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dtrace: fix normalization of stddev aggregation

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To be upstreamed.

Discussed with:	Bryan Cantrill <bryancantrill@gmail.com>
MFC after:	2 weeks
Sponsored by:	Panzura
This commit is contained in:
Andriy Gapon 2017-04-14 15:31:04 +00:00
parent f4247773fa
commit 5ad79d9b20
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=316853
1 changed files with 2 additions and 0 deletions
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cddl/contrib/opensolaris/lib/libdtrace/common/dt_consume.c
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@ -389,8 +389,10 @@ dt_stddev(uint64_t *data, uint64_t normal)
* The standard approximation for standard deviation is
* sqrt(average(x**2) - average(x)**2), i.e. the square root
* of the average of the squares minus the square of the average.
* When normalizing, we should divide the sum of x**2 by normal**2.
*/
dt_divide_128(data + 2, normal, avg_of_squares);
dt_divide_128(avg_of_squares, normal, avg_of_squares);
dt_divide_128(avg_of_squares, data[0], avg_of_squares);
norm_avg = (int64_t)data[1] / (int64_t)normal / (int64_t)data[0];