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Fix Xm mode calculation to properly average N=10 modes.

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This is still fast enough. ~100usec on my laptop with 1000 build times.
This commit is contained in:
Mike Perry 2020-11-06 16:30:16 -06:00
parent 406400a74d
commit ed9d60cb92
2 changed files with 27 additions and 38 deletions
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63
src/core/or/circuitstats.c
View File

@ -853,58 +853,49 @@ circuit_build_times_create_histogram(const circuit_build_times_t *cbt,
* Return the Pareto start-of-curve parameter Xm.
*
* Because we are not a true Pareto curve, we compute this as the
* weighted average of the N most frequent build time bins. N is either
* 1 if we don't have enough circuit build time data collected, or
* determined by the consensus parameter cbtnummodes (default 3).
* weighted average of the 10 most frequent build time bins. This
* heuristic allowed for the actual timeout rate to be closest
* to the chosen quantile cutoff, for quantiles 60-80%, out of
* many variant approaches (see #40157 for analysis).
*/
static build_time_t
circuit_build_times_get_xm(circuit_build_times_t *cbt)
{
build_time_t i, nbins;
build_time_t nbins = 0;
build_time_t *nth_max_bin;
int32_t bin_counts=0;
build_time_t ret = 0;
uint32_t *histogram = circuit_build_times_create_histogram(cbt, &nbins);
int n=0;
build_time_t xm_total = 0;
build_time_t Xm = 0;
int32_t xm_counts=0;
int num_modes = circuit_build_times_default_num_xm_modes();
uint32_t *histogram = circuit_build_times_create_histogram(cbt, &nbins);
tor_assert(nbins > 0);
tor_assert(num_modes > 0);
// Only use one mode if < 1000 buildtimes. Not enough data
// for multiple.
if (cbt->total_build_times < CBT_NCIRCUITS_TO_OBSERVE)
num_modes = 1;
nth_max_bin = tor_calloc(num_modes, sizeof(build_time_t));
/* Determine the N most common build times */
for (i = 0; i < nbins; i++) {
if (histogram[i] >= histogram[nth_max_bin[0]]) {
nth_max_bin[0] = i;
}
for (n = 1; n < num_modes; n++) {
if (histogram[i] >= histogram[nth_max_bin[n]] &&
(!histogram[nth_max_bin[n-1]]
|| histogram[i] < histogram[nth_max_bin[n-1]])) {
/* Determine the N most common build times, by selecting the
* nth largest mode, counting it, and removing it from the histogram. */
for (int n = 0; n < num_modes; n++) {
/* Get nth mode */
for (build_time_t i = 0; i < nbins; i++) {
if (histogram[i] > histogram[nth_max_bin[n]]) {
nth_max_bin[n] = i;
}
}
/* Update average */
xm_counts += histogram[nth_max_bin[n]];
xm_total += CBT_BIN_TO_MS(nth_max_bin[n])*histogram[nth_max_bin[n]];
/* Prevent from re-counting this value */
histogram[nth_max_bin[n]] = 0;
}
for (n = 0; n < num_modes; n++) {
bin_counts += histogram[nth_max_bin[n]];
ret += CBT_BIN_TO_MS(nth_max_bin[n])*histogram[nth_max_bin[n]];
log_info(LD_CIRC, "Xm mode #%d: %u %u", n, CBT_BIN_TO_MS(nth_max_bin[n]),
histogram[nth_max_bin[n]]);
}
/* bin_counts can become zero if all of our last CBT_NCIRCUITS_TO_OBSERVE
/* xm_counts can become zero if all of our last CBT_NCIRCUITS_TO_OBSERVE
* circuits were abandoned before they completed. This shouldn't happen,
* though. We should have reset/re-learned a lower timeout first. */
if (bin_counts == 0) {
ret = 0;
if (xm_counts == 0) {
log_warn(LD_CIRC,
"No valid circuit build time data out of %d times, %u modes, "
"have_timeout=%d, %lfms", cbt->total_build_times, num_modes,
@ -912,15 +903,13 @@ circuit_build_times_get_xm(circuit_build_times_t *cbt)
goto done;
}
tor_assert(bin_counts > 0);
ret /= bin_counts;
Xm = xm_total / xm_counts;
done:
tor_free(histogram);
tor_free(nth_max_bin);
return ret;
return Xm;
}
/**

2
src/core/or/circuitstats.h
View File

@ -58,7 +58,7 @@ void circuit_build_times_mark_circ_as_measurement_only(origin_circuit_t *circ);
#define CBT_BIN_WIDTH ((build_time_t)10)
/** Number of modes to use in the weighted-avg computation of Xm */
#define CBT_DEFAULT_NUM_XM_MODES 3
#define CBT_DEFAULT_NUM_XM_MODES 10
#define CBT_MIN_NUM_XM_MODES 1
#define CBT_MAX_NUM_XM_MODES 20