Set thread QoS to USER_INITIATED on Apple Silicon

On Apple Silicon Macs, TBB worker threads are created with the default
QoS class, which macOS may schedule to efficiency cores even when
performance cores are available. This significantly degrades parallel
performance.

This adds a pthread_set_qos_class_self_np() call in on_scheduler_entry()
to set USER_INITIATED QoS, signaling to macOS that these are compute
threads the user is waiting for. This causes macOS to prefer performance
cores when available.

Fixes #3277

Author: ssp3nc3r

stan/math/rev/core/init_chainablestack.hpp

@@ -11,6 +11,11 @@
 #include <thread>
 #include <tuple>
 
+#ifdef __APPLE__
+#include <pthread.h>
+#include <sys/qos.h>
+#endif
+
 namespace stan {
 namespace math {
 
@@ -20,6 +25,10 @@ namespace math {
  * hook ensures that each worker thread has an initialized AD tape
  * ready for use.
  *
+ * On Apple Silicon, this also sets the thread QoS class to
+ * USER_INITIATED so that macOS prefers scheduling compute threads
+ * on performance cores rather than efficiency cores.
+ *
  * Refer to
  * https://software.intel.com/content/www/us/en/develop/documentation/tbb-documentation/top/intel-threading-building-blocks-developer-reference/task-scheduler/taskschedulerobserver.html
  * for details on the observer concept.
@@ -37,6 +46,13 @@ class ad_tape_observer final : public tbb::task_scheduler_observer {
   ~ad_tape_observer() { observe(false); }
 
   void on_scheduler_entry(bool worker) {
+#ifdef __APPLE__
+#if defined(__arm64__) || defined(__aarch64__)
+    // Set thread QoS to USER_INITIATED so macOS prefers scheduling
+    // TBB worker threads on performance cores rather than efficiency cores.
+    pthread_set_qos_class_self_np(QOS_CLASS_USER_INITIATED, 0);
+#endif
+#endif
     std::lock_guard<std::mutex> thread_tape_map_lock(thread_tape_map_mutex_);
     const std::thread::id thread_id = std::this_thread::get_id();
     if (thread_tape_map_.find(thread_id) == thread_tape_map_.end()) {