Predictive PEC: use the monotonic clock rather than the system wall clock (#1267)

GP guider needs accurate time interval measurements, and is not interested
in the wall clock.  std::chrono::steady_clock is a better choice than system_clock.

Testing:
 - gp guider unit tests pass

Author: agalasso

contributions/MPI_IS_gaussian_process/src/gaussian_process_guider.cpp

@@ -61,10 +61,10 @@
 #define HYSTERESIS 0.1 // for the hybrid mode
 
 GaussianProcessGuider::GaussianProcessGuider(guide_parameters parameters)
-    : start_time_(std::chrono::system_clock::now()), last_time_(std::chrono::system_clock::now()), control_signal_(0),
-      prediction_(0), last_prediction_end_(0), dither_steps_(0), dithering_active_(false), dither_offset_(0.0),
-      circular_buffer_data_(CIRCULAR_BUFFER_SIZE), covariance_function_(), output_covariance_function_(),
-      gp_(covariance_function_), learning_rate_(DEFAULT_LEARNING_RATE), parameters(parameters)
+    : start_time_(clock::now()), last_time_(clock::now()), control_signal_(0), prediction_(0), last_prediction_end_(0),
+      dither_steps_(0), dithering_active_(false), dither_offset_(0.0), circular_buffer_data_(CIRCULAR_BUFFER_SIZE),
+      covariance_function_(), output_covariance_function_(), gp_(covariance_function_), learning_rate_(DEFAULT_LEARNING_RATE),
+      parameters(parameters)
 {
     circular_buffer_data_.push_front(data_point()); // add first point
     circular_buffer_data_[0].control = 0; // set first control to zero
@@ -86,7 +86,7 @@ GaussianProcessGuider::~GaussianProcessGuider() { }
 
 void GaussianProcessGuider::SetTimestamp()
 {
-    auto current_time = std::chrono::system_clock::now();
+    auto current_time = clock::now();
     double delta_measurement_time = std::chrono::duration<double>(current_time - last_time_).count();
     last_time_ = current_time;
     get_last_point().timestamp = std::chrono::duration<double>(current_time - start_time_).count() -
@@ -238,7 +238,7 @@ double GaussianProcessGuider::PredictGearError(double prediction_location)
     // in the first step of each sequence, use the current time stamp as last prediction end
     if (last_prediction_end_ < 0.0)
     {
-        last_prediction_end_ = std::chrono::duration<double>(std::chrono::system_clock::now() - start_time_).count();
+        last_prediction_end_ = std::chrono::duration<double>(clock::now() - start_time_).count();
     }
 
     // prediction from the last endpoint to the prediction point
@@ -296,7 +296,7 @@ double GaussianProcessGuider::result(double input, double SNR, double time_step,
     // the starting time is set at the first call of result after startup or reset
     if (get_number_of_measurements() == 1)
     {
-        start_time_ = std::chrono::system_clock::now();
+        start_time_ = clock::now();
         last_time_ = start_time_; // this is OK, since last_time_ only provides a minor correction
     }
 
@@ -325,7 +325,7 @@ double GaussianProcessGuider::result(double input, double SNR, double time_step,
     {
         if (prediction_point < 0.0)
         {
-            prediction_point = std::chrono::duration<double>(std::chrono::system_clock::now() - start_time_).count();
+            prediction_point = std::chrono::duration<double>(clock::now() - start_time_).count();
         }
         // the point of highest precision shoud be between now and the next step
         UpdateGP(prediction_point + 0.5 * time_step);
@@ -380,7 +380,7 @@ double GaussianProcessGuider::deduceResult(double time_step, double prediction_p
     {
         if (prediction_point < 0.0)
         {
-            prediction_point = std::chrono::duration<double>(std::chrono::system_clock::now() - start_time_).count();
+            prediction_point = std::chrono::duration<double>(clock::now() - start_time_).count();
         }
         // the point of highest precision should be between now and the next step
         UpdateGP(prediction_point + 0.5 * time_step);
@@ -416,8 +416,8 @@ void GaussianProcessGuider::reset()
     circular_buffer_data_[0].control = 0; // set first control to zero
 
     last_prediction_end_ = -1.0; // the negative value signals we didn't predict yet
-    start_time_ = std::chrono::system_clock::now();
-    last_time_ = std::chrono::system_clock::now();
+    start_time_ = clock::now();
+    last_time_ = clock::now();
 
     dither_offset_ = 0.0;
     dither_steps_ = 0;
@@ -573,7 +573,7 @@ void GaussianProcessGuider::inject_data_point(double timestamp, double input, do
     last_prediction_end_ = timestamp;
     get_last_point().timestamp = timestamp; // overrides the usual HandleTimestamps();
 
-    start_time_ = std::chrono::system_clock::now() - std::chrono::seconds((int) timestamp);
+    start_time_ = clock::now() - std::chrono::seconds((int) timestamp);
 
     add_one_point(); // add new point here, since the control is for the next point in time
     HandleControls(control); // already store control signal

contributions/MPI_IS_gaussian_process/src/gaussian_process_guider.h

@@ -71,6 +71,8 @@ enum Hyperparameters
 class GaussianProcessGuider
 {
 public:
+    typedef std::chrono::steady_clock clock;
+
     struct data_point
     {
         double timestamp;
@@ -113,8 +115,8 @@ class GaussianProcessGuider
     };
 
 private:
-    std::chrono::system_clock::time_point start_time_; // reference time
-    std::chrono::system_clock::time_point last_time_;
+    clock::time_point start_time_; // reference time
+    clock::time_point last_time_;
 
     double control_signal_;
     double prediction_;

contributions/MPI_IS_gaussian_process/tests/gaussian_process/gp_guider_test.cpp

@@ -244,11 +244,11 @@ TEST_F(GPGTest, timer_test)
     GPG->result(1.0, 2.0, 3.0);
     std::this_thread::sleep_for(std::chrono::milliseconds(wait));
 
-    auto time_start = std::chrono::system_clock::now();
+    auto time_start = GaussianProcessGuider::clock::now();
     GPG->result(1.0, 2.0, 3.0);
     double first_time = GPG->get_second_last_point().timestamp;
     std::this_thread::sleep_for(std::chrono::milliseconds(wait));
-    auto time_end = std::chrono::system_clock::now();
+    auto time_end = GaussianProcessGuider::clock::now();
     GPG->result(1.0, 2.0, 3.0);
     double second_time = GPG->get_second_last_point().timestamp;
 

src/guide_algorithm_gaussian_process.cpp

@@ -1019,7 +1019,7 @@ void GuideAlgorithmGaussianProcess::GuidingStarted()
     bool need_reset = true;
     double ra_offset; // RA delta in SI seconds
 
-    auto now = std::chrono::system_clock::now();
+    auto now = std::chrono::steady_clock::now();
 
     double prev_ra = guiding_ra_;
     guiding_ra_ = CurrentRA();
@@ -1092,7 +1092,7 @@ void GuideAlgorithmGaussianProcess::GuidingStopped()
     double period_length = GPG->GetGPHyperparameters()[PKPeriodLength];
     pConfig->Profile.SetDouble(GetConfigPath() + "/gp_period_per_kern", period_length);
 
-    guiding_stopped_time_ = std::chrono::system_clock::now();
+    guiding_stopped_time_ = std::chrono::steady_clock::now();
 }
 
 void GuideAlgorithmGaussianProcess::GuidingPaused() { }

src/guide_algorithm_gaussian_process.h

@@ -122,7 +122,7 @@ class GuideAlgorithmGaussianProcess : public GuideAlgorithm
     bool block_updates_; // Don't update GP if guiding is disabled
     double guiding_ra_; // allow resuming guiding after guiding stopped if there is no change in RA
     PierSide guiding_pier_side_;
-    std::chrono::system_clock::time_point guiding_stopped_time_; // time guiding stopped
+    std::chrono::steady_clock::time_point guiding_stopped_time_; // time guiding stopped
 
 protected:
     double GetControlGain() const;