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 <mwilck@xxxxxxxx>
---
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
--
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