Getting Started with QNX Neutrino - QNX Software Systems

© 2009, QNX Software Systems GmbH & Co. KG. Advanced topics
An accurate timestamp
resolution for a little while yet!) The nsec member indicates how many nanoseconds
elapse between ticks of the base timing clock. The default is 10 milliseconds (1
millisecond on machines with CPU speeds of greater than 40 MHz), so the nsec
member (if you use the “get” form of the call by specifying the old parameter) will
show approximately 10 million nanoseconds. (As we discussed above, in “Clock
interrupt sources,” it’s not going to be exactly 10 millisecond.)
While you can certainly feel free to try to set the base timing resolution on your
system to something ridiculously small, the kernel will step in and prevent you from
doing that. Generally, you can set most systems in the 1 millisecond to hundreds of
microseconds range.
There is one timebase that might be available on your processor that doesn’t obey the
rules of “base timing resolution” we just described. Some processors have a
high-frequency (high-accuracy) counter built right into them, which Neutrino can let
you have access to via the ClockCycles() call. For example, on a Pentium processor
running at 200 MHz, this counter increments at 200 MHz as well, so it can give you
timing samples right down to 5 nanoseconds. This is particularly useful if you want to
figure out exactly how long a piece of code takes to execute (assuming of course, that
you don’t get preempted). You’d call ClockCycles() before your code and after your
code, and then compute the delta. See the Neutrino Library Reference for more
details.
Note that on an SMP system, you may run into a little problem. If your thread gets a
ClockCycles() value from one CPU and then eventually runs on another CPU, you
may get inconsistent results. This stems from the fact that the counters used by
ClockCycles() are stored in the CPU chips themselves, and are not synchronized
between CPUs. The solution to this is to use thread affinity to force the thread to run
on a particular CPU.
Advanced topics
Other clock sources
Now that we’ve seen the basics of timers, we’ll look at a few advanced topics:
1 the CLOCK_SOFTTIME and CLOCK_MONOTONIC timer types, and
2 kernel timeouts
We’ve seen the clock source CLOCK_REALTIME, and mentioned that a POSIX
conforming implementation may supply as many different clock sources as it feels
like, provided that it at least provides CLOCK_REALTIME.
What is a clock source? Simply put, it’s an abstract source of timing information. If
you want to put it into real life concepts, your personal watch is a clock source; it
measures how fast time goes by. Your watch will have a different level of accuracy
April 30, 2009 Chapter 3 • Clocks, Timers, and Getting a Kick Every So Often 159