add option to set max thread count

src/app/qgsoptions.cpp

@@ -548,6 +548,10 @@ QgsOptions::QgsOptions( QWidget *parent, Qt::WindowFlags fl ) :
   chkUseRenderCaching->setChecked( settings.value( "/qgis/enable_render_caching", false ).toBool() );
   chkParallelRendering->setChecked( settings.value( "/qgis/parallel_rendering", false ).toBool() );
   spinMapUpdateInterval->setValue( settings.value( "/qgis/map_update_interval", 250 ).toInt() );
+  chkMaxCores->setChecked( settings.value( "/qgis/max_cores", 0 ).toInt() != 0 );
+  spinMaxCores->setEnabled( chkMaxCores->isChecked() );
+  spinMaxCores->setRange( 1, QThread::idealThreadCount() );
+  spinMaxCores->setValue( settings.value( "/qgis/max_cores", QThread::idealThreadCount() ).toInt() );
 
   // Default simplify drawing configuration
   mSimplifyDrawingGroupBox->setChecked( settings.value( "/qgis/simplifyDrawingHints", ( int )QgsVectorSimplifyMethod::GeometrySimplification ).toInt() != QgsVectorSimplifyMethod::NoSimplification );
@@ -1072,6 +1076,10 @@ void QgsOptions::saveOptions()
   settings.setValue( "/qgis/enable_anti_aliasing", chkAntiAliasing->isChecked() );
   settings.setValue( "/qgis/enable_render_caching", chkUseRenderCaching->isChecked() );
   settings.setValue( "/qgis/parallel_rendering", chkParallelRendering->isChecked() );
+  if ( chkMaxCores->isChecked() )
+    settings.setValue( "/qgis/max_cores", int( spinMaxCores->value() ) );
+  else
+    settings.remove( "/qgis/max_cores" );
   settings.setValue( "/qgis/map_update_interval", spinMapUpdateInterval->value() );
   settings.setValue( "/qgis/legendDoubleClickAction", cmbLegendDoubleClickAction->currentIndex() );
   bool legendLayersCapitalise = settings.value( "/qgis/capitaliseLayerName", false ).toBool();

src/core/qgsmaprendererjob.cpp

@@ -5,6 +5,7 @@
 #include <QTime>
 #include <QTimer>
 #include <QtConcurrentMap>
+#include <QSettings>
 
 #include "qgscrscache.h"
 #include "qgslogger.h"
@@ -731,6 +732,14 @@ void QgsMapRendererParallelJob::start()
 
   mLayerJobs = prepareJobs( 0, mLabelingEngine );
 
+  // set max thread count
+  QSettings settings;
+  int max_cores = settings.value( "/qgis/max_cores", 0 ).toInt();
+  if ( max_cores <= 0 || max_cores > QThread::idealThreadCount() )
+    max_cores = QThread::idealThreadCount();
+  QThreadPool::globalInstance()->setMaxThreadCount( max_cores );
+  qDebug( "set max thread count to %d", QThreadPool::globalInstance()->maxThreadCount() );
+
   // start async job
 
   connect( &mFutureWatcher, SIGNAL( finished() ), SLOT( renderLayersFinished() ) );
@@ -829,6 +838,10 @@ QImage QgsMapRendererParallelJob::renderedImage()
 
 void QgsMapRendererParallelJob::renderLayersFinished()
 {
+  // restore max thread count
+  QThreadPool::globalInstance()->setMaxThreadCount( QThread::idealThreadCount() );
+  qDebug( "restored max thread count to ideal (%d)", QThreadPool::globalInstance()->maxThreadCount() );
+
   Q_ASSERT( mStatus == RenderingLayers );
 
   // compose final image

src/ui/qgsoptionsbase.ui

@@ -1628,13 +1628,47 @@
                     </property>
                    </widget>
                   </item>
+                  <item>
+                   <layout class="QHBoxLayout" name="horizontalLayout_26">
                     <item>
                      <widget class="QCheckBox" name="chkParallelRendering">
                       <property name="text">
-                     <string>Render layers in parallel using all available CPU cores</string>
+                       <string>Render layers in parallel using many CPU cores</string>
                       </property>
                      </widget>
                     </item>
+                    <item>
+                     <widget class="Line" name="line_6">
+                      <property name="orientation">
+                       <enum>Qt::Vertical</enum>
+                      </property>
+                     </widget>
+                    </item>
+                    <item>
+                     <widget class="QCheckBox" name="chkMaxCores">
+                      <property name="text">
+                       <string>Max cores to use:</string>
+                      </property>
+                     </widget>
+                    </item>
+                    <item>
+                     <widget class="QSpinBox" name="spinMaxCores"/>
+                    </item>
+                    <item>
+                     <spacer name="horizontalSpacer_41">
+                      <property name="orientation">
+                       <enum>Qt::Horizontal</enum>
+                      </property>
+                      <property name="sizeHint" stdset="0">
+                       <size>
+                        <width>40</width>
+                        <height>20</height>
+                       </size>
+                      </property>
+                     </spacer>
+                    </item>
+                   </layout>
+                  </item>
                   <item>
                    <layout class="QHBoxLayout" name="horizontalLayout_4">
                     <item>
@@ -4598,5 +4632,21 @@
     </hint>
    </hints>
   </connection>
+  <connection>
+   <sender>chkMaxCores</sender>
+   <signal>toggled(bool)</signal>
+   <receiver>spinMaxCores</receiver>
+   <slot>setEnabled(bool)</slot>
+   <hints>
+    <hint type="sourcelabel">
+     <x>589</x>
+     <y>110</y>
+    </hint>
+    <hint type="destinationlabel">
+     <x>689</x>
+     <y>110</y>
+    </hint>
+   </hints>
+  </connection>
  </connections>
 </ui>