How to get better precision out of getrusage on the ARM?

On Fri, Jan 1, 2016 at 1:14 PM, Corey Minyard [off-list ref] wrote:

Those numbers are statistical, if a tick occurs while something is running,
that something is assigned the entire timer tick.  So as you have found, you
can get some pretty unusual numbers on this, especially with short
intervals.

I have some patches at https://sourceforge.net/projects/microstate that add
the ability to do accurate accounting of time, if you really need that.

Thank you Corey,
That makes sense.  I have found that using
clock_gettime(CLOCK_THREAD_CPUTIME_ID, ...) provides the information I
require.  I was confused by the accounting of timer ticks, and
especially by the fact that the accounting changed from exactly 1000us
(or 10000us) to a range centered about 1000us as I fiddled with config
parameters.  I felt sure that one of those config options would result
in accounting only such time to a thread as the thread used.
clock_gettime() returns a thread specific timer that I can use.

At this point, I remain confused/disappointed by the 3us time interval
between calls to clock_gettime().  That feels like a long time for a
pair of context switches into the kernel to get the thread timer, but
I guess that's what it is (on my 500 MHz Cortex-A5).

--wpd