Geometry Nodes: improve evaluator with lazy threading

In large node setup the threading overhead was sometimes very significant.
That's especially true when most nodes do very little work.

This commit improves the scheduling by not using multi-threading in many
cases unless it's likely that it will be worth it. For more details see the comments
in `BLI_lazy_threading.hh`.

Differential Revision: https://developer.blender.org/D15976

2 files changed, 33 insertions, 0 deletions

diff --git a/source/blender/blenlib/intern/lazy_threading.cc b/source/blender/blenlib/intern/lazy_threading.cc
new file mode 100644
index 00000000000..803fd81a96d
--- /dev/null
+++ b/source/blender/blenlib/intern/lazy_threading.cc
@@ -0,0 +1,30 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
+#include "BLI_lazy_threading.hh"
+#include "BLI_vector.hh"
+
+namespace blender::lazy_threading {
+
+/**
+ * This is a #RawVector so that it can be destructed after Blender checks for memory leaks.
+ */
+thread_local RawVector<FunctionRef<void()>, 0> hint_receivers;
+
+void send_hint()
+{
+  for (const FunctionRef<void()> &fn : hint_receivers) {
+    fn();
+  }
+}
+
+HintReceiver::HintReceiver(const FunctionRef<void()> fn)
+{
+  hint_receivers.append(fn);
+}
+
+HintReceiver::~HintReceiver()
+{
+  hint_receivers.pop_last();
+}
+
+}  // namespace blender::lazy_threading
diff --git a/source/blender/blenlib/intern/task_range.cc b/source/blender/blenlib/intern/task_range.cc
index 7e405529f03..181b760bea1 100644
--- a/source/blender/blenlib/intern/task_range.cc
+++ b/source/blender/blenlib/intern/task_range.cc
@@ -12,6 +12,7 @@
 
 #include "DNA_listBase.h"
 
+#include "BLI_lazy_threading.hh"
 #include "BLI_task.h"
 #include "BLI_threads.h"
 
@@ -104,6 +105,8 @@ void BLI_task_parallel_range(const int start,
     const size_t grainsize = MAX2(settings->min_iter_per_thread, 1);
     const tbb::blocked_range<int> range(start, stop, grainsize);
 
+    blender::lazy_threading::send_hint();
+
     if (settings->func_reduce) {
       parallel_reduce(range, task);
       if (settings->userdata_chunk) {