Provide more information in job queue error and docs (#995)

* Provide more information in job queue error and docs

* format Java

* Trigger build

docs/configuration.md

@@ -173,7 +173,7 @@ Most of those properties are designed for performance tuning. Adjusting those nu
 * number_of_netty_threads: number frontend netty thread, default: number of logical processors available to the JVM.
 * netty_client_threads: number of backend netty thread, default: number of logical processors available to the JVM.
 * default_workers_per_model: number of workers to create for each model that loaded at startup time, default: available GPUs in system or number of logical processors available to the JVM.
-* job_queue_size: number inference jobs that frontend will queue before backend can serve, default 100.
+* job_queue_size: number inference jobs that frontend will queue before backend can serve, default 100. Useful in cases where certain requests take predictably longer to complete.
 * async_logging: enable asynchronous logging for higher throughput, log output may be delayed if this is enabled, default: false.
 * default_response_timeout: Timeout, in seconds, used for model's backend workers before they are deemed unresponsive and rebooted. default: 120 seconds.
 * unregister_model_timeout: Timeout, in seconds, used when handling an unregister model request when cleaning a process before it is deemed unresponsive and an error response is sent. default: 120 seconds.

frontend/server/src/main/java/com/amazonaws/ml/mms/http/InferenceRequestHandler.java

@@ -173,7 +173,9 @@ private void predict(
         Job job = new Job(ctx, modelName, WorkerCommands.PREDICT, input);
         if (!ModelManager.getInstance().addJob(job)) {
             throw new ServiceUnavailableException(
-                    "No worker is available to serve request: " + modelName);
+                    "No worker is available to serve request for model: "
+                            + modelName
+                            + ". Consider increasing job queue size.");
         }
     }