fix async mode, test=develop (#21367)

paddle/fluid/operators/distributed_ops/listen_and_serv_op.cc

@@ -464,23 +464,21 @@ void ListenAndServOp::RunImpl(const framework::Scope &scope,
   f(request_get_no_barrier_handler_.get());
   f(request_notify_handler_.get());
 
-  // start the server listening after all member initialized.
-  server_thread_.reset(new std::thread(RunServer, rpc_service_));
-  VLOG(3) << "wait server thread to become ready...";
-  rpc_service_->WaitServerReady();
-
   // register SIGINT(from ctrl+C) and SIGTERM(from kill) signal handlers
   signal(SIGINT, SignalHandler::StopAndExit);
   signal(SIGTERM, SignalHandler::StopAndExit);
 
-  // Cache the type of the received vars as `sparse_vars_` and `dense_vars_`
-  // so that we can reset them at the end of each iteration.
-  // NOTE: only used in sync update
-  CacheVarsType(inputs, recv_scope);
-
-  // Write to a file of server selected port for python use.
-  SavePort();
   if (sync_mode) {
+    // start the server listening after all member initialized.
+    server_thread_.reset(new std::thread(RunServer, rpc_service_));
+    VLOG(3) << "wait server thread to become ready...";
+    rpc_service_->WaitServerReady();
+
+    CacheVarsType(inputs, recv_scope);
+
+    // Write to a file of server selected port for python use.
+    SavePort();
+
     RunSyncLoop(&executor, program, &recv_scope, &dev_ctx,
                 prefetch_block_id_list, checkpoint_block_id);
   } else {
@@ -498,6 +496,15 @@ void ListenAndServOp::RunImpl(const framework::Scope &scope,
       split(grad_to_block_id_str[0], ':', &pieces);
       distributed::HeartBeatMonitor::Init(fan_in, pserver_id == 0, pieces[0]);
     }
+
+    // start the server listening after all member initialized.
+    server_thread_.reset(new std::thread(RunServer, rpc_service_));
+    VLOG(3) << "wait server thread to become ready...";
+    rpc_service_->WaitServerReady();
+
+    // Write to a file of server selected port for python use.
+    SavePort();
+
     RunAsyncLoop(&executor, program, &recv_scope);
   }
 }