The log standard deviation can be calculated much more simply by realizing sum_n (x_i - avg(x))^2 == sum_n x_i^2 - n * avg(x)^2
Also, use timespecsub rather than the custom timeval_to_usec, and avoid taking log(0). Signed-off-by: Martin Wilck <[email protected]> --- libmultipath/prioritizers/path_latency.c | 71 ++++++++++++++------------------ 1 file changed, 30 insertions(+), 41 deletions(-) diff --git a/libmultipath/prioritizers/path_latency.c b/libmultipath/prioritizers/path_latency.c index ce5c95dd7075..44472b77dd86 100644 --- a/libmultipath/prioritizers/path_latency.c +++ b/libmultipath/prioritizers/path_latency.c @@ -34,6 +34,7 @@ #include "prio.h" #include "structs.h" #include "util.h" +#include "time-util.h" #define pp_pl_log(prio, fmt, args...) condlog(prio, "path_latency prio: " fmt, ##args) @@ -56,14 +57,6 @@ #define DEF_BLK_SIZE 4096 -static double lg_path_latency[MAX_IO_NUM]; - -static inline long long timeval_to_us(const struct timespec *tv) -{ - return ((long long)tv->tv_sec * USEC_PER_SEC) + - (tv->tv_nsec / NSEC_PER_USEC); -} - static int prepare_directio_read(int fd, int *blksz, char **pbuf, int *restore_flags) { @@ -199,22 +192,6 @@ out: return 0; } -double calc_standard_deviation(double *lg_path_latency, int size, - double lg_avglatency) -{ - int index; - double sum = 0; - - for (index = 0; index < size; index++) { - sum += (lg_path_latency[index] - lg_avglatency) * - (lg_path_latency[index] - lg_avglatency); - } - - sum /= (size - 1); - - return sqrt(sum); -} - /* * Do not scale the prioriy in a certain range such as [0, 1024] * because scaling will eliminate the effect of base_num. @@ -234,20 +211,16 @@ int calcPrio(double lg_avglatency, double lg_maxavglatency, int getprio(struct path *pp, char *args, unsigned int timeout) { int rc, temp; - int index = 0; int io_num = 0; double base_num = 0; double lg_avglatency, lg_maxavglatency, lg_minavglatency; double standard_deviation; double lg_toldelay = 0; - long long before, after; - struct timespec tv; int blksize; char *buf; int restore_flags = 0; double lg_base; - long long sum_latency = 0; - long long arith_mean_lat; + double sum_squares = 0; if (pp->fd < 0) return -1; @@ -260,7 +233,6 @@ int getprio(struct path *pp, char *args, unsigned int timeout) pp->dev, io_num, base_num); } - memset(lg_path_latency, 0, sizeof(lg_path_latency)); lg_base = log(base_num); lg_maxavglatency = log(MAX_AVG_LATENCY) / lg_base; lg_minavglatency = log(MIN_AVG_LATENCY) / lg_base; @@ -269,8 +241,10 @@ int getprio(struct path *pp, char *args, unsigned int timeout) temp = io_num; while (temp-- > 0) { - (void)clock_gettime(CLOCK_MONOTONIC, &tv); - before = timeval_to_us(&tv); + struct timespec tv_before, tv_after, tv_diff; + double diff, reldiff; + + (void)clock_gettime(CLOCK_MONOTONIC, &tv_before); if (do_directio_read(pp->fd, timeout, buf, blksize)) { pp_pl_log(0, "%s: path down", pp->dev); @@ -278,25 +252,34 @@ int getprio(struct path *pp, char *args, unsigned int timeout) return -1; } - (void)clock_gettime(CLOCK_MONOTONIC, &tv); - after = timeval_to_us(&tv); + (void)clock_gettime(CLOCK_MONOTONIC, &tv_after); + + timespecsub(&tv_after, &tv_before, &tv_diff); + diff = tv_diff.tv_sec * 1000 * 1000 + tv_diff.tv_nsec / 1000; + + if (diff == 0) + /* + * Avoid taking log(0). + * This unlikely case is treated as minimum - + * the sums don't increase + */ + continue; + + /* we scale by lg_base here */ + reldiff = log(diff) / lg_base; + /* * We assume that the latency complies with Log-normal * distribution. The logarithm of latency is in normal * distribution. */ - lg_path_latency[index] = log(after - before) / lg_base; - lg_toldelay += lg_path_latency[index++]; - sum_latency += after - before; + lg_toldelay += reldiff; + sum_squares += reldiff * reldiff; } cleanup_directio_read(pp->fd, buf, restore_flags); lg_avglatency = lg_toldelay / (long long)io_num; - arith_mean_lat = sum_latency / (long long)io_num; - pp_pl_log(4, "%s: arithmetic mean latency is (%lld us), geometric mean latency is (%lld us)", - pp->dev, arith_mean_lat, - (long long)pow(base_num, lg_avglatency)); if (lg_avglatency > lg_maxavglatency) { pp_pl_log(2, @@ -340,8 +323,14 @@ int getprio(struct path *pp, char *args, unsigned int timeout) ".It is recommend to be set larger", pp->dev, base_num); + standard_deviation = sqrt((sum_squares - lg_toldelay * lg_toldelay) + / (io_num -1)); rc = calcPrio(lg_avglatency, lg_maxavglatency, lg_minavglatency); + pp_pl_log(3, "%s: latency avg=%.2e uncertainty=%.1f prio=%d\n", + pp->dev, exp(lg_avglatency * lg_base), + exp(standard_deviation * lg_base), rc); + return rc; } -- 2.15.0 -- dm-devel mailing list [email protected] https://www.redhat.com/mailman/listinfo/dm-devel
