fix(root): spawn state root task only if host has enough parallelism (#14555)

crates/engine/tree/src/tree/mod.rs

@@ -2431,8 +2431,11 @@ where
         // Atomic bool for letting the prewarm tasks know when to stop
         let cancel_execution = ManualCancel::default();
 
+        let use_legacy_state_root =
+            self.config.legacy_state_root() || !root::has_enough_parallelism();
+
         let (state_root_handle, state_root_task_config, state_root_sender, state_hook) =
-            if is_descendant_of_persisting_blocks && !self.config.legacy_state_root() {
+            if is_descendant_of_persisting_blocks && !use_legacy_state_root {
                 let consistent_view = ConsistentDbView::new_with_latest_tip(self.provider.clone())?;
 
                 // Compute trie input
@@ -2557,7 +2560,7 @@ where
         // a different database transaction per thread and it might end up with a
         // different view of the database.
         let (state_root, trie_output, root_elapsed) = if is_descendant_of_persisting_blocks {
-            if self.config.legacy_state_root() {
+            if use_legacy_state_root {
                 match self.compute_state_root_parallel(block.header().parent_hash(), &hashed_state)
                 {
                     Ok(result) => {

crates/engine/tree/src/tree/root.rs

@@ -52,6 +52,18 @@ pub(crate) fn thread_pool_size() -> usize {
     std::thread::available_parallelism().map_or(3, |num| (num.get() / 2).max(3))
 }
 
+/// Determines if the host has enough parallelism to run the state root task.
+///
+/// It requires at least 5 parallel threads:
+/// - Engine in main thread that spawns the state root task.
+/// - State Root Task spawned in [`StateRootTask::spawn`]
+/// - Sparse Trie spawned in [`run_sparse_trie`]
+/// - Multiproof computation spawned in [`MultiproofManager::spawn_multiproof`]
+/// - Storage root computation spawned in [`ParallelProof::multiproof`]
+pub(crate) fn has_enough_parallelism() -> bool {
+    std::thread::available_parallelism().is_ok_and(|num| num.get() >= 5)
+}
+
 /// Outcome of the state root computation, including the state root itself with
 /// the trie updates and the total time spent.
 #[derive(Debug)]