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Expand a comment regarding geometric sampling.

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This commit is contained in:
Jason Evans 2011-03-14 22:22:29 -07:00
parent fa5d245aef
commit ff7450727f
1 changed files with 16 additions and 2 deletions
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18
jemalloc/include/jemalloc/internal/prof.h
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@ -247,8 +247,22 @@ prof_sample_threshold_update(prof_tdata_t *prof_tdata)
double u; double u;
/* /*
* Compute prof_sample_threshold as a geometrically distributed random * Compute sample threshold as a geometrically distributed random
* variable with mean (2^opt_lg_prof_sample). * variable with mean (2^opt_lg_prof_sample).
*
* __ __
* | log(u) | 1
* prof_tdata->threshold = | -------- |, where p = -------------------
* | log(1-p) | opt_lg_prof_sample
* 2
*
* For more information on the math, see:
*
* Non-Uniform Random Variate Generation
* Luc Devroye
* Springer-Verlag, New York, 1986
* pp 500
* (http://cg.scs.carleton.ca/~luc/rnbookindex.html)
*/ */
prn64(r, 53, prof_tdata->prn_state, prn64(r, 53, prof_tdata->prn_state,
(uint64_t)6364136223846793005LLU, (uint64_t)1442695040888963407LLU); (uint64_t)6364136223846793005LLU, (uint64_t)1442695040888963407LLU);
@ -374,7 +388,7 @@ prof_sample_accum_update(size_t size)
/* Take care to avoid integer overflow. */ /* Take care to avoid integer overflow. */
if (size >= prof_tdata->threshold - prof_tdata->accum) { if (size >= prof_tdata->threshold - prof_tdata->accum) {
prof_tdata->accum -= (prof_tdata->threshold - size); prof_tdata->accum -= (prof_tdata->threshold - size);
/* Compute new prof_sample_threshold. */ /* Compute new sample threshold. */
prof_sample_threshold_update(prof_tdata); prof_sample_threshold_update(prof_tdata);
while (prof_tdata->accum >= prof_tdata->threshold) { while (prof_tdata->accum >= prof_tdata->threshold) {
prof_tdata->accum -= prof_tdata->threshold; prof_tdata->accum -= prof_tdata->threshold;