Add Back pressure section

README.md

@@ -1428,3 +1428,7 @@ The user is not blocked and the job is processed in the background.  During this
 Tasks queues receive tasks and their related data, runs them, then delivers their results.  They can support scheduling and can be used to run computationally-intensive jobs in the background.
 
 **Celery** has support for scheduling and primarily has python support.
+
+### Back pressure
+
+If queues start to grow significantly, the queue size can become larger than memory, resulting in cache misses, disk reads, and even slower performance.  [Back pressure](http://mechanical-sympathy.blogspot.com/2012/05/apply-back-pressure-when-overloaded.html) can help by limiting the queue size, thereby maintaining a high throughput rate and good response times for jobs already in the queue.  Once the queue fills up, clients get a server busy or HTTP 503 status code to try again later.  Clients can retry the request at a later time, perhaps with [exponential backoff](https://en.wikipedia.org/wiki/Exponential_backoff).