Cycles: Free Blender side data as soon as we don't need it

Currently this is possible after built-in images are loaded in memory.
Allows to save memory used by dependency graph and copy-on-write.

In practice this lowers peak system memory usage from 52GB to 42GB on
a production file of spring 03_035_A.lighting.

Note, that this only applies to F12 and command line renders.

Bigger note, that this optimization is currently only possible if
there are no grease pencil objects to be rendered.

2 files changed, 26 insertions, 0 deletions

diff --git a/intern/cycles/blender/blender_session.cpp b/intern/cycles/blender/blender_session.cpp
index 28b0f4faf63..17da8b43d29 100644
--- a/intern/cycles/blender/blender_session.cpp
+++ b/intern/cycles/blender/blender_session.cpp
@@ -470,6 +470,11 @@ void BlenderSession::render(BL::Depsgraph& b_depsgraph_)
 		                &python_thread_state);
 		builtin_images_load();
 
+		/* Attempt to free all data which is held by Blender side, since at this
+		 * point we knwo that we've got everything to render current view layer.
+		 */
+		free_blender_memory_if_possible();
+
 		/* Make sure all views have different noise patterns. - hardcoded value just to make it random */
 		if(view_index != 0) {
 			scene->integrator->seed += hash_int_2d(scene->integrator->seed, hash_int(view_index * 0xdeadbeef));
@@ -1406,4 +1411,16 @@ void BlenderSession::update_resumable_tile_manager(int num_samples)
 	session->tile_manager.range_num_samples = range_num_samples;
 }
 
+void BlenderSession::free_blender_memory_if_possible()
+{
+	if (!background) {
+		/* During interactive render we can not free anything: attempts to save
+		 * memory would cause things to be allocated and evaluated for every
+		 * updated sample.
+		 */
+		return;
+	}
+	b_engine.free_blender_memory();
+}
+
 CCL_NAMESPACE_END
diff --git a/intern/cycles/blender/blender_session.h b/intern/cycles/blender/blender_session.h
index 99b14a09a5c..fdeba7b2f81 100644
--- a/intern/cycles/blender/blender_session.h
+++ b/intern/cycles/blender/blender_session.h
@@ -106,6 +106,9 @@ public:
 	BL::BlendData b_data;
 	BL::RenderSettings b_render;
 	BL::Depsgraph b_depsgraph;
+	/* NOTE: Blender's scene might become invalid after call
+	 * free_blender_memory_if_possible().
+	 */
 	BL::Scene b_scene;
 	BL::SpaceView3D b_v3d;
 	BL::RegionView3D b_rv3d;
@@ -172,6 +175,12 @@ protected:
 
 	/* Update tile manager to reflect resumable render settings. */
 	void update_resumable_tile_manager(int num_samples);
+
+	/* Is used after each render layer synchronization is done with the goal
+	 * of freeing render engine data which is held from Blender side (for
+	 * example, dependency graph).
+	 */
+	void free_blender_memory_if_possible();
 };
 
 CCL_NAMESPACE_END