Looks Good.
Reviewed-by Guan Junxiong <guanjunxiong@xxxxxxxxxx>
On 2018/1/14 5:19, Martin Wilck wrote:
> 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).
> ---
> 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..e764f1dd8a21 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_avglatency)
> + / (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;
> }
>
--
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