[Interest] QTimer and Qt::PreciseTimer

[Joshua Grauman]

Thiago,

Thanks, this is really helpful. This particular code is only used by me 
(I've added a module to my program to allow screencasting of my program 
with alpha channel so that it can be overlaid over video of me. Hence the 
need for the clock to be roughly synched to the camera clock so the video 
and screencast are in synch). So I am just trying to optimize the 
situation to get the best clock for me. So when I was reading about ntp, 
it sounded like it would gently adjust the time (speed up / slow down 
internal clock to synch with time), and thereby correct for drift over 
time. But it sounds like switching to QElapsedTimer is better.

Do you know offhand about how accurate typical PC system clocks are? I'm 
wondering about how much drift there would be after an hour of recording 
screencasts.

Josh


> On Friday, 28 July 2017 16:15:27 PDT Joshua Grauman wrote:
>> I have a related but different question.
>>
>> I am reading the time with QDateTime::currentMSecsSinceEpoch(). How
>> susceptible is this time to clock drift/jitter/etc. on Linux (Kubuntu
>> 16.04.2)?
>
> It's susceptible to complete jumps. You're using the realtime clock.
>
>> I know ntpd is constantly updating the time, and there is some
>> error involved in this.
>
> How do you know that all your users are using ntpd? How do you know that they
> are in a network that allows NTP packets (it's UDP)?
>
> And how sure are you that the system won't run ntpdate on a whim once you
> connect to a network? (it shouldn't, but a lot of poorly-configured systems
> exist)
>
>> If I am reading the time from
>> QDateTime::currentMSecsSinceEpoch() and using it to set a QTimer every
>> 40ms, how does ntpd constantly correcting the time going to affect this,
>> if at all?
>
> QTimer has nothing to do with the current wall-clock time. The Qt event loop
> uses the monotonic clock exclusively.
>
>> FYI: My timer slot basically looks like this:
>>
>> void myClass::animationTimerSlot(void)
>> {
>>    if(firstTime)
>>      startTime=QDateTime::currentMSecsSinceEpoch();
>>
>>    dowork();
>>
>>    frameCnt++;
>>    int t = startTime + ((double)frameCnt*40.0) -
>> QDateTime::currentMSecsSinceEpoch(); if(t<0)
>>      t=0;
>>    animTimer->start(t);
>> }
>
> Your code is susceptible to time jumps. If the clock went backwards for a
> second (for example), your t would be about 1000 and your animation would stop
> for a second.
>
> Use the monotonic clock instead.
>
> -- 
> Thiago Macieira - thiago.macieira (AT) intel.com
>  Software Architect - Intel Open Source Technology Center
>
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