[dm-devel] [PATCH v2 19/20] libmultipath: path latency: simplify getprio()

[Guan Junxiong]

Looks Good.

Reviewed-by Guan Junxiong <guanjunxiong at huawei.com>

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;
>  }
>