On 05/07/12 17:30, Luis Claudio R. Goncalves wrote:
> On Mon, May 07, 2012 at 03:00:17PM -0700, Frank Rowand wrote:
> | This is the resulting message on the ARM panda with a 'bad'
> | 32khz timer:
> |
> |
> | # cyclictest -q -p 80 -t -n -l 10 -h ${hist_bins} -R 100
> | reported clock resolution: 1 nsec
> | measured clock resolution less than: 30517 nsec
>
> How about using a fixed loop size (say 1000000 clock reads) to define
> the average cost of reading the clock (the second value presented above)
> instead of a variable amount of iterations? Reading the clock twice and
> calculating the average could lead to wrong impressions. Also, it would
> be interesting to run such test under a real time priority (FIFO:2, maybe?)
> to avoid too much external interference on the readings, mainly involuntary
> context switches.
>
> Having too different values called 'clock resolution' may be a good source
> of confusion. The value of clock_getres() is the resolution, as per the
> system jargon, and the second value should be granularity, reading cost,
> the-average-time-it-takes-to-read-the-clock or something alike.
The 'measured clock resolution' is not intended to be representative of the
time it takes for clock_gettime() to execute. It really is a measure of
the resolution that the clock is actually delivering. It is the smallest
amount that the actual clock actually increments.
Look at the patch again:
If consecutive calls to clock_gettime() return the same value, then the
clock resolution is larger than the execution time of clock_gettime().
If this is the case, then when the clock moves to a new value, the
difference from the old value is representative of the clock resolution.
The patch calls clock_gettime() a number of times in a loop, collecting
timestamps. The timestamps are then scanned, looking for:
1) does a difference of zero occur?
2) what is the minimum non-zero difference?
If a difference of zero is found, then the clock resolution is larger
than the execution time of clock_gettime(). If this resolution is
_also_ greater than the resolution that the clock claims from
clock_getres(), then this is reported.
Reporting the minimum non-zero difference is being generous.
Saying that the measured resolution is "less than" is also
being generous. What can I say, I'm trying to be optimistic? :-)
This might be more obvious if I show you the actual clock_gettime()
values from a run on an ARM panda:
# cyclictest -q -p 80 --smp -l 1 -R 50 -v
For consecutive calls to clock_gettime():
time, delta time (nsec)
8720.940737610 0
8720.940737610 0
8720.940737610 0
8720.940737610 0
8720.940737610 0
8720.940737610 0
8720.940737610 0
8720.940768124 30514
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940768124 0
8720.940798638 30514
8720.940798638 0
8720.940798638 0
reported clock resolution: 1 nsec
measured clock resolution less than: 30514 nsec
>
> | A possible follow on patch would be to generate a hard
> | error (fail the test) if the measured resolution was
> | above some unreasonable value (perhaps > 1 msec), but
> | allow the hard fail to be overridden with yet another
> | command line option. Any opinions about that?
>
> My suggestion is to keep the current behavior and add an option to
> stop/complain case the clock has a poor resolution or has a reading
> cost too high.
>
> Luis
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