[PATCH v3 3/5] latencystats: do not use floating point

[Tyler Retzlaff]

On Wed, Apr 17, 2024 at 10:07:25AM -0700, Stephen Hemminger wrote:
> The cycle counts do not need to be stored as floating point.
> Instead keep track of latency in cycles, and convert to
> nanoseconds when read.
> 
> Change Exponential Weighted Moving Average weight from .2 to .25
> to avoid use of floating point for that.
> 
> Signed-off-by: Stephen Hemminger <stephen at networkplumber.org>
> ---
>  lib/latencystats/rte_latencystats.c | 37 +++++++++++------------------
>  1 file changed, 14 insertions(+), 23 deletions(-)
> 
> diff --git a/lib/latencystats/rte_latencystats.c b/lib/latencystats/rte_latencystats.c
> index fe8c3c563a..11bd0ea4ae 100644
> --- a/lib/latencystats/rte_latencystats.c
> +++ b/lib/latencystats/rte_latencystats.c
> @@ -47,10 +47,10 @@ static uint64_t timer_tsc;
>  static uint64_t prev_tsc;
>  
>  struct rte_latency_stats {
> -	float min_latency; /**< Minimum latency in nano seconds */
> -	float avg_latency; /**< Average latency in nano seconds */
> -	float max_latency; /**< Maximum latency in nano seconds */
> -	float jitter; /** Latency variation */
> +	uint64_t min_latency; /**< Minimum latency */
> +	uint64_t avg_latency; /**< Average latency */
> +	uint64_t max_latency; /**< Maximum latency */
> +	uint64_t jitter; /** Latency variation */
>  	rte_spinlock_t lock; /** Latency calculation lock */
>  };
>  
> @@ -82,13 +82,12 @@ int32_t
>  rte_latencystats_update(void)
>  {
>  	unsigned int i;
> -	float *stats_ptr = NULL;
>  	uint64_t values[NUM_LATENCY_STATS] = {0};
>  	int ret;
>  
>  	for (i = 0; i < NUM_LATENCY_STATS; i++) {
> -		stats_ptr = RTE_PTR_ADD(glob_stats,
> -				lat_stats_strings[i].offset);
> +		const uint64_t *stats_ptr = RTE_PTR_ADD(glob_stats,
> +							lat_stats_strings[i].offset);
>  		values[i] = floor(*stats_ptr / latencystat_cycles_per_ns());

will this need a cast to uint64_t?

>  	}
>  
> @@ -105,11 +104,10 @@ static void
>  rte_latencystats_fill_values(struct rte_metric_value *values)
>  {
>  	unsigned int i;
> -	float *stats_ptr = NULL;
>  
>  	for (i = 0; i < NUM_LATENCY_STATS; i++) {
> -		stats_ptr = RTE_PTR_ADD(glob_stats,
> -				lat_stats_strings[i].offset);
> +		const uint64_t *stats_ptr = RTE_PTR_ADD(glob_stats,
> +							lat_stats_strings[i].offset);
>  		values[i].key = i;
>  		values[i].value = floor(*stats_ptr / latencystat_cycles_per_ns());
>  	}
> @@ -156,15 +154,8 @@ calc_latency(uint16_t pid __rte_unused,
>  		void *_ __rte_unused)
>  {
>  	unsigned int i;
> -	uint64_t now;
> -	float latency;
> -	static float prev_latency;
> -	/*
> -	 * Alpha represents degree of weighting decrease in EWMA,
> -	 * a constant smoothing factor between 0 and 1. The value
> -	 * is used below for measuring average latency.
> -	 */
> -	const float alpha = 0.2;
> +	uint64_t now, latency;
> +	static uint64_t prev_latency;
>  

if this is merged i can abandon this series
https://patchwork.dpdk.org/project/dpdk/list/?series=31747

Acked-by: Tyler Retzlaff <roretzla at linux.microsoft.com>