Run one worker on main thread

core/threaded/reactor_threaded.c

@@ -982,18 +982,6 @@ void lf_print_snapshot(environment_t* env) {
 }
 #endif // NDEBUG
 
-// Start threads in the thread pool.
-void start_threads(environment_t* env) {
-  assert(env != GLOBAL_ENVIRONMENT);
-
-  LF_PRINT_LOG("Starting %u worker threads in environment", env->num_workers);
-  for (int i = 0; i < env->num_workers; i++) {
-    if (lf_thread_create(&env->thread_ids[i], worker, env) != 0) {
-      lf_print_error_and_exit("Could not start thread-%u", i);
-    }
-  }
-}
-
 /**
  * @brief Determine the number of workers.
  */
@@ -1124,23 +1112,48 @@ int lf_reactor_c_main(int argc, const char* argv[]) {
     _lf_initialize_start_tag(env);
 
     lf_print("Environment %u: ---- Spawning %d workers.", env->id, env->num_workers);
-    start_threads(env);
+
+    for (int j = 0; j < env->num_workers; j++) {
+      if (i == 0 && j == 0) {
+        // The first worker thread of the first environment will be
+        // run on the main thread, rather than creating a new thread.
+        // This is important for bare-metal platforms, who can't
+        // afford to have the main thread sit idle.
+        continue;
+      }
+      if (lf_thread_create(&env->thread_ids[j], worker, env) != 0) {
+        lf_print_error_and_exit("Could not start thread-%u", j);
+      }
+    }
+
     // Unlock mutex and allow threads proceed
     LF_MUTEX_UNLOCK(&env->mutex);
   }
 
+  // main thread worker (first worker thread of first environment)
+  void* main_thread_exit_status = NULL;
+  if (num_envs > 0 && envs[0].num_workers > 0) {
+    environment_t* env = &envs[0];
+    main_thread_exit_status = worker(env);
+  }
+
   for (int i = 0; i < num_envs; i++) {
     // Wait for the worker threads to exit.
     environment_t* env = &envs[i];
     void* worker_thread_exit_status = NULL;
     int ret = 0;
-    for (int i = 0; i < env->num_workers; i++) {
-      int failure = lf_thread_join(env->thread_ids[i], &worker_thread_exit_status);
-      if (failure) {
-        lf_print_error("Failed to join thread listening for incoming messages: %s", strerror(failure));
+    for (int j = 0; j < env->num_workers; j++) {
+      if (i == 0 && j == 0) {
+        // main thread worker
+        worker_thread_exit_status = main_thread_exit_status;
+      } else {
+        int failure = lf_thread_join(env->thread_ids[j], &worker_thread_exit_status);
+        if (failure) {
+          lf_print_error("Failed to join thread listening for incoming messages: %s", strerror(failure));
+        }
       }
       if (worker_thread_exit_status != NULL) {
-        lf_print_error("---- Worker %d reports error code %p", i, worker_thread_exit_status);
+        lf_print_error("---- Worker %d reports error code %p", j, worker_thread_exit_status);
         ret = 1;
       }
     }

low_level_platform/impl/src/lf_zephyr_support.c

@@ -94,7 +94,9 @@ int lf_enable_interrupts_nested() {
 #define NUMBER_OF_WATCHDOGS 0
 #endif
 
-#define NUMBER_OF_THREADS (NUMBER_OF_WORKERS + USER_THREADS + NUMBER_OF_WATCHDOGS)
+// Number of additional threads that will be created
+// One worker will run on the main thread, so for N workers, only (N - 1) worker threads should be created
+#define NUMBER_OF_THREADS ((NUMBER_OF_WORKERS - 1) + USER_THREADS + NUMBER_OF_WATCHDOGS)
 
 K_MUTEX_DEFINE(thread_mutex);