Task scheduler: Remove per-pool threads limit

This feature was adding extra complexity to task scheduling
which required yet extra variables to be worried about to be
modified in atomic manner, which resulted in following issues:

- More complex code to maintain, which increases risks of
  something going wrong when we modify the code.

- Extra barriers and/or locks during task scheduling, which
  causes extra threading overhead.

- Unable to use some other implementation (such as TBB) even for
  the comparison tests.

Notes about other changes.

There are two places where we really had to use that limit.

One of them is the single threaded dependency graph. This will
now construct a single-threaded scheduler at evaluation time.
This shouldn't be a problem because it only happens when using
debugging command line arguments and the code simply don't
run in regular Blender operation.

The code seems a bit duplicated here across old and new
depsgraph, but think it's OK since the old depsgraph is already
gone in 2.8 branch and i don't see where else we might want
to use such a single-threaded scheduler.

When/if we'll want to do so, we can move it to a centralized
single-threaded scheduler in threads.c.

OpenGL render was a bit more tricky to port, but basically we
are using conditional variables to wait background thread to
do all the job.

1 files changed, 17 insertions, 1 deletions

diff --git a/source/blender/editors/render/render_opengl.c b/source/blender/editors/render/render_opengl.c
index 90dac7c34ce..1d0f433ba38 100644
--- a/source/blender/editors/render/render_opengl.c
+++ b/source/blender/editors/render/render_opengl.c
@@ -715,7 +715,6 @@ static bool screen_opengl_render_init(bContext *C, wmOperator *op)
 			oglrender->task_scheduler = task_scheduler;
 			oglrender->task_pool = BLI_task_pool_create_background(task_scheduler,
 			                                                       oglrender);
-			BLI_pool_set_num_threads(oglrender->task_pool, 1);
 		}
 		else {
 			oglrender->task_scheduler = NULL;
@@ -747,6 +746,23 @@ static void screen_opengl_render_end(bContext *C, OGLRender *oglrender)
 	int i;
 
 	if (oglrender->is_animation) {
+		/* Trickery part for movie output:
+		 *
+		 * We MUST write frames in an exact order, so we only let background
+		 * thread to work on that, and main thread is simply waits for that
+		 * thread to do all the dirty work.
+		 *
+		 * After this loop is done work_and_wait() will have nothing to do,
+		 * so we don't run into wrong order of frames written to the stream.
+		 */
+		if (BKE_imtype_is_movie(scene->r.im_format.imtype)) {
+			BLI_mutex_lock(&oglrender->task_mutex);
+			while (oglrender->num_scheduled_frames > 0) {
+				BLI_condition_wait(&oglrender->task_condition,
+				                   &oglrender->task_mutex);
+			}
+			BLI_mutex_unlock(&oglrender->task_mutex);
+		}
 		BLI_task_pool_work_and_wait(oglrender->task_pool);
 		BLI_task_pool_free(oglrender->task_pool);
 		/* Depending on various things we might or might not use global scheduler. */