Replace wall clock with monotonic clock

src/headless.cpp

@@ -78,7 +78,6 @@ int main(int argc, char *argv[])
     std::uniform_real_distribution<float> acc_gen(-3.0,2.0);
     std::uniform_real_distribution<float> steer_gen(-0.7,0.7);
 
-    auto start = std::chrono::system_clock::now();
     auto action_printer = mgr.actionTensor().makePrinter();
     auto model_printer = mgr.bicycleModelTensor().makePrinter();
     auto self_printer = mgr.selfObservationTensor().makePrinter();
@@ -132,6 +131,7 @@ int main(int argc, char *argv[])
 	mgr.getShapeTensorFromDeviceMemory(exec_mode, num_worlds);
 
 
+    const auto start = std::chrono::steady_clock::now();
     for (CountT i = 0; i < (CountT)num_steps; i++) {
         if (rand_actions) {
             for (CountT j = 0; j < (CountT)num_worlds; j++) {
@@ -153,9 +153,8 @@ int main(int argc, char *argv[])
         mgr.step();
         printObs();
     }
-
-    auto end = std::chrono::system_clock::now();
-    std::chrono::duration<double> elapsed = end - start;
+    const auto end = std::chrono::steady_clock::now();
+    const std::chrono::duration<double> elapsed= end - start;
 
     float fps = (double)num_steps * (double)num_worlds / elapsed.count();
     printf("FPS %f\n", fps);