add example of using multi-thread tokio runtime (#294)

* add example of using multi-thread tokio runtime

* Update multi_thread_system.rs

Co-authored-by: Rob Ede <robjtede@icloud.com>

actix-rt/examples/multi_thread_system.rs

@@ -0,0 +1,60 @@
+//! An example on how to build a multi-thread tokio runtime for Actix System.
+//! Then spawn async task that can make use of work stealing of tokio runtime.
+
+use actix_rt::System;
+
+fn main() {
+    System::with_tokio_rt(|| {
+        // build system with a multi-thread tokio runtime.
+        tokio::runtime::Builder::new_multi_thread()
+            .worker_threads(2)
+            .enable_all()
+            .build()
+            .unwrap()
+    })
+    .block_on(async_main());
+}
+
+// async main function that acts like #[actix_web::main] or #[tokio::main]
+async fn async_main() {
+    let (tx, rx) = tokio::sync::oneshot::channel();
+
+    // get a handle to system arbiter and spawn async task on it
+    System::current().arbiter().spawn(async {
+        // use tokio::spawn to get inside the context of multi thread tokio runtime
+        let h1 = tokio::spawn(async {
+            println!("thread id is {:?}", std::thread::current().id());
+            std::thread::sleep(std::time::Duration::from_secs(2));
+        });
+
+        // work stealing occurs for this task spawn
+        let h2 = tokio::spawn(async {
+            println!("thread id is {:?}", std::thread::current().id());
+        });
+
+        h1.await.unwrap();
+        h2.await.unwrap();
+        let _ = tx.send(());
+    });
+
+    rx.await.unwrap();
+
+    let (tx, rx) = tokio::sync::oneshot::channel();
+    let now = std::time::Instant::now();
+
+    // without additional tokio::spawn, all spawned tasks run on single thread
+    System::current().arbiter().spawn(async {
+        println!("thread id is {:?}", std::thread::current().id());
+        std::thread::sleep(std::time::Duration::from_secs(2));
+        let _ = tx.send(());
+    });
+
+    // previous spawn task has blocked the system arbiter thread
+    // so this task will wait for 2 seconds until it can be run
+    System::current().arbiter().spawn(async move {
+        println!("thread id is {:?}", std::thread::current().id());
+        assert!(now.elapsed() > std::time::Duration::from_secs(2));
+    });
+
+    rx.await.unwrap();
+}