Problem with QTimer under Win XP

[wzielj]

I use QTimer to schedule events every 20ms. Unfortunately under Win XP scheduling doesn't work.



timer = new QTimer(this);
//Co 1/50s event
timer->start(20);
QObject::connect( timer, SIGNAL( timeout() ), this,SLOT( timerEvent() ) );

I think timer not always send signal - i mean if i put line mesurement += 0.02 in timerEvent method than after 10 sec mesurement is 8.6..

I've tried with qt_win_use_simple_timers=true; but i received compilation's error. If this solution might work please explain me step by step how to fix it...I use QT v 4.4.3

[seneca]

From the docs:


Note that QTimer's accuracy depends on the underlying operating system and hardware. Most platforms support an accuracy of 1 millisecond, but Windows 98 supports only 55. If Qt is unable to deliver the requested number of timer clicks, it will silently discard some.

Could this be the case?

[wzielj]

Propably yes but is there any other way i can call ops every 20ms ?

[seneca]

I think the problem is that sometimes the slot is running 20 ms or longer. This is a basic problem that you can only to a certain extent solve by optimizing, but you can never solve it completely because XP is not a realtime operating system where you have guarantee about the processing time assigned to a certain thread or process.

Maybe you should explain more what your goal is, there could be a better approach than timers, unless you really need realtime capability.

[wzielj]

My aim is to call an event exactly every 20ms

[wysota]

My aim is to call an event exactly every 20ms

Just for fun or is there anything more into it? :)

[Lesiok]

Event recurrence in time is warranted only in RTOS. Windows XP is not RTOS.

[The Storm]

You can try with events, because they are 10 function calls faster than the signals/slots, perhaps you will get better results. For more info - http://doc.trolltech.com/4.4/qobject.html#timerEvent

[wysota]

Timers (regardless of how you use them) do not guarantee they will called exactly every interval miliseconds. If you set a timeout to 100ms, it can be called from 0 to 10 times within a second and there is nothing you can do about it, it's just the way timers work on non-realtime operating systems.

[faldzip]

and if you really want that event every 20ms you have to use some real time operating system, like QNX.