From 21100ccc77f731dc7cba5a496001027e8a73a089 Mon Sep 17 00:00:00 2001
From: "Steinar H. Gunderson" <sgunderson@bigfoot.com>
Date: Mon, 12 Jun 2017 21:52:09 +0200
Subject: [PATCH] Expose the start time point instead of the uptime, as per
 Prometheus recommendations.

---
 metrics.cpp | 7 +++++++
 metrics.h   | 6 ++++++
 mixer.cpp   | 5 +++--
 mixer.h     | 2 +-
 4 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/metrics.cpp b/metrics.cpp
index 1f3f8a1..fd5e54c 100644
--- a/metrics.cpp
+++ b/metrics.cpp
@@ -4,13 +4,20 @@
 #include <math.h>
 
 #include <algorithm>
+#include <chrono>
 #include <locale>
 #include <sstream>
 
 using namespace std;
+using namespace std::chrono;
 
 Metrics global_metrics;
 
+double get_timestamp_for_metrics()
+{
+	return duration<double>(system_clock::now().time_since_epoch()).count();
+}
+
 namespace {
 
 string serialize_name(const string &name, const vector<pair<string, string>> &labels)
diff --git a/metrics.h b/metrics.h
index a815658..bd506a9 100644
--- a/metrics.h
+++ b/metrics.h
@@ -15,6 +15,12 @@
 
 class Histogram;
 
+// Prometheus recommends the use of timestamps instead of “time since event”,
+// so you can use this to get the number of seconds since the epoch.
+// Note that this will be wrong if your clock changes, so for non-metric use,
+// you should use std::chrono::steady_clock instead.
+double get_timestamp_for_metrics();
+
 class Metrics {
 public:
 	enum Type {
diff --git a/mixer.cpp b/mixer.cpp
index 24f2a80..400fdd1 100644
--- a/mixer.cpp
+++ b/mixer.cpp
@@ -402,9 +402,11 @@ Mixer::Mixer(const QSurfaceFormat &format, unsigned num_cards)
 		set_output_card_internal(global_flags.output_card);
 	}
 
+	metric_start_time_seconds = get_timestamp_for_metrics();
+
 	global_metrics.add("frames_output_total", &metric_frames_output_total);
 	global_metrics.add("frames_output_dropped", &metric_frames_output_dropped);
-	global_metrics.add("uptime_seconds", &metric_uptime_seconds);
+	global_metrics.add("start_time_seconds", &metric_start_time_seconds);
 	global_metrics.add("memory_used_bytes", &metrics_memory_used_bytes);
 	global_metrics.add("metrics_memory_locked_limit_bytes", &metrics_memory_locked_limit_bytes);
 }
@@ -937,7 +939,6 @@ void Mixer::thread_func()
 
 		metric_frames_output_total = frame_num;
 		metric_frames_output_dropped = stats_dropped_frames;
-		metric_uptime_seconds = elapsed;
 
 		if (frame_num % 100 == 0) {
 			printf("%d frames (%d dropped) in %.3f seconds = %.1f fps (%.1f ms/frame)",
diff --git a/mixer.h b/mixer.h
index e2490ed..8504d5e 100644
--- a/mixer.h
+++ b/mixer.h
@@ -543,7 +543,7 @@ private:
 	// Metrics.
 	std::atomic<int64_t> metric_frames_output_total{0};
 	std::atomic<int64_t> metric_frames_output_dropped{0};
-	std::atomic<double> metric_uptime_seconds{0.0};
+	std::atomic<double> metric_start_time_seconds{0.0 / 0.0};
 	std::atomic<int64_t> metrics_memory_used_bytes{0};
 	std::atomic<double> metrics_memory_locked_limit_bytes{0.0 / 0.0};
 };
-- 
2.39.2