trade num threads and speed for stability

api/app.py

@@ -45,7 +45,10 @@ def create_app() -> FastAPI:
         Evaluate the correctness of the solutions in the given samples data.
         """
         if parallel < 1:
-            n_workers = max(1, multiprocessing.cpu_count() // 2)
+            n_workers = min(4, multiprocessing.cpu_count() // 2)
+            if n_workers < 1:
+                n_workers = 1
+
         else:
             n_workers = parallel