gnss_synchro_monitor: reduce CPU consumption

For some unknown reaso gnss_synchro_monitor stays in a tight loop trying
to catch all syncros and sometimes consumes the whole CPU core.
Triggering the gnss_syncro_monitor from sample counter as it is done
with observables greatly reduces CPU usage from ~60...90% to <1%.

Signed-off-by: Vladislav P <vladisslav2011@gmail.com>

src/core/monitor/gnss_synchro_monitor.cc

@@ -55,11 +55,12 @@ gnss_synchro_monitor::gnss_synchro_monitor(int n_channels,
 
 void gnss_synchro_monitor::forecast(int noutput_items __attribute__((unused)), gr_vector_int& ninput_items_required)
 {
-    for (int32_t channel_index = 0; channel_index < d_nchannels; channel_index++)
+    for (int32_t channel_index = 0; channel_index < d_nchannels - 1; channel_index++)
         {
             // Set the required number of inputs to 0 so that a lone input on any channel can be pushed to UDP
             ninput_items_required[channel_index] = 0;
         }
+    ninput_items_required[d_nchannels - 1] = 1;
 }
 
 
@@ -70,7 +71,7 @@ int gnss_synchro_monitor::general_work(int noutput_items __attribute__((unused))
     const auto** in = reinterpret_cast<const Gnss_Synchro**>(&input_items[0]);
 
     // Loop through each input stream channel
-    for (int channel_index = 0; channel_index < d_nchannels; channel_index++)
+    for (int channel_index = 0; channel_index < d_nchannels - 1; channel_index++)
         {
             // Loop through each item in each input stream channel
             int count = 0;
@@ -91,6 +92,7 @@ int gnss_synchro_monitor::general_work(int noutput_items __attribute__((unused))
                         }
                 }
         }
+    consume(d_nchannels - 1, ninput_items[d_nchannels - 1]);
 
     // Not producing any outputs
     return 0;

src/core/receiver/gnss_flowgraph.cc

@@ -206,7 +206,7 @@ void GNSSFlowgraph::init()
             udp_addr_vec.erase(std::unique(udp_addr_vec.begin(), udp_addr_vec.end()), udp_addr_vec.end());
 
             // Instantiate monitor object
-            GnssSynchroMonitor_ = gnss_synchro_make_monitor(channels_count_,
+            GnssSynchroMonitor_ = gnss_synchro_make_monitor(channels_count_ + 1,
                 configuration_->property("Monitor.decimation_factor", 1),
                 configuration_->property("Monitor.udp_port", 1234),
                 udp_addr_vec, enable_protobuf);
@@ -850,6 +850,7 @@ int GNSSFlowgraph::connect_sample_counter()
             ch_out_sample_counter_ = gnss_sdr_make_sample_counter(fs, observable_interval_ms, sig_conditioner_.at(0)->get_right_block()->output_signature()->sizeof_stream_item(0));
             top_block_->connect(sig_conditioner_.at(0)->get_right_block(), 0, ch_out_sample_counter_, 0);
             top_block_->connect(ch_out_sample_counter_, 0, observables_->get_left_block(), channels_count_);  // extra port for the sample counter pulse
+            top_block_->connect(ch_out_sample_counter_, 0, GnssSynchroMonitor_, channels_count_);             // extra port for the sample counter pulse
         }
     catch (const std::exception& e)
         {