Getting Started with QNX Neutrino - QNX Software Systems

Clocks and timers
© 2009, QNX Software Systems GmbH & Co. KG.
Base timing resolution
Getting more precision
The point of this story is that regardless of the nice round value shown, the real value
is selected to be the next faster value.
Let’s say that the timer tick is operating at just slightly faster than 10 ms. Can I
reliably sleep for 3 milliseconds?
Nope.
Consider what happens in the kernel. You issue the C-library delay() call to go to sleep
for 3 milliseconds. The kernel has to set the variable in the ISR to some value. If it
sets it to the current time, this means the timer has already expired and that you should
wake up immediately. If it sets it to one tick more than the current time, this means
that you should wake up on the next tick (up to 10 milliseconds away).
The moral of this story is: “Don’t expect timing resolution any better than the input
timer tick rate.”
Under Neutrino, a program can adjust the value of the hardware divisor component in
conjunction with the kernel (so that the kernel knows what rate the timer tick ISR is
being called at). We’ll look at this below in the “Getting and setting the realtime
clock” section.
Timing jitter
There’s one more thing you have to worry about. Let’s say the timing resolution is
10 ms and you want a 20 ms timeout.
Are you always going to get exactly 20 milliseconds worth of delay from the time that
you issue the delay() call to the time that the function call returns?
Absolutely not.
There are two good reasons why. The first is fairly simple: when you block, you’re
taken off the running queue. This means that another thread at your priority may now
be using the CPU. When your 20 milliseconds have expired, you’ll be placed at the
end of the READY queue for that priority so you’ll be at the mercy of whatever thread
happens to be running. This also applies to interrupt handlers running or
higher-priority threads running — just because you are READY doesn’t mean that
you’re consuming the CPU.
The second reason is a bit more subtle. The following diagram will help explain why:
140 Chapter 3 • Clocks, Timers, and Getting a Kick Every So Often April 30, 2009