i change the interval to 50 msec then CPU usage decrease to 25 percent !
i am now working on do something to avoid repaint all of pixmap every 50 msec .

now my question is 25 percent just for drawing 800 line is not too high although i use opengl widget ?
my program is not too efficient or its is common usage for this job .

another interesting result is happens . when i change my widget from QGLWidget to QWidget and re run the program the result do not change and CPU usage is also about 25. i am sure that this usage is just for drawing because when i comment the line of drawing pixmap the usage then decrease to 3 percent.

are you sure that Qt doing something extra about using GPU in use of QGLWidget ?