[PATCH v4 3/6] latencystats: do not use floating point

[Stephen Hemminger <stephen@networkplumber.org>]

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.

The average latency took too long to "warm up".
Do what RFC 6298 suggests and initialize on first sample.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Tyler Retzlaff <roretzla@linux.microsoft.com>
---
 lib/latencystats/rte_latencystats.c | 88 +++++++++++++++--------------
 1 file changed, 45 insertions(+), 43 deletions(-)

diff --git a/lib/latencystats/rte_latencystats.c b/lib/latencystats/rte_latencystats.c
index 55a099c818..6ef8e344bf 100644
--- a/lib/latencystats/rte_latencystats.c
+++ b/lib/latencystats/rte_latencystats.c
@@ -4,6 +4,7 @@
 
 #include <math.h>
 
+#include <rte_common.h>
 #include <rte_string_fns.h>
 #include <rte_mbuf_dyn.h>
 #include <rte_log.h>
@@ -42,10 +43,10 @@ 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 */
 };
 
@@ -77,13 +78,12 @@ 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 / cycles_per_ns);
 	}
 
@@ -100,11 +100,10 @@ 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 / cycles_per_ns);
 	}
@@ -151,15 +150,9 @@ 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;
+	static bool first_sample = true;
 
 	now = rte_rdtsc();
 
@@ -170,32 +163,41 @@ calc_latency(uint16_t pid __rte_unused,
 
 		latency = now - *timestamp_dynfield(pkts[i]);
 
-		/*
-		 * The jitter is calculated as statistical mean of interpacket
-		 * delay variation. The "jitter estimate" is computed by taking
-		 * the absolute values of the ipdv sequence and applying an
-		 * exponential filter with parameter 1/16 to generate the
-		 * estimate. i.e J=J+(|D(i-1,i)|-J)/16. Where J is jitter,
-		 * D(i-1,i) is difference in latency of two consecutive packets
-		 * i-1 and i.
-		 * Reference: Calculated as per RFC 5481, sec 4.1,
-		 * RFC 3393 sec 4.5, RFC 1889 sec.
-		 */
-		glob_stats->jitter +=  (fabsf(prev_latency - latency)
-					- glob_stats->jitter)/16;
-		if (glob_stats->min_latency == 0)
-			glob_stats->min_latency = latency;
-		else if (latency < glob_stats->min_latency)
+		if (unlikely(first_sample)) {
+			first_sample = false;
+
 			glob_stats->min_latency = latency;
-		else if (latency > glob_stats->max_latency)
 			glob_stats->max_latency = latency;
-		/*
-		 * The average latency is measured using exponential moving
-		 * average, i.e. using EWMA
-		 * https://en.wikipedia.org/wiki/Moving_average
-		 */
-		glob_stats->avg_latency +=
-			alpha * (latency - glob_stats->avg_latency);
+			glob_stats->avg_latency = latency;
+			glob_stats->jitter = latency / 2;
+		} else {
+			/*
+			 * The jitter is calculated as statistical mean of interpacket
+			 * delay variation. The "jitter estimate" is computed by taking
+			 * the absolute values of the ipdv sequence and applying an
+			 * exponential filter with parameter 1/16 to generate the
+			 * estimate. i.e J=J+(|D(i-1,i)|-J)/16. Where J is jitter,
+			 * D(i-1,i) is difference in latency of two consecutive packets
+			 * i-1 and i.
+			 * Reference: Calculated as per RFC 5481, sec 4.1,
+			 * RFC 3393 sec 4.5, RFC 1889 sec.
+			 */
+			glob_stats->jitter += ((prev_latency - latency)
+					       - glob_stats->jitter) / 16;
+			if (latency < glob_stats->min_latency)
+				glob_stats->min_latency = latency;
+			if (latency > glob_stats->max_latency)
+				glob_stats->max_latency = latency;
+			/*
+			 * The average latency is measured using exponential moving
+			 * average, i.e. using EWMA
+			 * https://en.wikipedia.org/wiki/Moving_average
+			 *
+			 * Alpha is .25
+			 */
+			glob_stats->avg_latency += (latency - glob_stats->avg_latency) / 4;
+		}
+
 		prev_latency = latency;
 	}
 	rte_spinlock_unlock(&glob_stats->lock);
-- 
2.43.0