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RE: [lmi] Problem of the week: testing a testing tool
RE: [lmi] Problem of the week: testing a testing tool
Wed, 10 Jan 2007 14:42:49 -0500
On 2007-01-09 7:44 Zulu, Greg Chicares wrote:
> Let's focus on this one first:
>> c) On those that don't have the '1.#IO' and appear correct,
>> scientific notation and millisecond counts don't match exactly.
> In the data shown,
> (i) the LHS is greater than the RHS, and
> (ii) their difference is exactly one unit in the last place.
> Would exactly those conditions always obtain when a test like
> this is rerun?
Not always. Sometimes they match and sometimes they don't. I
ran 20-odd tests consecutively and every one but one [see last
one here] had at least 1 that was off. Usually it was 2 that
were off. The least frequently correct was 'Overhead:'.
Overhead: [3.961e+000] 1 iteration took 3960 milliseconds
Vector : [9.221e+000] 1 iteration took 9220 milliseconds
Write : [7.738e+000] 1 iteration took 7738 milliseconds
Overhead: [3.986e+000] 1 iteration took 3985 milliseconds
Vector : [9.165e+000] 1 iteration took 9165 milliseconds
Write : [8.159e+000] 1 iteration took 8158 milliseconds
Overhead: [3.945e+000] 1 iteration took 3944 milliseconds
Vector : [9.030e+000] 1 iteration took 9030 milliseconds
Write : [7.691e+000] 1 iteration took 7690 milliseconds
Overhead: [4.376e+000] 1 iteration took 4376 milliseconds
Vector : [9.697e+000] 1 iteration took 9697 milliseconds
Write : [8.414e+000] 1 iteration took 8413 milliseconds
Overhead: [3.966e+000] 1 iteration took 3966 milliseconds
Vector : [9.387e+000] 1 iteration took 9387 milliseconds
Write : [7.971e+000] 1 iteration took 7971 milliseconds
Erratic nature traditionally points to a rounding problem.
>> For c) Mismatched values are result of differing methods of
>> mathematical manipulation. The scientific notation keeps its
>> floating-point value and is divided by the number of iterations
>> [z] where the milliseconds are multiplied by 1000.0 and cast as
>> an int.
> Is that enough information to answer the new question posed
> under (0) above?
Not completely. Additional testing with display of intermediate
values showed that the millisecond value was being truncated
not rounded. The truncated value explains the differences.
> Anyway, what would be better?
Round the millisecond value to 0 decimals after multiplying
by 1000 instead of casting as an int. See patch below.
Giving this consistently correct output:
Overhead: [3.984e+000] 1 iteration took 3984 milliseconds
Vector : [9.105e+000] 1 iteration took 9105 milliseconds
Write : [7.814e+000] 1 iteration took 7814 milliseconds
Overhead: [3.976e+000] 1 iteration took 3976 milliseconds
Vector : [9.153e+000] 1 iteration took 9153 milliseconds
Write : [7.797e+000] 1 iteration took 7797 milliseconds
Overhead: [4.088e+000] 1 iteration took 4088 milliseconds
Vector : [9.713e+000] 1 iteration took 9713 milliseconds
Write : [8.196e+000] 1 iteration took 8196 milliseconds
Overhead: [5.386e-005] 10 iterations took 1 milliseconds
Vector : [2.243e-003] 10 iterations took 22 milliseconds
Read : [1.079e-002] 1 iteration took 11 milliseconds
Write : [2.770e-003] 10 iterations took 28 milliseconds
'cns' io: [5.345e-002] 1 iteration took 53 milliseconds
'ill' io: [1.961e-002] 1 iteration took 20 milliseconds
Overhead: [7.241e-005] 10 iterations took 1 milliseconds
Vector : [3.697e-003] 10 iterations took 37 milliseconds
Read : [1.751e-002] 1 iteration took 18 milliseconds
Write : [1.043e-002] 1 iteration took 10 milliseconds
'cns' io: [8.790e-002] 1 iteration took 88 milliseconds
'ill' io: [3.257e-002] 1 iteration took 33 milliseconds
Overhead: [4.290e-005] 100 iterations took 4 milliseconds
Vector : [2.158e-003] 100 iterations took 216 milliseconds
Read : [7.445e-003] 10 iterations took 74 milliseconds
Write : [2.581e-003] 100 iterations took 258 milliseconds
'cns' io: [5.219e-002] 10 iterations took 522 milliseconds
'ill' io: [1.946e-002] 10 iterations took 195 milliseconds
[3.245e-003] 100 iterations took 325 milliseconds
[3.296e-002] 10 iterations took 330 milliseconds
[4.964e-001] 1 iteration took 496 milliseconds
Ran 30-odd tests and all produced matching output.
>>> 3. How could those defects have been prevented?
>> Classify, standardize, document, disseminate and utilize rules
>> [protocols if you like that better] for various implementation
>> situations. Ex: "Be sure a numeric value is not negative before
>> casting as unsigned."
> Do we have any that would apply here?
"If rounded values are to be compared, they must be the result of
similar rounding methods and precision."
"Be sure a numeric value is not negative before casting as unsigned."
"Always check divisors for zero before they are used in division."
>> Rigorous unit testing at coding time [e.g. matrix of all possible
>> conditions encountered for that operation.]
> How big would that matrix be in this situation?
Potentially huge. In general, when performing floating point
math you need to at least cover all precisions possible for
the size number you are using. For rounding, cover all digits
[0-9] in the most significant decimal place with results
depending on the rounding method. Consider the results of
similar yet not exact rounding methods when their values will
be compared either visually or mathematically [e.g.
materially_equal()]. Concerning signed vs. unsigned, consider
the possible results of a calculation [e.g. plus, zero, minus,
overflowed, undefined] and its effect on subsequent casting.
>> Use an interactive debugger.
>>> "4. the virtues of stepping through every line of code using
>>> the debugger."
> BTW, he means for every single line of code you write, not
> just lines suspected of being erroneous.
> Would you actually do that?
I have done that in the past. Would try to now if I could get
the debugger to work.
>> From a critical review:
>> How thorough should your testing be? Maguire talks about
>> 'coverage' and explains that you should step through both arms
>> of each if statement to ensure statement coverage - step through
>> with the debugger, by the way!
>> Code review by others.
> Which (one or more) of those practices should we adopt?
Why not all?
Rigorous unit testing at coding time. Stepping through with
the debugger can be coupled with this.
Code review by others.
>>> 4. How should those defects be removed?
> I.e., what patch would you propose for this?
+ #include "round_to.hpp"
std::string Timer::elapsed_msec_str() const
- oss << static_cast<int>(1000.0 * elapsed_usec());
+ round_to<double> const RoundToNearestWhole(0, r_to_nearest);
+ oss << RoundToNearestWhole(1000.0 * elapsed_usec());
oss << " milliseconds";
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