From 729fe5c0334df5d2eca01258c146680118767b4e Mon Sep 17 00:00:00 2001
From: Andrew Noyes <andrew@weaselab.dev>
Date: Wed, 20 Aug 2025 15:45:38 -0400
Subject: [PATCH] Measure latency

---
 tools/load_tester.cpp | 63 ++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 60 insertions(+), 3 deletions(-)

diff --git a/tools/load_tester.cpp b/tools/load_tester.cpp
index ece392d..bffcc08 100644
--- a/tools/load_tester.cpp
+++ b/tools/load_tester.cpp
@@ -145,6 +145,12 @@ sem_t connectionLimit;
 std::atomic<bool> g_shutdown{false};
 std::atomic<int> g_connect_threads{0};
 
+// Latency statistics
+std::atomic<uint64_t> g_total_requests{0};
+std::atomic<double> g_total_latency{0.0};
+std::atomic<double> g_min_latency{1e9};
+std::atomic<double> g_max_latency{0.0};
+
 void signal_handler(int sig) {
   if (sig == SIGTERM || sig == SIGINT) {
     g_shutdown.store(true, std::memory_order_relaxed);
@@ -215,6 +221,7 @@ struct Connection {
   Connection(int fd, int64_t id) : fd(fd), id(id) {
     llhttp_init(&parser, HTTP_RESPONSE, &settings);
     parser.data = this;
+    requestStartTime = now(); // Record when first request starts
     initRequest();
   }
 
@@ -297,6 +304,9 @@ struct Connection {
   const int fd;
   const int64_t id;
 
+  // Latency tracking
+  double requestStartTime = 0.0;
+
 #if __has_feature(thread_sanitizer)
   void tsan_acquire() { tsan_sync.load(std::memory_order_acquire); }
   void tsan_release() { tsan_sync.store(0, std::memory_order_release); }
@@ -355,8 +365,38 @@ private:
   int on_message_complete() {
     assert(responseId == requestId);
     TRACE_EVENT("http", "Receive response", perfetto::Flow::Global(responseId));
+
+    // Calculate and record latency
+    if (requestStartTime > 0.0) {
+      double responseTime = now();
+      double latency = responseTime - requestStartTime;
+
+      // Update global statistics atomically
+      g_total_requests.fetch_add(1, std::memory_order_relaxed);
+      g_total_latency.fetch_add(latency, std::memory_order_relaxed);
+
+      // Update min/max latency with compare-and-swap
+      double current_min = g_min_latency.load(std::memory_order_relaxed);
+      while (latency < current_min &&
+             !g_min_latency.compare_exchange_weak(current_min, latency,
+                                                  std::memory_order_relaxed)) {
+        // Retry if another thread updated min_latency
+      }
+
+      double current_max = g_max_latency.load(std::memory_order_relaxed);
+      while (latency > current_max &&
+             !g_max_latency.compare_exchange_weak(current_max, latency,
+                                                  std::memory_order_relaxed)) {
+        // Retry if another thread updated max_latency
+      }
+    }
+
     responseId = 0;
     ++responsesReceived;
+
+    // For subsequent requests, start timing from when the last response was
+    // received
+    requestStartTime = now();
     initRequest();
     return 0;
   }
@@ -709,9 +749,26 @@ int main(int argc, char *argv[]) {
 
     double currTime = now();
     double currConnections = connectionId.load(std::memory_order_relaxed);
-    printf("req/s: %f\n", (currConnections - prevConnections) /
-                              (currTime - prevTime) *
-                              g_config.requests_per_connection);
+    double throughput = (currConnections - prevConnections) /
+                        (currTime - prevTime) *
+                        g_config.requests_per_connection;
+
+    // Get latency statistics
+    uint64_t total_requests = g_total_requests.load(std::memory_order_relaxed);
+    double total_latency = g_total_latency.load(std::memory_order_relaxed);
+    double min_latency = g_min_latency.load(std::memory_order_relaxed);
+    double max_latency = g_max_latency.load(std::memory_order_relaxed);
+
+    printf("req/s: %.2f", throughput);
+
+    if (total_requests > 0) {
+      double avg_latency = total_latency / total_requests;
+      printf(" | latency: avg=%.3fms min=%.3fms max=%.3fms (n=%lu)",
+             avg_latency * 1000.0, min_latency * 1000.0, max_latency * 1000.0,
+             total_requests);
+    }
+
+    printf("\n");
 
     // Check if we should exit based on duration
     if (g_config.duration > 0 && (currTime - startTime) >= g_config.duration) {