h2load: Reservoir sampling

src/h2load.cc

@@ -151,33 +151,12 @@ std::mt19937 gen(rd());
 } // namespace
 
 namespace {
-void sampling_init(Sampling &smp, size_t total, size_t max_samples) {
+void sampling_init(Sampling &smp, size_t max_samples) {
   smp.n = 0;
-
-  if (total <= max_samples) {
-    smp.interval = 0.;
-    smp.point = 0.;
-    return;
-  }
-
-  smp.interval = static_cast<double>(total) / max_samples;
-
-  std::uniform_real_distribution<> dis(0., smp.interval);
-
-  smp.point = dis(gen);
+  smp.max_samples = max_samples;
 }
 } // namespace
 
-namespace {
-bool sampling_should_pick(Sampling &smp) {
-  return smp.interval == 0. || smp.n == ceil(smp.point);
-}
-} // namespace
-
-namespace {
-void sampling_advance_point(Sampling &smp) { smp.point += smp.interval; }
-} // namespace
-
 namespace {
 void writecb(struct ev_loop *loop, ev_io *w, int revents) {
   auto client = static_cast<Client *>(w->data);
@@ -361,10 +340,7 @@ Client::~Client() {
     SSL_free(ssl);
   }
 
-  if (sampling_should_pick(worker->client_smp)) {
-    sampling_advance_point(worker->client_smp);
-    worker->sample_client_stat(&cstat);
-  }
+  worker->sample_client_stat(&cstat);
   ++worker->client_smp.n;
 }
 
@@ -726,10 +702,7 @@ void Client::on_stream_close(int32_t stream_id, bool success, bool final) {
       ++worker->stats.req_failed;
     }
 
-    if (sampling_should_pick(worker->request_times_smp)) {
-      sampling_advance_point(worker->request_times_smp);
-      worker->sample_req_stat(req_stat);
-    }
+    worker->sample_req_stat(req_stat);
 
     // Count up in successful cases only
     ++worker->request_times_smp.n;
@@ -1190,8 +1163,8 @@ Worker::Worker(uint32_t id, SSL_CTX *ssl_ctx, size_t req_todo, size_t nclients,
   stats.req_stats.reserve(std::min(req_todo, max_samples));
   stats.client_stats.reserve(std::min(nclients, max_samples));
 
-  sampling_init(request_times_smp, req_todo, max_samples);
-  sampling_init(client_smp, nclients, max_samples);
+  sampling_init(request_times_smp, max_samples);
+  sampling_init(client_smp, max_samples);
 }
 
 Worker::~Worker() {
@@ -1224,14 +1197,28 @@ void Worker::run() {
   ev_run(loop, 0);
 }
 
+namespace {
+template <typename Stats, typename Stat>
+void sample(Sampling &smp, Stats &stats, Stat *s) {
+  ++smp.n;
+  if (stats.size() < smp.max_samples) {
+    stats.push_back(*s);
+    return;
+  }
+  auto d = std::uniform_int_distribution<>(0, smp.n - 1);
+  auto i = d(gen);
+  if (i < smp.max_samples) {
+    stats[i] = *s;
+  }
+}
+} // namespace
+
 void Worker::sample_req_stat(RequestStat *req_stat) {
-  stats.req_stats.push_back(*req_stat);
-  assert(stats.req_stats.size() <= max_samples);
+  sample(request_times_smp, stats.req_stats, req_stat);
 }
 
 void Worker::sample_client_stat(ClientStat *cstat) {
-  stats.client_stats.push_back(*cstat);
-  assert(stats.client_stats.size() <= max_samples);
+  sample(client_smp, stats.client_stats, cstat);
 }
 
 void Worker::report_progress() {
@@ -1313,14 +1300,10 @@ process_time_stats(const std::vector<std::unique_ptr<Worker>> &workers) {
   size_t nclient_times = 0;
   for (const auto &w : workers) {
     nrequest_times += w->stats.req_stats.size();
-    if (w->request_times_smp.interval != 0.) {
-      request_times_sampling = true;
-    }
+    request_times_sampling = w->request_times_smp.n > w->stats.req_stats.size();
 
     nclient_times += w->stats.client_stats.size();
-    if (w->client_smp.interval != 0.) {
-      client_times_sampling = true;
-    }
+    client_times_sampling = w->client_smp.n > w->stats.client_stats.size();
   }
 
   std::vector<double> request_times;

src/h2load.h

@@ -217,13 +217,10 @@ enum ClientState { CLIENT_IDLE, CLIENT_CONNECTED };
 
 struct Client;
 
-// We use systematic sampling method
+// We use reservoir sampling method
 struct Sampling {
-  // sampling interval
-  double interval;
-  // cumulative value of interval, and the next point is the integer
-  // rounded up from this value.
-  double point;
+  // maximum number of samples
+  size_t max_samples;
   // number of samples seen, including discarded samples.
   size_t n;
 };