Here's a hypothetical scenario to illustrate the use of timer map.
A video frame is updated between frame_refresh_start() and
frame_refresh_end(), in most cases, the interval between these two
functions is less than 40ms, but occasionally over 200ms.
We can set a timer which alarm after the frame_refresh_start() has
been executed 42ms(slightly larger than 40ms) and destory this timer
if frame_refresh_end() is called. So, for most cases, the timer is not
triggered, this can significantly reduce the amount of trace data.