eFk==4VX-+ui0kVtrV_L@mail.gmail.com>
Date:	Wed, 29 Dec 2010 15:50:51 +0530
From:	Mohan V <mohanvforum@...il.com>
To:	Yong Zhang <yong.zhang0@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Drift when measuring time using add_timer

On Wed, Dec 29, 2010 at 2:15 PM, Yong Zhang <yong.zhang0@...il.com> wrote:
> On Tue, Dec 28, 2010 at 8:05 PM, Mohan V <mohanvforum@...il.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
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>
>
>
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