docs: show how to use total argument for generators

README.md

@@ -147,12 +147,14 @@ from taskqueue import GreenTaskQueue
 
 tasks = ( PrintTask(i) for i in range(1000000) ) 
 with GreenTaskQueue('sqs-queue-name') as tq:
-  tq.insert_all(tasks)
+  tq.insert_all(tasks, total=(end - start))
 ```
 
 In Listing 5, we replace TaskQueue with GreenTaskQueue. Under the hood, TaskQueue relies on Python kernel threads to achieve concurrent IO. However, on systems with mutliple cores, especially those in a virutalized or NUMA context, the OS will tend to distribute the threads fairly evenly between cores leading to high context-switching overhead. Ironically, a more powerful multicore system can lead to lower performance. To remedy this issue, we introduce a user-space cooperative threading model (green threads) using gevent (which depending on your system is uses either libev or libuv for an event loop).  
 
-This can result in a substantial performance increase on some systems. Typically a single core will be fully utilized with extremely low overhead. However, using cooperative threading with networked IO in Python requires monkey patching the standard library (!!). Refusing to patch the standard library will result in single threaded performance. Thus, using GreenTaskQueue can introduce problems into many larger applications (we've seen problems with multiprocessing and ipython). However, often the task upload script can be isolated from the rest of the system and this allows monkey patching to be safely performed. To give users more control over when they wish to accept the risk of monkey patching, it is not performed automatically and a warning will appear with instructions for amending your program. 
+This can result in a substantial performance increase on some systems. Typically a single core will be fully utilized with extremely low overhead. However, using cooperative threading with networked IO in Python requires monkey patching the standard library (!!). Refusing to patch the standard library will result in single threaded performance. Thus, using GreenTaskQueue can introduce problems into many larger applications (we've seen problems with multiprocessing and ipython). However, often the task upload script can be isolated from the rest of the system and this allows monkey patching to be safely performed. To give users more control over when they wish to accept the risk of monkey patching, it is not performed automatically and a warning will appear with instructions for amending your program.  
+
+As generators do not support the `len` operator, we manually pass in the number of items to display a progress bar.
 
 ```python 
 # Listing 6: 1000s-10000 per second, low memory usage, near zero latency, efficient multiprocessing
@@ -166,7 +168,7 @@ def upload(args):
   start, end = args
   tasks = ( PrintTask(i) for i in range(start, end) ) 
   with GreenTaskQueue('sqs-queue-name') as tq:
-    tq.insert_all(tasks) 
+    tq.insert_all(tasks, total=(end - start)) 
 
 task_ranges = [ (0, 250000), (250000, 500000), (500000, 750000), (750000, 1000000) ]
 with ProcessPoolExecutor(max_workers=4) as pool: