bp/UringGlue: limit io_uring workers to 64+64

These numbers have been tested on a machine with 96 CPU cores.

debian/changelog

@@ -9,6 +9,7 @@ cm4all-beng-proxy (19.3) unstable; urgency=low
   * bp: remove delegate support
   * io_uring: handle EAGAIN to full pipe
   * io_uring: add settings "io_uring_sqpoll", "io_uring_sq_thread_cpu"
+  * io_uring: limit io_uring workers to 64+64
   * do not resolve IPv6 scope ids to interface names
 
  --   

src/bp/UringGlue.cxx

@@ -51,6 +51,17 @@ UringGlue::UringGlue([[maybe_unused]] EventLoop &event_loop,
 		fprintf(stderr, "Failed to initialize io_uring: ");
 		PrintException(std::current_exception());
 	}
+
+	try {
+		/* limit the number of io_uring worker threads; having
+		   too many only leads to lock contention inside the
+		   kernel */
+		// TODO make configurable?
+		uring->SetMaxWorkers(64, 64);
+	} catch (...) {
+		fprintf(stderr, "Failed to set up io_uring: ");
+		PrintException(std::current_exception());
+	}
 #endif
 }