Performance counters: Estimate RMS for elapsed counters. Allow to use a perf counter across processes, deal with overruns and other resulting inconsistencies from cross-process use.

src/modules/systemlib/perf_counter.c

@@ -39,6 +39,7 @@
 
 #include <stdlib.h>
 #include <stdio.h>
+#include <string.h>
 #include <sys/queue.h>
 #include <drivers/drv_hrt.h>
 #include <math.h>
@@ -67,10 +68,13 @@ struct perf_ctr_count {
 struct perf_ctr_elapsed {
 	struct perf_ctr_header	hdr;
 	uint64_t		event_count;
+	uint64_t		event_overruns;
 	uint64_t		time_start;
 	uint64_t		time_total;
 	uint64_t		time_least;
 	uint64_t		time_most;
+	float			mean;
+	float			M2;
 };
 
 /**
@@ -126,6 +130,28 @@ perf_alloc(enum perf_counter_type type, const char *name)
 	return ctr;
 }
 
+perf_counter_t
+perf_alloc_once(enum perf_counter_type type, const char *name)
+{
+	perf_counter_t handle = (perf_counter_t)sq_peek(&perf_counters);
+
+	while (handle != NULL) {
+		if (!strcmp(handle->name, name)) {
+			if (type == handle->type) {
+				/* they are the same counter */
+				return handle;
+			} else {
+				/* same name but different type, assuming this is an error and not intended */
+				return NULL;
+			}
+		}
+		handle = (perf_counter_t)sq_next(&handle->link);
+	}
+
+	/* if the execution reaches here, no existing counter of that name was found */
+	return perf_alloc(type, name);
+}
+
 void
 perf_free(perf_counter_t handle)
 {
@@ -213,18 +239,30 @@ perf_end(perf_counter_t handle)
 			struct perf_ctr_elapsed *pce = (struct perf_ctr_elapsed *)handle;
 
 			if (pce->time_start != 0) {
-				hrt_abstime elapsed = hrt_absolute_time() - pce->time_start;
+				int64_t elapsed = hrt_absolute_time() - pce->time_start;
 
-				pce->event_count++;
-				pce->time_total += elapsed;
+				if (elapsed < 0) {
+					pce->event_overruns++;
+				} else {
 
-				if ((pce->time_least > elapsed) || (pce->time_least == 0))
-					pce->time_least = elapsed;
+					pce->event_count++;
+					pce->time_total += elapsed;
 
-				if (pce->time_most < elapsed)
-					pce->time_most = elapsed;
+					if ((pce->time_least > (uint64_t)elapsed) || (pce->time_least == 0))
+						pce->time_least = elapsed;
 
-				pce->time_start = 0;
+					if (pce->time_most < (uint64_t)elapsed)
+						pce->time_most = elapsed;
+
+					// maintain mean and variance of the elapsed time in seconds
+					// Knuth/Welford recursive mean and variance of update intervals (via Wikipedia)
+					float dt = elapsed / 1e6f;
+					float delta_intvl = dt - pce->mean;
+					pce->mean += delta_intvl / pce->event_count;
+					pce->M2 += delta_intvl * (dt - pce->mean);
+
+					pce->time_start = 0;
+				}
 			}
 		}
 		break;
@@ -310,14 +348,17 @@ perf_print_counter_fd(int fd, perf_counter_t handle)
 
 	case PC_ELAPSED: {
 		struct perf_ctr_elapsed *pce = (struct perf_ctr_elapsed *)handle;
+		float rms = sqrtf(pce->M2 / (pce->event_count-1));
 
-		dprintf(fd, "%s: %llu events, %lluus elapsed, %lluus avg, min %lluus max %lluus\n",
-		       handle->name,
-		       pce->event_count,
-		       pce->time_total,
-		       pce->time_total / pce->event_count,
-		       pce->time_least,
-		       pce->time_most);
+		dprintf(fd, "%s: %llu events, %llu overruns, %lluus elapsed, %lluus avg, min %lluus max %lluus %5.3fus rms\n",
+			handle->name,
+			pce->event_count,
+			pce->event_overruns,
+			pce->time_total,
+			pce->time_total / pce->event_count,
+			pce->time_least,
+			pce->time_most,
+			(double)(1e6f * rms));
 		break;
 	}
 
@@ -325,14 +366,13 @@ perf_print_counter_fd(int fd, perf_counter_t handle)
 		struct perf_ctr_interval *pci = (struct perf_ctr_interval *)handle;
 		float rms = sqrtf(pci->M2 / (pci->event_count-1));
 
-		dprintf(fd, "%s: %llu events, %lluus avg, min %lluus max %lluus %5.3f msec mean %5.3f msec rms\n",
-		       handle->name,
-		       pci->event_count,
-		       (pci->time_last - pci->time_first) / pci->event_count,
-		       pci->time_least,
-		       pci->time_most,
-			   (double)(1000 * pci->mean),
-			   (double)(1000 * rms));
+		dprintf(fd, "%s: %llu events, %lluus avg, min %lluus max %lluus %5.3fus rms\n",
+			handle->name,
+			pci->event_count,
+			(pci->time_last - pci->time_first) / pci->event_count,
+			pci->time_least,
+			pci->time_most,
+			(double)(1e6f * rms));
 		break;
 	}
 

src/modules/systemlib/perf_counter.h

@@ -56,7 +56,7 @@ typedef struct perf_ctr_header	*perf_counter_t;
 __BEGIN_DECLS
 
 /**
- * Create a new counter.
+ * Create a new local counter.
  *
  * @param type			The type of the new counter.
  * @param name			The counter name.
@@ -65,6 +65,16 @@ __BEGIN_DECLS
  */
 __EXPORT extern perf_counter_t	perf_alloc(enum perf_counter_type type, const char *name);
 
+/**
+ * Get the reference to an existing counter or create a new one if it does not exist.
+ *
+ * @param type			The type of the counter.
+ * @param name			The counter name.
+ * @return			Handle for the counter, or NULL if a counter
+ *				could not be allocated.
+ */
+__EXPORT extern perf_counter_t	perf_alloc_once(enum perf_counter_type type, const char *name);
+
 /**
  * Free a counter.
  *